CQD Special Seminars

Optical tweezers and tightly focused beams are now central tools in atomic and molecular quantum science, but many current platforms operate in regimes where plane-wave, paraxial, and point-particle approximations are no longer sufficient. In this talk, I will outline how our previous work on structured light-matter interaction provides the technical basis for a research program that treats the microscopic structure of optical fields as a genuine control resource, rather than as a hidden experimental detail.

The first part of the program focuses on atoms and molecules in realistic high-numerical-aperture tweezer fields. Longitudinal field components, polarization gradients, tensor light shifts, and spatial phase structure can generate internal-motion entanglement, decoherence, leakage, and heating. A predictive microscopic theory is therefore needed to identify dominant error channels and design mitigation strategies. For molecules, the rotational degree of freedom amplifies these effects, but also opens new possibilities for rotational control and engineered dipolar interactions. The second part applies these ideas to the trap-resolved assembly of microwave-dressed molecular complexes. Here, structured optical confinement and microwave-induced interactions define light-induced potential energy surfaces for coherent dimer and trimer formation. I will discuss how wavepacket methods, especially multilayer MCTDH, can be used to model and optimize these processes, and why Heidelberg provides an ideal environment for developing this direction.

The dynamics of interacting, disordered quantum systems is a central topic in many-body physics. Dense ensembles of nitrogen-vacancy centres (NVs) in diamond provide a unique platform to realise a strongly interacting, intrinsically disordered spin systems with long-range dipolar interactions at room temperature. This enables access to rich many-body physics including thermalisation, non-equilibrium dynamics, and dimensional crossovers.

In this talk, I will first introduce the NV as a controllable spin platform and discuss how interactions within an ensemble give rise to complex many-body behaviour. I will then explain how we can probe the spin environment and selectively control interactions with Hamiltonian engineering techniques. A central challenge in using these systems as quantum simulators is the lack of precise knowledge of the interaction within a sample. I will present my work on using tailored pulse sequences as spectroscopic tools to characterise the spin bath and extract key parameters governing the system dynamics. This provides a route towards quantitative control of disordered spin ensembles and the exploration of emergent many-body physics in solid-state quantum simulators.

We theoretically investigate supersolids in a cylindrical tube under finite supercurrent and show that both the
contrast of the density modulation and superfluid fraction depend sensitively on the imposed flow. By imposing a
phase twist on the condensate wave function, we demonstrate the existence of stationary supersolid states carrying
finite current. Furthermore, we find that a superfluid near the roton instability can be driven into the supersolid
phase via phase twisting, providing an alternative route to supersolidity that does not rely on modifying interparticle
interactions, as is commonly done in experiments. At sufficiently large currents, Landau and dynamical instabilities
emerge, beyond which stationary solutions cease to exist and the system evolves into time-dependent states. For
strong phase twists, phase slips can occur, leading to current reversal.

Chromium atoms in their ground state have a large spin and a large permanent magnetic dipole moment. The
long-range and anisotropic dipole-dipole interactions between the atoms confer unique properties to ultracold
chromium gases (chromium BECs). At LPL, we experimentally explore how isolated quantum systems with long-
range interactions evolve after being prepared out of equilibrium and ultimately thermalize.
In a 3D optical lattice, we have demonstrated quantum thermalization toward a high-temperature thermal state
[1], and we have measured the growth of quantum spin correlations during thermalization [2,3]. Using dynamical
decoupling techniques, the team has recently studied spin coherence and itinerant magnetism across the
superfluid–Mott transition [4,5].
The chromium experimental apparatus at LPL, originally built in 2004, now requires significant upgrades to
achieve our new scientific objectives. In this talk, I will present the main limitations of the former apparatus, the
current design of the new experimental platform, and discuss the new scientific goals.

In 2021, the design and construction of the ROYMAGE (Mobile Ytterbium Optical Clock Applied to Geodesic
Exploration) neutral ytterbium optical clock were initiated at the Paris Observatory within the LTE (Laboratoire
Temps Espace), formerly SYRTE. Following caesium, rubidium, strontium and mercury, a fifth species is now
being studied in the context of the emerging redefinition of the second.
As predicted by Albert Einstein, time is coupled to the gravitational potential, an effect known as gravitational
time dilation. For an atomic clock, this results in a direct link between frequency and the local geopotential
experienced by the atoms.
The ROYMAGE instrument is therefore designed to be transportable. It will be connectable to the
approximately 60 outputs of the REFIMEVE fiber network, which disseminates an ultrastable carrier at 1542
nm, enabling comparisons with about 12 stationary optical clocks across Europe. These measurements are
expected to contribute to altitude referencing and geoid determination, particularly in regions where
conventional geodetic methods such as levelling or satellite techniques are not well suited to abrupt variations
in gravitational potential.

Sensing applications with ultracold atoms range from gravitational wave detection to timekeeping with optical clocks, as well as probing fundamental physical constants. In my presentation, I will focus on the Atom Interferometer Observatory Network (AION) project, a current UK-based collaboration aiming to build ultra-sensitive quantum sensors in search of new physics, including detection of mid-frequency gravitational waves and ultra-light dark matter. The talk will summarize the ongoing work with preparing fermionic strontium for interferometric measurements at the University of Cambridge as part of the AION collaboration. Specifically, I will highlight my contributions to the optical dipole trapping and red magneto-optical trap (1S0 -> 3P1) stages during my master's thesis there.

Mobile impurities interacting with a quantum medium form quasiparticles known as polarons, a central concept in many-body physics. While the impurity problem has been widely studied with ultracold atoms, repulsive polarons in the strongly correlated regime remain difficult to access because they rapidly decay into molecular states before a well-defined dressing cloud can form.

In this talk, I will present the realization of a long-lived, strongly interacting repulsive Bose polaron in a two-dimensional system. Using a Bose–Einstein condensate of ⁶Li dimers, we create impurities by exciting a small fraction of dimers into higher vibrational states of the tightly confined potential, effectively mapping the polaron problem into synthetic spins. This approach circumvents the intrinsic metastability that has so far limited access to the repulsive branch. Using trap-modulation and Bragg spectroscopy, we probe the polaron spectrum and measure both the energy and effective mass, revealing strong many-body dressing with a polaron mass exceeding twice that of a free dimer. 

In addition, I will briefly discuss a new experimental setup at IGFAE - Santiago de Compostela aimed at exploring the BEC–BCS crossover with dipolar interactions, opening the door to investigating fermionic pairing and many-body systems in the presence of long-range interactions.

Scale invariance lies at the foundation of modern statistical physics and underpins the description of continuous phase transitions. Its most striking manifestation is the universal probability distribution function (PDF) of the order parameter, which encapsulates the complete statistical structure of critical fluctuations—beyond what traditional quantities such as averages or critical exponents can reveal. However, this universal distribution is exceptionally challenging to measure, as it reflects the non-Gaussian and scale-invariant nature of critical fluctuations.

We will report on the experimental study of the statistics of the condensate order parameter across the superfluid–Mott transition in a gas of 3D lattice bosons, making use of single-atom-resolved detection in momentum space [1]. First, we observe non-Gaussian statistics of the order parameter near the transition, distinguished by non-zero and oscillating high-order cumulants [2]. We provide direct experimental evidence that these oscillations are universal. Second, crossing the Mott transition for various entropies and collapsing the cumulant oscillations, we obtain the non-universal coefficients required to reconstruct the universal PDF [3]. Finally, this universal scaling function determined experimentally is shown to yield algebraic scaling laws whose exponents are consistent with the critical exponents of the (expected) 3D XY universality class.

I will review recent works about how nonthermal fixed points are approached and what happens if one breaks (on average) spatial homogeneity and discuss new thoughts on these subjects. Based on 2502.01622, 2504.18754 and new results with Matisse De Lescluze

One of the great challenges of atomic physics is to accurately prepare, manipulate and measure the quantum-mechanical state of a physical system. One particular property of multi-particles quantum states is entanglement. This property is of high interest for performing non-classical calculations for the use in quantum information or for sensitivity enhanced measurements. Spin entangled states of many body atomic ensembles have been engineered and validated. Isolating a single atomic pair thanks to optical tweezers allows to deeply investigate spin-changing collision at the particle level and the entangled state. So far, the spin entanglement of an atomic pair have been successful for groundstate- cooled atoms. Being able to maintain it at a higher temperature would be a step forward to robust measurements into real-world field implementations. Here, we study hot spin-exchange collision as a route to entanglement. In previous works, we observed the population dynamics of the magnetic sublevels of an atomic pair of ⁸⁵Rb prepared separately in two microtraps undergoing a collision in an optical tweezer. The spin-changing collision of two thermal atoms initially prepared in a m = 0 state leads to strong spin pair correlations between the magnetic states m = 1 and m = −1. To probe the entanglement of the pair, a Raman transition pulse couple the two magnetic sublevels, leading to a destructive interference when the pair is entangled. Our measurements and a simulation taking into account the full level structure of the atom while applying the Raman pulse, show that the spin exchange collision successfully create an entangled pair from two thermal atoms. Applying a magnetic gradient that the atomic pair experiences, introduces a bias between the two magnetic states and therefore destroy the entanglement of the pair.  As a proof of principle, we show that this resulting entanglement could be useful for magnetic fields measurements beyond the standard quantum limit.

Persistent currents in annular geometries have played a key role in disclosing the quantum phase coherence of superconductors while keeping a tight connection with a hydrodynamic interpretation. Recently, ultracold fermionic gases joined exhibiting long-lived supercurrents in annular geometries, and have attracted much interest for fundamental studies of superfluid dynamics, such as the interaction between two adjacent supercurrents. Here, we observe how the contact interface between two counter-rotating atomic superflows develops into an ordered circular array of quantized vortices when merging the supercurrents. The vortex array loses stability and rolls up into ever-increasing cluster size. We extract the instability growth rates and find they obey the same scaling relations across different superfluid regimes, ranging from weakly interacting bosonic to strongly correlated fermionic pair condensates. These results establish interesting connections between vortex arrays and shear flow instabilities, suggesting a possible interpretation of the observed quantized vortex dynamics as a manifestation of the underlying un stable flow. Moreover,they open the way for exploring out-of-equilibrium phenomena.

https://www.nature.com/articles/s41567-024-02466-4

Cold atomic gases have proven a valuable medium in which to study early universe phase transitions. Here, we make use of a two-dimensional, three-component spin-1 gas to model first-order false vacuum decay. We identify a metastable state within the phase structure of components and examine its evolution using the stochastic projected Gross-Pitaevskii equation. We explore the dependence of the rate of vacuum decay on density and temperature and compare our numerical findings with instanton predictions. We then investigate the consequences of introducing an optical box trap.

We measure the dynamical growing of quantum correlations of a large ensemble of dipolar chromium atoms, during an out-of-equilibrium dynamic, taking place in a 3D deep optical lattice. Two-point correlators associated with the magnetization are measured from ensemble measurements, assuming homogeneity. While collective measurements show that globally anti-correlations develop in our system, the implementation of a bipartite protocol allows to investigate the correlation landscape, and to demonstrate a strong anisotropy of correlations, linked to the anisotropic nature of the dipolar interaction. Our various theoretical models offer a description of the system throughout the dynamics. In particular, at long time, where quantum thermalization leads to a stationary state with thermal properties, we can point thermalization at a high negative spin temperature. Recent results regarding measurements of the norm of the collective spin of the atoms using  the Dynamical Decoupling technique will be also discussed.
 

Recent experiments on ultracold atoms offer the exciting possibility to probe few-body quantum gases with exceptional accuracy: For example, the particle number can be varied, the excitation spectrum can be probed precisely, and even the position of single atoms can be measured [1]. This makes cold quantum gases an ideal platform to test the emergent behavior of many-particle systems.  

In this talk, I present the consequences of a joint scale and conformal invariance in mesoscopic  two-dimensional  Fermi  gases  at  weak  interactions.  This  system  had previously been overlooked since a quantum anomaly was assumed to the invariance, but we show that a conformal invariance remains at weak interactions. The presented results provide evidence for the conformal tower structure in the energy spectrum of a nonrelativistic conformally invariant interacting system [2]. I will discuss different ways of exciting states in the energy spectrum. Furthermore, the conformal symmetry predicts the  hyperradial  distribution  function  of  the  many-body  wavefunctions  in  closed analytical form, which we have confirmed using Metropolis importance sampling. 

Moreover, I will show that the symmetries persists in rotating Fermi gases, where the interpretation of excited states in a conformal tower are breathing modes that leave the center of mass intact, and two different center-of-mass excitations describing cyclotron and guiding-center motion of the total particle cloud in analogy with the Foucault pendulum [3]. Since the Coriolis force due to rotation is mathematically equivalent to the magnetic Lorentz force, rotating Fermi gases mimic electrons in a magnetic field. I will discuss how lowest Landau level states are present already at finite rotation frequencies before the centrifugal force deconfines the particles. 

Quantum gases trapped in a box offer a versatile playground for exploring many-body dynamics. The box trap features homogeneous bulk potential with sharp walls at the boundary, allowing us to access intricate many-body states and dynamics inaccessible with samples in conventional harmonic traps.  

Here, I will present a set of studies with atomic superfluids confined in two-dimensional (2D) optical boxes, investigated at Purdue University. I will first discuss nonequilibrium dynamics observed in repulsive interaction regimes. By studying the interaction of a repulsive gas with a sharp circular wall, we show how a sudden wall reduction leads to the generation of ring dark solitons (RDSs). We observe transverse (snake) instability at discrete azimuthal angles, which clearly results in a self-patterned formation of a circular vortex dipole array [1]. In addition, by introducing a particle sink with strong losses in a homogeneous gas, we observe a convergent supersonic flow [2], triggering Landau instability, which manifests as a periodic RDS emission. The observed flow indicates intriguing quasi-periodic bursts of superluminal signals at a periodicity consistent with expected soliton oscillations.  

Second, I will discuss our recent studies of box-trapped 1D Bose gases quenched from repulsive to attractive interaction, leading to modulation instability (MI). It is known that MI amplifies initial density fluctuations, resulting in the formation of solitonic excitations recently observed with samples in a 1D harmonic trap or a 2D box. Unlike phase-incoherent solitonic excitations, however, we observe multi-mode breathers, for the first time, resulting from the nonlinear stage of MI in the integrability-preserving box potential [3]. We demonstrate a form of dynamical crystallization in which periodic density modulations recur dynamically in a time evolution intertwined with the reduction and recovery of global phase coherence. These studies have shown that spatial homogeneity and sharp boundaries in box-trapped gases not only lead to unexpected outcomes but also open various applications, such as forming complex 2D vortex matters and rich instability-induced dynamics. 

Lastly, if time remains, I will introduce a novel pathway toward controlling many-body systems developed at the Institute for Molecular Science. It involves closely packed ultracold atoms, either in optical lattices or optical tweezers, excited with an ultrashort laser pulse to a Rydberg state far beyond the Rydberg blockade regime, developing into correlated matter on an ultrafast timescale.

[1] H. Tamura, C.-A. Chen, C.-L. Hung, Phys. Rev. X 13, 031029 (2023). 
[2] H. Tamura, S. Khlebnikov, C.-A. Chen, C.-L. Hung, arXiv 2304.10667 (2023). 
[3] H. Tamura et al., in preparation.

Programmable arrays of neutral atoms trapped in optical tweezers and lattices have emerged as a powerful tool for studies of competitive optical atomic clocks, as well as the generation of entangled quantum states with the use of Rydberg interactions and methods from both analog quantum simulation and digital quantum information processing. In this talk, I will discuss our efforts to merge these two capabilities and use Rydberg interactions to generate entanglement that can be applied to optical-frequency measurements on a platform compatible with state-of-the-art frequency precision. First, I will describe work in which we create spin squeezed states with almost 4 dB of metrological gain. We use these states to perform synchronous optical-frequency comparisons between independent ensembles of atoms in our array and realize a fractional-frequency stability of 1.087(1)x10^-15 after one second of averaging time. This stability represents a 1.94(1) dB improvement over the theoretically achievable precision for this measurement when performed with the same number of unentangled atoms, known as the standard quantum limit. Second, I will present results on generating Greenberger-Horne-Zeilinger (GHZ) states on the clock transition in strontium. We investigate the possibility of leveraging cascades of GHZ states with different sizes for performing measurements that might outperform comparable classical states, even in the presence of frequency noise that would typically lead to phase excursions beyond the invertible regime for the largest GHZ states.

Symmetry-breaking quantum phase transitions lead to the production of topological defects or domain walls in a wide range of physical systems. In second-order transitions, these exhibit universal scaling laws described by the Kibble-Zurek mechanism, but for first-order transitions a similarly universal approach is still lacking. Here we propose a spinor Bose-Einstein condensate as a testbed system where critical scaling behavior in a first-order quantum phase transition can be understood from generic properties. We generalize the Kibble-Zurek mechanism to determine the critical exponents for: (1) the onset of the decay of the metastable state on short times scales, and (2) the number of resulting phase-separated ferromagnetic domains at longer times, as a one-dimensional spin-1 condensate is ramped across a first-order quantum phase transition. The predictions are in excellent agreement with mean-field numerical simulations and provide a paradigm for studying the decay of metastable states in experimentally accessible systems.

False vacuum decay plays a vital role in many models of the early Universe. However, we lack a satisfying theoretical understanding of this process, with existing approaches working only in imaginary (Euclidean) time, and relying on crucial assumptions that have yet to be empirically tested. An exciting route forward is to use cold-atom systems which undergo first-order phase transitions that are analogous to vacuum decay. In this talk, I will present recent theoretical work to understand this analogy using semiclassical lattice simulations, and will discuss possibilities and challenges for realising these analogues in the laboratory.

Rapidly rotating quantum gases realize the physics of charged particles in high magnetic fields. We developed a novel protocol, geometric squeezing, that enables to create Bose-Einstein condensates in a single Landau gauge wavefunction of the lowest Landau level. Based on the non-commutativity of guiding center X and Y coordinates, geometric squeezing in a saddle potential is a real space analogue to squeezing in phase space of an inverted 1D harmonic oscillator. The condensate’s transverse width shrinks to the Heisenberg-limited ground-state extent of cyclotron motion. Removing the saddle enables studying the evolution of a Landau gauge condensate in "flat land" under the sole influence of interactions. Surprisingly, we find that Landau gauge condensates are unstable towards crystallization into arrays of droplets. This instability of states in the lowest Landau level has its classical analogy in the Kelvin-Helmholtz instability of counterflowing liquids. We explore the crossover of this instability from the lowest Landau level to the Thomas-Fermi regime. I will discuss experiments on observing edge states in confined geometries and prospects to extend this work beyond mean-field quantum Hall states of bosons.

Rydberg atoms, with their giant electronic orbitals, exhibit dipole-dipole interaction reaching the GHz range at a distance of a micron (C₃ ~ GHz.μm³), making them a prominent contender for realizing ultrafast quantum operations. However, such strong interactions have never been harnessed so far because of the stringent requirements on the fluctuation of the atom positions and the necessary excitation strength. Here, we introduce novel techniques to enter this regime and explore it with two strongly-interacting single atoms [1].

First, we trap ⁸⁷Rb atoms in holographic tweezers focused with a high-NA lens (0.75), allowing to bring two atoms at distance as close as 1.2 µm. The atoms are then cooled to the motional ground-state of the tweezers and thus localized with a quantum-limited precision of 30 nm, which allows to unlock coherent ultrastrong interaction. Then, we use ultrashort, picosecond, laser pulses to excite a pair of these close-by atoms to a Rydberg state simultaneously [2], far beyond the Rydberg blockade regime.

Following excitation, atoms experience the dipole-dipole interaction, which, for our particular choice of Rydberg state, gives rise to an energy exchange between the two atoms [3]. We observe this coherent dynamic occurring on the nano-second timescale. After a full exchange, the atoms are back in their initial orbitals with a π-phase shift. We measured this phase shift by probing the superposition of a ground and Rydberg orbital by Ramsey interferometry with attosecond precision. This phase shift is the key to the realization of an ultrafast two-qubit C-Z gate. The techniques demonstrated here opens the path for ultrafast quantum simulation and computation operating at the speed-limit set by dipole-dipole interactions.

References

[1] Y. Chew et al., “Ultrafast energy exchange between two single Rydberg atoms on a nanosecond timescale”, Nat. Photonics 16, 724 (2022).

[2] Mizoguchi et al., “Ultrafast creation of overlapping Rydberg electrons in an atomic BEC and Mott-insulator lattice”, Phys. Rev. Lett. 124, 253201 (2020).

[3] Ravets, S. et al. “Coherent dipole–dipole coupling between two single Rydberg atoms at an electrically-tuned Förster resonance”, Nat. Physics 10, 914 (2014).

I will present experimental studies of the interaction of light and matter from two different point of views. First, I will discuss the decoherence of one-dimensional bosons in a lattice when subjected to light scattering. Then, I will report on the optical response of a two-dimensional gas of two-level atoms in an optically-dense regime.

Understanding the fundamental limits of information processing with quantum indistinguishable particles requires a theoretical toolkit for quantifying their potential nonclassical properties. I will present a selection of recent advances in the description of quantum correlations between indistinguishable particles, as well as novel thermodynamical properties resulting from indistinguishability with no classical analogue.

I begin this talk by reviewing the existence of supersolid and melted supersolid phases (hexatic superfluids) in two-dimensional continuum dipolar boson systems [1]. Immediately after that, I discuss the emergence of supersolid phases of dipolar and spin-orbit coupled bosons in optical lattices. For dipolar systems, we show that the ground state phase diagram is very sensitive to the direction of an externally applied field with respect to the normal to the plane of a two-dimensional square optical lattice, and that supersolids are stabilized by dipolar interactions [2]. We find that the phase diagram, at high filling factors, is very rich with various supersolid (e.g., checkerboard and striped) phases emerging out of superfluid regions [2]. For spin-orbit coupled systems in two-dimensional square optical lattices, we show that the competition between the optical lattice period and the spin-orbit coupling length – which can be made comparable in experiments – along with spin hybridization induced by a transverse field (i.e., Rabi coupling) and local interparticle interactions, create a rich variety of quantum phases including uniform and phase separated superfluids and supersolids [3]. Finally, I present recent results describing the existence of a Devil’s staircase of supersolid phases, when the spin-orbit coupling momentum transfer is not aligned with the principal axis of the square lattice [4].

[1] “Hexatic, Wigner Crystal, and Superfluid Phases of Dipolar Bosons”, K. Mitra, C. J. Williams, and C. A. R. Sá de Melo, arXiv:0903.4665v1 (26 Mar 2009).

[2] “Stability of Superfluid and Supersolid Phases of Dipolar Bosons in Optical Lattices”, I. Danshita and C. A. R. Sá de Melo, Phys. Rev. Lett. 103, 225301 (2009).

[3] “Quantum Phases of Two-Component Bosons with Spin-Orbit Coupling in Optical Lattices”, D. Yamamoto, I. B. Spielman, and C. A. R. Sá de Melo, Phys. Rev. A 96, 061603(R) (2017).

[4] “Supersolid Devil’s Staircases of Spin-Orbit-Coupled Bosons in Optical Lattices”, D. Yamamoto, K. Bannai, N. Furukawa, and C. A. R. Sá de Melo, Phy. Rev. Res. 4, L032023 (2022)

Over the last two decades, NV centers have gained interest in the life sciences due to their nanoscale sensing and imaging abilities. Real-space imaging techniques with NV centers are either limited by the optical diffraction limit of approximately 400 nm or require cumbersome point-by-point scanning probe techniques for nanoscale resolution. An alternative technique in Fourier imaging from conventional magnetic resonance imaging (MRI) has been shown to go beyond this limit, however, with scanning probe microscopy. This thesis provides a proof of concept of the Fourier imaging technique with widefield microscopy. The design is simulated with the use of COMSOL Multiphysics, and the theoretical spatial resolution is discussed.

We consider a mixture of two Bose-Einstein condensates, one with antidipolar interactions and second nondipolar component, radially confined in a harmonic potential (quasi-infinite tube). We characterize the phase diagram of this binary system and predict a phase transition from a uniform miscible phase to an antidipolar supersolid, induced by a roton instability. We also show the dynamic formation of the supersolid after a quench across the phase transition.

Dark matter(DM) halos composed of ultralight bosons exhibit wavy behaviour with de Broglie wavelength in cosmological scales, known as fuzzy DM (FDM), wave DM or BECDM. To the leading order of the space-time metric, the effective equation of motion is the Schrodinger-Poison system of equation, a classical-field wavefunction coupled to Newtonian gravity, and is reminiscent of the universal phenomenon of Bose-Einstein condensation (BEC), described by a macroscopic condensate wavefunction. This model reproduces the density distribution in large length scales in the cold DM model, called Navarro–Frenk–White profile, and can be a candidate to resolve the missing-satellite, too-big-to-fail and cusp-core problems with a compact solitonic core in the centre of a halo. Here inspired by widely-studied laboratory atomic systems we systematically examine the BEC concept by examining the field fluctuations in fuzzy dark matter halos, generated by our merger simulations, via probing the spatial phase-phase and density-density correlation functions to unveil the FDM halo properties. We find out that the solitonic core is fully coherent and coincides with the Penrose-Onsager condensate mode, exhibiting off-diagonal-long-range order, of a virialized halo. Moving outward from the core, fluctuations enhance and the bimodal fit of the core-halo profile can nicely capture the crossover length scale. By looking at the energy distribution, we demonstrate that these fluctuations are mainly sourced by a large number of quantized vortices, indicating a turbulence-like state, which is persistent in our simulation. In addition, the intervortex distance scale matches the granule one by comparing the vortex energy and overdensity power spectra. This work provides a new picture to investigate the FDM halos.

Mixtures of Bose-Einstein condensate offer situations where the usual mean-field interaction is reduced and higher-order terms may play a dominant role in the equation of state. In this context, the case of coherently coupled two component Bose-Einstein condensate will be addressed. First, we demonstrate a method to engineer large attractive three body interactions with striking consequences on the system properties [1]. Second, we measure the beyond-mean field equation of state and show that it is modified as compared to the uncoupled case [2]. 

[1] A. Hammond, L. Lavoine, and T. Bourdel, ‘’Tunable three-body interactions in driven two-component Bose-Einstein condensates’’, Phys. Rev. Lett. 128, 083401
[2] L. Lavoine, A. Hammond, A. Recati, D. S. Petrov, and T. Bourdel, ‘’Beyond-Mean-Field Effects in Rabi-Coupled Two-Component Bose-Einstein Condensate ‘’, Phys. Rev. Lett. 127, 203402, 2021

Rydberg atom arrays are promising candidates for high-quality quantum computation and quantum simulation. However, long state preparation times limit the amount of measurement data that can be generated at reasonable timescales. This restriction directly affects the estimation of operator expectation values, as well as the reconstruction and characterization of quantum states. Over the last years, neural networks have been explored as a powerful and systematically tuneable ansatz to represent quantum wave functions. Via tomographical state reconstruction, such numerical models can significantly reduce the amount of necessary measurements to accurately reconstruct operator expectation values. At the same time, neural networks can find ground state wave functions of given Hamiltonians via variational energy minimization. In this talk, I will apply both the data-driven and the Hamiltonian-driven training procedures to reconstruct the ground state of a two-dimensional array of Rydberg atoms in the vicinity of a quantum phase transition. I will demonstrate the limitations of the individual approaches and show that a combination of the two leads to a significant enhancement in the variational ground state search by naturally finding an improved network initialization from a limited amount of measurement data.

Quantum metrology has concentrated almost exclusively on using integrable systems as sensors, such as precessing spins or harmonic oscillators prepared in non-classical states.  Here we show that large benefits can be drawn from rendering integrable quantum sensors chaotic, both in terms of achievable sensitivity as well asrobustness to noise, while avoiding the challenge of preparing and protecting large-scale entanglement.  In the presence of dissipation, a stationary non-equilibrium state can be reached for large times that contains substantial amount of quantum Fisher information about the parameter to be measured, while without chaotic driving the system has long decayed to its ground state.  Classically, such a state corresponds to a strange attractor with a filigrane, fractal structure. 

After demonstrating the principles at the hand of the “kicked top”, we apply the method to spin-precession magnetometry and show that the sensitivity of state-of-the-art magnetometers can be further enhanced by subjecting the spin-precession to non-linear kicks that renders the dynamics chaotic [1,2].  Going beyond periodic kicks, we demonstrate that further improvements can be achieved by optimizing the individual kicking strengths with reinforcement learning [3].

References

[1] Lukas J. Fiderer and Daniel Braun, Nature Communications 9, 1351 (2018).

[2] Lukas J. Fiderer and Daniel Braun, Conf. Proceedings „Optical, Opto-Atomic, and Entanglement-Enhanced Precision Metrology”, 10934, 10934S (2019); arXiv:1903.02393 [quant-ph]

[3] Jonas Schuff, Lukas J. Fiderer, and Daniel Braun, NJP 22, 035001 (2020).

Rare earth ions as a dopant in nanocrystals are promising candidates for quantum information processing. The long coherence time of their nuclear spin and their optical interface are adequate for quantum networking. In my talk I report on the development of an optical resonator platform out of two machined optical fibers. This platform paths the way for single qubit control and readout. The presented work handles the crucial problem of the integration of a nanocrystal into an all-fiber cavity.

Quantum computers prepare a fiducial state, manipulate the quantum information using quantum gates, and are able to perform a read out. Until now several systems have been engineered to form a viable quantum computer, and even demonstrated quantum supremacy. Examples include photonics, neutral atoms, cavity quantum electrodynamics, trapped ions, nuclear magnetic resonance, and solid-state systems. In this talk I present how to employ an ultracold mixture of two atomic species for universal quantum computation on qudits. To this end, one atomic species realizes the effective spin, which forms the fundamental unit of information in this setup and the second atomic species forms a phonon bath, which is used to entangle the effective spins. We demonstrate the possibility of universal quantum computation with qudits and discuss how to use this platform to implement a quantum error correcting code.

I will present a heuristic to simulate quantum circuits based on a probabilistic representation of the quantum state as the outcome distribution of a positive operator valued measure. In this language, unitary evolution translates into evolution of probability distributions subject to "somewhat" stochastic matrices, which are a generalization of stochastic matrices. I approximate the evolution of the quantum state using a transformer architecture and provide a proof-of-principle demonstration of the approach on simple quantum circuits.

The Kibble-Zurek mechanism takes place when a system is slowly driven through a second order phase transition. This produces a diabatic freeze out of critical fluctuations and cuts off the divergence of the correlation length. Recasting this problem in a systematic RG formulation, we show that the slow drive can be used to activate not only the leading critical exponents of the underlying equilibrium problem, but the full critical exponent spectrum. We thus uncover an aspect of the Kibble-Zurek phenomenology, where the underlying equilibrium critical physics provides multiple universal scaling regimes.

Quantum simulation is the idea to overcome the problem of quantum complexity by using special purpose quantum computers to emulate the quantum many-body dynamics of interest. Such quantum simulation experiments have been realized in the last decade using various platforms, including trapped ions and ultracold atoms. Nevertheless, many open questions remain about how exactly these systems can contribute to our understanding of the dynamics of strongly interacting quantum systems and how we can use them to make faithful predictions. I will discuss recent advances in detecting coherence and entanglement in artificial quantum magnets and ultracold atomic clouds.

We develop a framework that connects learning with classical and quantum control, and this framework yields adaptive quantum-control policies that beat the standard quantum limit, inspires new methods for improving quantum-gate design for quantum computing, and suggest new ways to apply classical and quantum machine learning to control.

Despite a growing number of realizations in experiment the efficient numerical simulation of real time evolution of isolated quantum many-body systems far from equilibrium remains challenging. Especially, systems of intermediate spatial dimensions are still largely elusive to the established approaches. In this work we demonstrate that combining a time-dependent variational principle with deep neural networks as ansatz for the wave function yields a versatile and reliable method in the sense that it is not tailored to the specific problem and the error can be quantified and systematically reduced. A deep network architecture is particularly well suited to exploit the locality of physical dynamics for the representation of the time-evolved wave function. As a concrete example, we simulate the dynamics of the paradigmatic and experimentally relevant two-dimensional transverse field Ising model. The maximal times reached are comparable to or exceed the capabilities of state-of-the-art tensor network methods.

In recent years, propelled by the progress in the field of quantum simulations with ultracold atoms, there has been an increasing interest of the condensed matter community in what is generally called quasicrystal lattices, long-range ordered but non-periodic structures. Besides retaining intrinsic relevant questions that range from the stability of tiled structures at zero temperature to their relation to fractal lattices, quasicrystals have also shown to support quantum phases of matter such as superconductors and Bose-Einstein condensates. Nonetheless, in spite of important works that address the emergence of quasicrystalline order in classical systems, a deeper understanding of the role of quantum fluctuations in these structures still lacks. Here we present our proposal to realize quasi-crystalline states in ultracold setups with nonlocal interactions.

In the last two decades, quantum simulators based on ultracold atoms in
optical lattices have successfully emulated strongly correlated
condensed matter systems. With the recent development of quantum gas
microscopes, these quantum simulators can now control such systems with
single-site resolution. Within the same time period, atomic clocks have
also started to take advantage of optical lattices by trapping alkaline
earth metal atoms such as Sr, and interrogating them with precision and
accuracy at the 1e-18 level. Here, we report on progress towards a new
quantum simulator that combines quantum gas microscopy with optical
lattice clock technology. We aim to trap ultracold Sr atoms in
large-mode-volume and state-dependent optical lattices to emulate
strongly-coupled light-matter-interfaces in parameter regimes that are
unattainable in real photonic systems.

Towards this goal, we report on

(1) A narrow-line magneto-optical trapping technique that outperforms
standard techniques in terms of speed, robustness, and capture
fraction;

(2) A monolithic in-vacuum optical buildup cavity with two crossed
modes with mode diameters of 0.8 mm.

(3) The most precise measurement of a tuneout wavelength to date which
in combination with state-of-the-art atomic structure calculations
improves the dominant systematic uncertainty of Sr lattice clocks;

(4) A proof-of-principle experiment where we demonstrate stable
trapping of 3P0 atoms in a one-dimensional optical lattice at the
ground state tuneout wavelength.

Dynamic correlations of quantum observables are a useful theoretical tool appearing in fluctuation-dissipation theorems, the theory of optical coherence, and glassy dynamics to name a few. Experimentally, however, these correlations are challenging to measure due to measurement backaction incurred at early times. Within the context of spin-lattices we show that ancilla-based weak measurements are able to reduce this backaction, allowing for dynamic correlations of arbitrary observables to be measured.

We further analyse the dynamics generated by these ancilla-based measurements by considering their representation in terms of positive operator-valued measures. Within this framework we prove the existence of a special class of observables for which measurement backaction is of no concern, so that dynamic correlations of these can be obtained without making use of ancillas.

A discussion of experimental implementations will show that measuring dynamic correlations of these special observables nevertheless remains challenging. To mitigate these challenges we propose a modified measurement protocol, and we rigorously estimate its accuracy by means of Lieb-Robinson bounds. On the basis of these bounds we identify a parameter regime in which this modified protocol allows for accurate measurements of the desired two-time correlations.

References:

P. Uhrich et al., Phys. Rev. A, 96:022127 (2017)

M. Kastner and P. Uhrich, Eur. Phys. J. Spec. Top. (2018) 227: 365

P. Uhrich et al., Quantum Sci. Technol. 4 024005 (2019)

Angular momentum plays a central role in a plethora of quantum processes, from nuclear collisions to decoherence in quantum dots to ultrafast magnetic switching. Here we consider a single molecule embedded in a superfluid Helium nanodroplet as a prototype of a fully controllable many-body system in which to reveal angular momentum dynamics: an ultrashort, high-intensity laser pulse can induce molecular axis alignment, creating extreme out-of-equilibrium conditions, while imaging of molecular fragments after Coulomb explosion allows to obtain time-resolved measurements of molecular alignment [1].

The rotational dynamics of a molecule in superfluid Helium cannot be simply understood in terms of interference of rotational molecular states due to the strong interactions with many-body environment: we show that this scenario can be described in terms of the angulon quasiparticle [2,3]—a quantum rotor dressed by a field of many-body excitations—with a very good agreement with experimental data [4] for several molecular species and across a wide range of laser fluences. The dynamical theory we develop contributes to advancing the understanding of angular momentum dynamics in a many-body environment, with applications ranging from ultracold molecules to condensed matter.

1. D. Pantlehner et al., Phys. Rev. Lett. 110, 093002 (2013).

2. R. Schmidt and M. Lemeshko,  Phys. Rev. Lett. 114, 203001 (2015).

3. M. Lemeshko, Phys. Rev. Lett. 118, 095301 (2015).

4. I.N. Cherepanov, G. Bighin, L. Christiansen, A.V. Jørgensen, R. Schmidt, H. Stapelfeldt, M. Lemeshko, submitted

In recent years, several studies of cold and ultracold hybrid systems involving atomic and molecular ions interacting with neutral atoms or molecules have led to rapid progress towards reaching the quantum regime, where a few partial waves contribute to the behavior of the system. In this work, we explore the effect of long-range interactions on the inelastic processes taking place at ultracold temperatures. We study how these long-range interactions couple to the shorter-range potential energy surfaces (PES) and can be used to explain /control the outcome of scattering events at low energy. In particular, we explore how the state of the projectile can influence the type of long-range interaction, leading to barriers that reduce or even prevent reactions in some cases, or accentuate the attractive polarization interaction that increase reaction rates in other cases. We present results on two polyatomic molecular ions reacting with excited Ca atoms, namely BaOCH3+  and BaCl+. For reactions to take place, Ca needs to be in an excited state, and the reaction rate depends strongly on the spin state of the excited state of Ca, i.e. either 1P or 3P.

We also discuss a different approach to affect charge exchange in atom-ion collision, namely using Feshbach resonance. This is a different example of using spin-states to affect reactions.

Finally, we present a simple formulation for the charge exchange in the case of resonant processes, linking the s-wave regime to higher temperatures. The expression is valid for resonant scattering processes in general (charge transfer, spin-flip, excitation exchange) under appropriate conditions, and could be used for quasi-resonant processes as well.

Partially supported by the MURI US Army Research Office Grant No. W911NF-14-1-0378.

For abstract click below

For perturbative scalar field theories, the late-time-limit of the out-of-time-ordered correlation function that measures (quantum) chaos is shown to be equal to a Boltzmann-type kinetic equation that measures the total gross (instead of net) particle exchange between phase space cells, weighted by a function of energy. This derivation gives a concrete form to numerous attempts to derive chaotic many-body dynamics from ad hoc kinetic equations. As in conventional Boltzmann transport, which follows from the dynamics of the net particle number density exchange, the kernel of this kinetic integral equation is also set by the 2-to-2 scattering rate. This provides a mathematically precise statement of the known fact that in dilute weakly coupled gases late-time transport and early-time scrambling (or ergodicity) are controlled by the same physics.

Surprisingly infinitely strongly coupled, large-Nc theories with a holographic dual also possess this relation between early- and late-time physics. The gravitational shock wave computation used to extract the scrambling rate in strongly coupled quantum theories with a holographic dual is directly related to probing the system's hydrodynamic sound modes. At a special point along the sound dispersion relation curve, the residue of the retarded longitudinal stress-energy tensor two-point function vanishes. This pole-skipping point encodes the Lyapunov exponent of quantum chaos.

With the advent of quantum technologies, a quest toward the manipulation of mechanical oscillators in the quantum regime has been launched. I will present the experimental research of my group at the boundary between ultrafast spectroscopy, quantum optics and nanoscience, in which we prepare non-classical states of vibrations in nano- and molecular scale oscillators.

I will show how we can create a single quantum of vibration involving the collective motion of billions of atoms in a crystal [1,2], and how we can engineer a quantum superposition between two of these vibrational modes.  This non-classical state of oscillation features Bell correlations [3], the strongest form of correlation allowed by quantum mechanics.

I will explain how our technique can be extended to manipulate a broader range of nanoscale oscillators in the quantum regime, enabling new ways to process quantum information at ultrafast timescales, and opening a new window into quantum phenomena occurring in molecular and solid-state systems.

[1]  M. D. Anderson, S. T. Velez, K. Seibold, H. Flayac, V. Savona, N. Sangouard, and C. Galland, “Two-Color Pump-Probe Measurement of Photonic Quantum Correlations Mediated by a Single Phonon,” Phys. Rev. Lett. 120, 233601 (2018).

[2]  S. Tarrago Velez, K. Seibold, N. Kipfer, M. D. Anderson, V. Sudhir, C. Galland. “Birth and death of a single quantum of vibration” arXiv preprint arXiv:1811.03038v2 (2018).

[3]  S. Tarrago Velez et al., in preparation (2019).

We consider one- and two-dimensional Ising models with varying interaction ranges. Using matrix product state techniques, we study the dynamics of these systems and show a direct connection between the type of lowest-energy quasiparticles in the spectrum of the quench Hamiltonian and the type of nonanalyticities occuring in the Loschmidt return rate, a dynamical analog of the free energy. Our results also show a clear connection between the type of nonanalyticities and the phase of the long-time steady state in addition to how the order parameter decays at intermediate times. In particular, we discuss anomalous nonanalyticities that occur with no underlying local signature in the order parameter dynamics, unlike the traditional regular nonanalyticities that always correspond to a zero crossing of the order parameter. We demonstrate how dynamical quantum phase transitions can be used to extract the equilibrium physics of the model from short-time dynamics.

Quantum information networks will deliver the capability for long-distance, provably-secure communications via quantum key distribution, as well as optical quantum computing. Our work aims to provide components for these quantum networks: our specific design makes use of hollow-core photonic crystal fibres (HCPCFs) filled with rubidium atoms, and are amenable to direct integration with current optical fibre technology. The tight transverse confinement (diameter of tens of microns) and extended interaction lengths (centimetres) of the HCPCFs provides an extremely optically dense medium, ideal for efficient quantum information storage and for achieving strong atom-mediated photon-photon interactions. I will present results from our experiments aiming for efficient, coherent and noiseless storage of high-bandwidth optical pulses in warm rubidium-filled HCPCFs using the off-resonance cascade absorption (ORCA) technique. We have also recently demonstrated the ability to load a record number of laser-cooled atoms into a hollow-core optical fibre and I will present our latest results towards achieving high efficiency, long-lived storage. 

In this talk, I will review several Rydberg EIT experiments we have been working on in our group. This includes EIT spectral shifts and dephasing in an interacting Rydberg gas, microwave assisted Rydberg EIT, and imaging ions with Rydberg EIT.

In this talk, we will review the identical spin rotation effect (ISRE) at the microscopic origin of many collective spin phenomena in cold atomic gases. The ISRE occurs during the collision between two indistinguishable atoms with internal levels (pseudo-spin). It leads to the appearance of an exchange mean-field term in the kinetic equation. The latter leads to phenomena such as spin waves, anomalous segregation between internal levels, spin synchronization in atomic clocks, etc. The consequences of ISRE have mainly been studied for non-condensed Bose or Fermi gases. However they are also expected to be important for partially condensed gases.

Ultracold atom systems are well-controlled and tunable quantum systems, and thereby enable us to explore quantum many-body effects,  such as superfluidity, or second sound. In this talk, I will examine second sound and superfluidity in ultracold quantum gases using analytical and simulation techniques. I will report on the second sound measurements in the BEC-BCS crossover and provide a theoretical description of the second sound velocity on the BEC side of the system [1]. Here, I will demonstrate that the second sound velocity vanishes at the superfluid-thermal boundary, which is a defining feature of second sound. In the second part of this talk, I will investigate superfluidity of ultracold quantum gases via laser stirring. I will present the stirring experiments in the BEC- BCS crossover and provide a quantitative analysis of the breakdown of superfluidity [2]. I will then investigate superfluidity of 2D Bose gases across the Kosterlitz-Thouless transition and provide a quantitative understanding of the experiments performed in the Dalibard group [3]. I will also present the noise correlations of 2D Bose gases in short time of flight and use them to determine the superfluid phase of the recent experiments at Hamburg [4].

[1] D. Hoffmann, V. P. Singh, T. Paintner, W. Limmer, L. Mathey, and J. H. Denschlag, Second sound in the BEC-BCS crossover, forthcoming.

[2] W. Weimer, K. Morgener, V. P. Singh, J. Siegl, K. Hueck, N. Luick, L. Mathey, and H. Moritz, Phys. Rev. Lett. 114, 095301 (2015); V. P.  Singh et al., Phys. Rev. A 93, 023634 (2016).

[3] V. P. Singh, C. Weitenberg, J. Dalibard, and L. Mathey, Phys. Rev. A 95, 043631 (2017).

[4] V. P. Singh and L. Mathey, Phys. Rev. A 89, 053612 (2014).

We consider one- and two-dimensional Ising models with varying interaction ranges. Using matrix product state techniques, we study the  dynamics of these systems and show a direct connection between the type of lowest-energy quasiparticles in the spectrum of the quench  Hamiltonian and the type of nonanalyticities occuring in the Loschmidt return rate, a dynamical analog of the free energy. Our results also  show a clear connection between the type of nonanalyticities and the phase of the long-time steady state in addition to how the order parameter decays at intermediate times. In particular, we discuss anomalous nonanalyticities that occur with no underlying local signature in the order parameter dynamics, unlike the traditional regular nonanalyticities that always correspond to zero crossings of the order parameter. Moreover, we demonstrate how dynamical quantum phase transitions can be used to extract the equilibrium physics of the model from short-time dynamics.

We show that quantum confinement can induce spatial quasi-localization of excitations and slow dynamics even in the absence of quenched disorder. By means of numerical computations based on matrix product states and exact diagonalization, we study the nonequilibrium evolution in quantum Ising chains with longitudinal fields, in long-range quantum Ising chains, and in U(1) lattice gauge theories in one dimension. We demonstrate the emergence of regimes characterized by quasi-many-body localization and long-lived excitations at high energy. We capture these anomalous nonequilibrium dynamics via effective analytical descriptions or via exact mappings to models exhibiting weak ergodicity breaking. These phenomena can be tested in quantum simulators with trapped ions and Rydberg atoms.

References: arXiv:1806.09674, arXiv:1811.05513, and work in preparation (Feb 2019).

A quantum system exhibits off-diagonal long-range order (ODLRO) when the largest eigenvalue λ0 of the one-body-density matrix scales as λ0 ~ N, where N is the total number of particles. Putting λ0 ~ N^C to define the scaling exponent C, then C=1 corresponds to ODLRO and C=0 to the single-particle occupation of the density matrix orbitals. When 0<C<1, C can be used to quantify deviations from ODLRO. In this talk I will present the study of the exponent C in a variety of one-dimensional bosonic and anyonic systems.

Trapped ions are ideal candidates for engineering quantum systems with individual resolution. Qubits are encoded with the internal levels of the ions, and controlled with laser-driven interactions. Such a system present an excellent coherence time and can find wide applications in quantum simulations and quantum computing. I will present recent experiments using these systems to study non-equilibrium matter, including discrete time-crystals [1], as well as dynamical phases [2]. A spin chain with individual resolution for more than 50 qubits enables many applications such as quantum sampling and optimization.

[1]           J. Zhang, et al., Nature 543, 217–220 (2017).
[2]           J. Zhang, et al., Nature 551, 601–604 (2017).

I determine the quantum Cramér-Rao bound for the precision with which the oscillator frequency, encoded in a general single-mode Gaussian state, which is time-evolved with a driven damped harmonic oscillator, can be estimated explicitly. More precisely, I present a scheme for calculating the quantum Fisher information for a measurement of the oscillator frequency. Using this scheme, I determine the quantum Fisher information, which corresponds to the lower bound of the quantum Cramér-Rao inequality, for a time-evolved single-mode Gaussian state. Based on these results, I investigate which Gaussian states provide a large quantum Fisher information, i.e. which Gaussian states are particularly suitable for estimations of frequency. Finally, I give an outlook for the quantum Fisher Information of a damped harmonic oscillator relevant for estimating the damping constant.

We consider a quantum critical one-dimensional model of bosons with attractive interactions that displays a generic critical separatrix structure in the phase space governing the mean field dynamics. A semiclassical quantization of the latter, where a large but finite number of particles plays the role of a small quantum of action, allows us to account for many-body correlations crucial to the behavior at criticality. In particular, it enables analytical quantification of the spectral gap and a logarithmic scaling of local level spacings. The latter become asymptotically constant, resembling harmonicity, while associated with a unique Ehrenfest-like time scale that mimicks chaos. This interplay results in the counterintuitive coexistence of initial fast information scrambling and asymptotically perfect memory, observed as quasiperiodic revivals, e.g. present in the one-body entropy. By identifying the emergence of local Ehrenfest-like time scales during separatrix quantization as the generic decicive mechanism this constitutes a hallmark of criticality in integrable many-body systems.

The investigation of the scrambling of information in interacting quantum systems has recently attracted a lot of attention as a manifestation of quantum chaos. To capture the effect, one can make use of the so-called out-of-time-ordered correlators (OTOCs) whose short-time behavior can be directly related to the instability of a corresponding classical chaotic system with characteristic sensitivity to initial conditions given by the Lyapunov exponent. We show that local instability of the mean-field dynamics can be sufficient to reproduce the short-time behavior of the OTOCs as expected for chaotic systems, where the classical stability exponent takes the role of the Lyapunov exponent. We further investigate the transition from integrability to chaos in a hallmark system. We find that the onset of chaos strongly affect the long-time behavior of the OTOC while the short-time behavior remains dominated by criticality.

An interacting quantum system that is subject to disorder may cease to thermalize due to localization of its constituents, thereby marking the breakdown of thermodynamics. The key to our understanding of this phenomenon lies in the system’s entanglement, which is experimentally challenging to measure.

We realize such a many-body-localized system in a disordered Bose-Hubbard chain and characterize its entanglement properties. We observe that the particles become localized, suppressing transport and preventing the thermalization of subsystems. Notably, we measure the development of non-local correlations, whose evolution is consistent with a logarithmic growth of entanglement entropy–the hallmark of many-body localization. Our work experimentally establishes many-body localization as a qualitatively distinct phenomenon from localization in non-interacting, disordered systems.

The behaviour of a mobile impurity particle interacting with a quantum-mechanical medium is of fundamental importance in physics. Ultracold atomic gases have greatly improved our understanding of the impurity problem owing to the high degree of control over experimental parameters such as interactions and atom population. We will discuss recent theoretical and experimental progress in exploring the properties of impurities interacting with bosonic and fermonic mediums. In particular, we will discuss the effects of finite temperature on the Fermi polaron and the nature of the Bose polaron.

The search for environmentally benign energy sources and efficient energy storage materials remains a primary focus of scientists from various disciplines and backgrounds. Nature has devised complex systems such as enzymes that capture and store energy in different forms. There are several (chemical and biological) approaches to obtain catalytic systems which require costly machinery, rendering them inefficient on the level of the recent energy demand. Photovoltaic devices which convert sun light into electrical energy serves as a crucial way of producing sustainable alternative energy. In this talk, I will present computationally-guided spectroscopic characterization and design of a-) bio-inspired catalysts and b-) Si-based solar cells fabricated by means of effusion cell equipped electron-beam evaporation technique followed by solid-phase, aluminum-induced or laser crystallization. In addition to typical characterization methods (FT-IR, ToF-SIMS, Raman, XRD, etc.), since it gives the chance to study different mechanisms with coherent spin control, we have employed Electron Paramagnetic Resonance Spectroscopy as well as calculations with Density Functional Theory. Working principles and electronic structure-function relationship of bio-inspired Fe catalysts will be presented. Moreover, optimization of quantitatively-evaluated unpaired electrons due to defect centers (dangling bonds, oxygen vacancies, interphase defects, etc.) in poly-Si thin films will be discussed.

The angulon quasiparticle, formalizes the concept of a composite, rotating impurity in a quantum many-body environment and has proven useful in the description of several experimental settings, from ultracold molecules in a BEC to molecules in He nanodroplets. I introduce a diagrammatic formalism, merging Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, describing angular momentum redistribution in a many-body system. Then, motivated by recent experiments on laser-induced alignment of molecules in He nanodroplets, I introduce a finite-temperature variational approach to angulon dynamics, showing that the far-from-equilibrium dynamical response of molecular impurities can be rationalized in terms of angulons.

When an impurity is immersed into an environment, it changes its properties due to its interactions with the surrounding medium. The impurity is dressed by many-body excitations and forms a quasiparticle, the polaron. Depending on the character of the environment and the form of interactions, different types of polarons are created. In this talk, I will review recent experimental and theoretical progress on studying the many-body physics of polarons in ultracold atomic systems [1], and discuss related polaronic phenomena encountered in two-dimensional semiconductors [2] and the study of rotating molecules in superfluid Helium [3]. In the second part of the talk I will then focus on impurities interacting with bosonic quantum gases. Specifically, I will discuss progress on the theoretical description of Rydberg excitations coupled to Bose-Einstein condensates. In such systems the interaction between the Rydberg atom and the Bose gas is mediated by the Rydberg electron. This gives rise to a new polaronic dressing mechanisms, where instead of collective  excitations, molecules of gigantic size dress the Rydberg impurity. We develop a functional determinant approach [4] to describe the dynamics of such Rydberg systems which incorporates atomic and many-body theory. Using this approach we predict the appearance of a superpolaronic state which has recently been observed in experiments [5,6].  

[1] R. Schmidt, M. Knap, D. A. Ivanov, J.-S. You, M. Cetina, and E. Demler, Rep. Prog. Phys. 81, 024401 (2018).

[2] M. Sidler et al., Nature Physics 13, 255 (2017).

[3] R. Schmidt, and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015).

[4] R. Schmidt, H. Sadeghpour, and E. Demler, Phys. Rev. Lett. 116, 105302 (2016).

[5] F. Camargo et al., Phys. Rev. Lett. 120, 083401 (2018).

[6] R. Schmidt et al., Phys. Rev. A 97, 022707 (2018).

Quantum theory predicts that a quantum system will collapse from several of its possible states of existence, to just one, the moment it is measured. As quantum systems are never isolated from their surrounding environment (quantum bath), its measurement and the associated collapse should also affect the environment coupled to it. The extent to which a quantum bath should collapse in its own Hilbert space strongly depends on its coupling strength to the quantum system and its equilibration time. Thus a strong measurement of the quantum system may result only in a weak measurement on its macroscopic partner, the quantum bath. Here we use repetitive strong (projective) measurements of a quantum system, the NV centers in diamond, to gradually collapse an unknown, and arbitrary sized spin-bath to a state with very low fluctuation noise. Such projected quantum bath leads to an extended spin coherence time of the NV center by over 5 orders of magnitude. Our results demonstrate how quantum state engineering of an unknown mesoscopic environment and its tomography can be achieved only by measuring a nanoscaled object coupled to it. Moreover, our experiments also pave the way for validating the foundational aspects of measurement problem, in quantum mechanics, and its role in quantum information science.

In recent years the Nitrogen-Vacancy (NV) center has emerged as a promising sensor for magnetic fields. Due to their properties as a quantum system, NV center based sensors are inherently calibrated, exhibit high magnetic susceptibility, bandwidth, temperature working range, durability, and functionality at high magnetic fields. 

In order to miniaturize and improve such sensors, different sensor architectures and sensing techniques have been investigated, to achieve high magnetic field sensitivity while maintaining a small diameter.

We investigate the dynamical equilibration of an uncorrelated thermal Bose gas passing through a Bose-Einstein condensate phase transition. During such crossing, the system breaks its symmetry resulting in numerous uncorrelated regions separated by the spontaneously generated defects. The emergence of spontaneous defects formation obeys a universal scaling law with quench duration, well known as Kibble-Zurek mechanism. The ensuing re-equilibration stage is govern by the evolution and interactions of such defects under system-specific and external constraints. We perform a detailed numerical characterization of the entire non-equilibrium process in an elongated ultra-cold Bose gas with fully three-dimensional classical-field simulations, addressing subtle issues and demonstrating the quenched-induced decoupling of condensate atom number and coherence growth during the re-equilibration process. Our findings agree with experimental observations made at the later stage of the quench, and provide the information via useful dynamical visualization in currently experimentally inaccessible regimes.

Over the last years, network science has been established as an independent theory between mathematics and computer science. A lot of physicists took part in the discovery and description of network phenomena. This is why names like phase transition and Bose-Einstein condensation have found their way into network science.
But how can we use network science to improve our understanding of quantum many-body systems? This question stands at the heart of this thesis: The 1D Ising model is a magnetic model from solid state physics and has a so called quantum phase transition at T=0K between ferro- and paramagnetic spin arrangement. Finding the critical point, at which the phase transition occurs, with methods from network science is one of the main challenges of this thesis. For this, the 1D Ising model with different kinds of interactions is translated into a complex weighted network which in turn is analyzed. The results are promising and the translation scheme from physical model to the network can be used for other theoretical models.

Fractionalisation is a term used with respect to many-body phenomena in which quasiparticles do not share the same quantum numbers (statistics) as the system constituents. In this seminar, I will first recall several examples of fractionalisation in one-dimensional systems: spin-charge separation, quasiparticle deconfinement and Fractional Exclusion Statistics (FES). Then I will discuss how such phenomena appear in systems of correlated fermions and bosons we study in the context of quantum simulators.

The interplay between particles and lattice degrees of freedom is crucial to understand many paradigmatic phenomena in condensed matter, ranging from polaron physics to superconductivity. The study of these effects using atomic systems, however, remains a challenging task. Quantum simulations with ultracold atoms, in particular, rely on static optical lattices. The particles do not influence the lattice structure and, therefore, the effects of phonons are usually not taken into account.

In this talk, I will present a model of interacting bosons coupled to a set of two-level systems. The latter provides a minimal model to describe a dynamical lattice, and presents relevant phenomena such as a bosonic version of the well known Peierls transition. I will also show other properties of the system, including different types of ordered phases, topological defects and symmetry-protected topological phases. Finally, I will discuss the possibility of realizing the model using ultracold atoms.

Spin-orbit coupled cold atom systems, governed by Hamiltonians that contain quadratic kinetic energy terms typical for a particle's motion in the usual Schroedinger equation and linear kinetic energy terms typical for a particle's motion in the usual Dirac equation, have attracted a great deal of attention recently since they provide an alternative route for realizing fractional quantum Hall physics, topological insulators, and spintronics physics. This talk will discuss selected few-body aspects of spin-orbit coupled cold atom systems. Considering the experimentally most frequently realized 1D spin-orbit coupling, two topics will be discussed. 1) It will be shown that weak spin-orbit coupling terms can notably modify the two-body scattering properties. 2) It will be discussed what happens to Efimov's famous radial scaling law if single-particle spin-orbit coupling terms are added to the three-boson Hamiltonian with two-body short-range interactions.

Recent experiments with ultracold atoms offer platforms for studying non-equilibrium spin-dynamics of large quantum many-body models in controlled environments. Thus, also numerical methods for simulating such dynamics are of great importance. Here, I first present the DTWA, a semi-classical method based on the well-known truncated Wigner approximation. This method has been surprisingly successful in predicting dynamics of lattice models. I show how this method can be generalized to study dynamics of arbitrary discrete lattice models and useful to model an experimental setup with Chromium atoms in  an optical lattice (arXiv:1803.02628).

A particular application of non-equilibrium dynamics is transport. Transport of physical quantities such as energy, charge, or information plays a crucial role in a variety of scientific fields. Here, in a second part I present schemes of how the transport efficiencies of energy and charge in materials can be dramatically enhanced by coupling it to a cavity.

Thermodynamic machines can be reduced to the ultimate atomic limit [1],
using a single ion as a working agent. The confinement in a linear Paul
trap with tapered geometry allows for coupling axial and radial modes of
oscillation.The heat-engine is driven thermally by coupling it
alternately to hot and cold reservoirs, using the output power of the
engine to drive a harmonic oscillation [2].From direct measurements of
the ion dynamics, the thermodynamic cycles for various temperature
differences of the reservoirs can be determined [3] and the efficiency
compared with analytical estimates. I will describe how the engine
principle can be exploited to implement a differential probe for
non-classical baths.


[1] J. Rossnagel et al., "A single-atom heat engine", Science 352, 325
(2016).

[2] O. Abah et al., Phys. Rev. Lett. 109, 203006 (2012).

[3] J.  Rossnagel et al., New J. Phys. 17, 045004 (2015)

Optical lattices systems, driven by periodical laser beams or magnetic fields, provide exceptional platforms for quantum simulation of non-trivial many-body systems. In our doubly modulated model, by applying Floquet theory, we are able to create a dominant nearest neighbor interaction in the first order effective Hamiltonian and suppress other tunneling events by choosing suitable driving forces. As a consequence, it will be possible to observe density wave phase in our driven system.

We consider a repulsively interacting multicomponent Fermi gas under harmonic confinement, as recently realized in the experiment of Pagano et al. [Nat. Phys. 10, 198 (2014)]. This setup realizes a gas with tunable $SU(N)$ symmetry. In this talk, we concentrate on the density- and momentum-distributions of particles in such a setup, and present results both for the strongly-interacting limit and for finite interactions.

A particular focus will be on the so-called Tan's contact - the weight of a $k^{-4}-scaling which is observed in the tails of momentum distributions of general contact-interacting systems.
We exploit an exact solution at infinite repulsion to show a direct correspondence between the value of the Tan's contact for each of the N components of the gas and the Young tableaux for the $S_N$ permutation symmetry group identifying the magnetic structure of the ground-state. This opens an alternative route for the experimental determination of magnetic configurations in cold atomic gases, employing only standard (spin-resolved) time-of-flight techniques.

Departing from the exact solution in the infinitely-interacting regime, we then present an analytical scaling prediction for the Tan's contact at finite interactions with respect to the number of fermions, the number of components and the interaction strength and show its qualitative agreement with recent experiments. Along the way, we introduce the analytical (low density approximation, Bethe-ansatz) and numerical techniques (MPS/DMRG) used in the investigation.

Finally, we briefly discuss extensions of the previous approach to multi-component quantum mixtures (bosonic, fermionic, or both) and we show that the ground state of the system always displays the most symmetric spatial wave function allowed by the type of mixture.

References:
J. Decamp, J. Jünemann, M. Albert, M. Rizzi, A. Minguzzi, and P. Vignolo, 
„High-momentum tails as magnetic structure probes for strongly-correlated SU(k) fermionic mixtures in one-dimensional traps”,  PRA 94, 053614 (2016)
"Strongly correlated one-dimensional Bose-Fermi quantum mixtures: symmetry and correlations”, New J. Phys. 19 125001 (2017)

A wide range of stochastic methods has been applied to approach the non-relativistic fermionic many-body problem. Despite the huge success of these approaches, the sign problem prohibits exploration of a large class of systems due to exponential scaling of computational effort. Recently the complex Langevin method, known from relativistic lattice models, was adapted to non-relativistic theories. With this method at hand, we are able to extract properties for spin-polarized Fermi mixtures of arbitrary masses in the ground state as well as at finite temperature. More specifically, we are able to compute equations of state for Fermi mixtures of arbitrary mass and polarization as a function of interaction strength in one, two and three spatial dimensions. Additionally, we discuss pairing correlations in spin-polarized Fermi gases, ultimately aiming at a detection of the formation of an inhomogeneous superfluid condensate.

Substantial progress in the preparation of cold atom-ion hybrid systems has been achieved. With respect to quantum computation and quantum simulation, control of collisions is required. In particular, for a variety of experiments unwanted chemical reactions between atoms and ions such as charge exchange or the formation of molecular ions  need to be suppressed.
We present a method to control the cold collision between an ultracold atom and a trapped ion. A laser is used to excite the ground state atom to a repulsive Rydberg potential level once it approaches the ion to a certain distance. In this way the ion is effectively surrounded by a potential wall that the atom cannot cross. Once the atom leaves the interaction area, it is de-excited back to its original level. The adiabaticity of the scheme is analyzed as a function of different parameters such as laser frequency, laser power, initial atom-ion collision energy, as well as the direction of the collisional process with respect to the light field. By controlling e.g. the laser power and the laser frequency, as well as by addressing different Rydberg states, the properties of this shielding effect can be widely tuned. In particular, unwanted chemical reactions between atoms and ion can efficiently be suppressed, which is an important step towards realization of diverse quantum technological applications for hybrid atom-ion systems.

Quantized vortices are a hallmark of superfluids and superconductors. In this seminar I will talk about the orbital angular momentum Lz of an s-wave paired superfluid in the presence of an axisymmetric multiply quantised vortex. For vortices with winding number |k| > 1, I will argue that in the weak-pairing BCS regime, Lz is significantly reduced from its value Lz=\hbar N k/2 in the BEC regime, where N is the total number of fermions. This deviation results from the presence of unpaired fermions in the BCS ground state, which arise as a consequence of spectral flow along the vortex sub-gap states.

In this talk I will describe the experimental apparatus for the production of ultracold (~1 µK) strontium atoms which was built in Shanghai in the last 3 years. In particular I will present our laser cooling scheme, which consists of a side-loaded two-dimensional magneto-optical trap (2D-MOT), a broadband and a narrowband 3D-MOT. Also the Rydberg excitation laser system will be presented. In the end, recent progress in triplet Rydberg state spectroscopy will be shown.

I shall discuss two examples of interaction effects in quantum gases.

First, I shall discuss the interaction of a collective quantum object - a soliton in a one-dimensional Bose gas - with its thermal environment. Intuitively, one could think of this object as a large pollen in a fluid, expecting Brownian motion to affect the soliton dynamics. Yet, because of the underlying integrability of the problem, it was long thought that such an interaction does not exist. It turns out, however, that there remains a more subtle interplay between soliton and thermal gas which gives rise to a damping force similar to the radiation force exerted on an accelerated charge in electrodynamics, called the Abraham-Lorentz force.

The second part of the talk will discuss interaction effects in mesoscopic Fermi gases relevant to ongoing experiments in Heidelberg as well as experiments on SrTiO3 nanostructures. While Fermi gases with a variable interaction typically realize a BEC-BCS crossover, finite particle number or confinement can give rise to additional fluctuation effects. I will introduce some aspects of mesoscopic superfluids and discuss how fluctuation effects show up in experiments.

We give an overview of entanglement certification methods for multipartite and/or high-dimensional systems. In particular, we also show how we can leverage prior knowledge about the likely structure of states produced in the lab to design optimal protocols, that manage to quantify entanglement in an assumption-free setting without resorting to state tomography.

Heat engines were the basis of the industrial revolution and are still indispensable in our modern world. Originally designed as macroscopic machines that convert heat into mechanical work, the question naturally arises whether their operational principles can also be applied to the quantum domain and whether their performance can benefit from possible quantum advantages. In this overview talk I will present recent theoretical and experimental progress on quantum heat engines.

Ultra-cold atoms confined in the coherent electromagnetic fields have gained considerable attention in the research community. These systems provide a high degree of exibility and parameter tunability that makes them excellent candidates for modelling quantum systems. They allow us to investigate fundamental behaviour of quantum matter in a pristine fashion. To this end, one needs to implement several experimental techniques for harnessing the true potential of cold atoms. Along with the cooling and trapping techniques, the precise characterization methods allow one to investigate the properties of these systems with high accuracy. In this talk, I will describe the techniques we have used for studying our cold-atomic samples and controlling our experiments.

Keywords : Doppler Cooling, Saturation Absorption Spectroscopy (SAS), Magneto-Optical Trap (MOT), Absorption Imaging, LabVIEW FPGA.

The FORCA-G project aims to develop a quantum-sensor for probing short range forces, i.e forces at a length scale of typically few micrometers. The sensor relies on a trapped atom interferometer using an ultra-cold ensemble of 87Rb trapped in a vertical optical lattice (l = 532nm). For shallow depths of the lattice, stimulated Raman transitions are used to induce a coherent coupling between different lattice sites, allowing us to realize atom interferometers capable of probing with very high sensitivity and accuracy the local potential experienced by the atoms. By using a symmetrized Ramsey-Raman interferometer, our force quantum-sensor reaches a state-of-the-art relative sensitivity of 1.8x10 -6 at 1 s on the Bloch frequency, and thus on the local gravitational field.

In a recent work, we studied the impact of atomic interactions arising from the use of a dense and small ultra-cold atomic ensemble as a source for our trapped interferometer. The purpose of using such an atomic source is to reduce inhomogeneous dephasing and to obtain better addressability of the lattice sites and ultimately to populate only one of them. At densities of typically 1012 atoms/cm3, we observe an unexpected behavior of the contrast of Ramsey interferometers, when applying a p-pulse to symmetrize the interferometer. These results are interpreted as a competition between the spin-echo technique and a spin self-rephasing (SSR) mechanism based on the identical spin rotation effect (ISRE). Originating from particle indistinguishability, SSR has

been observed in trapped atomic clocks, where it can enhance the clock’s coherence up to several seconds. The study of these mechanisms due to atomic interactions seems thus to be of great interest for metrology and for developing more compact quantum-sensors based on trapped atomic ensembles, and capable of probing the external fields experienced by the atoms with a spatial resolution better that 1mm.

Tracing the evolution of baryonic matter from atoms in space to stars such as our Sun hinges on an accurate understanding of the underlying physics controlling the properties of the gas at every step along this pathway.  Here I will explain some of the key epochs in this cosmic cycle of gas and highlight our laboratory studies into the underlying atomic, molecular, and plasma physics which control the observed properties of the gas.

Ultracold atoms are exceptional tools to explore the physics of quantum matter. In fact, the high degree of tunability of ultracold Bose and Fermi gases makes them ideal systems for quantum simulation and for investigating macroscopic manifestations of quantum effects, such as superfluidity.

In ultracold gas research, a central role is played by collective oscillations. They can be used to study different dynamical regimes, such as superfluid, collisional, or collisionless limits or to test the equation of state of the system. In this talk, I will present a unified description of collective oscillations in low dimensions covering both Bose and Fermi statistics, different trap geometries and zero as well as finite temperature, based on the formalism of hydrodynamics and sum rules.

I will discuss the different behaviour exhibited by the second excited breathing mode in the collisional regime at low temperature and in the collisionless limit at high temperature in a one-dimensional (1D) trapped Bose gas with repulsive contact interaction. I will show how this mode exhibits a single-valued excitation spectrum in the collisional regime and two different frequencies in the collisionless limit. Our predictions could be important for future research related to the thermalization and damping phenomena in this low-dimensional system. I will show that 1D uniform Bose gases exhibit a non-monotonic temperature dependence of the chemical potential characterized by an increasing-with-temperature behaviour at low temperature. This is due to the thermal excitation of phonons and reveals an interesting analogy with the behaviour of superfluids. Finally, I will discuss our research on a gas with a finite number N of atoms in a ring geometry at zero temperature. I will discuss explicitly the deviations of the thermodynamic behaviour in the ring from the one in the large N limit.

[1] G. De Rosi and S. Stringari, Collective oscillations of a trapped quantum gas in low dimensions, Phys. Rev. A 92, 053617 (2015).

[2] G. De Rosi and S. Stringari, Hydrodynamic versus collisionless dynamics of a one-dimensional harmonically trapped Bose gas, Phys. Rev. A 94, 063605 (2016).

[3] G. De Rosi, G. E. Astrakharchik, and S. Stringari, Thermodynamic behavior of a one-dimensional Bose gas at low temperature, Phys. Rev. A 96, 013613 (2017).

The interesting features of Rydberg atoms have made them very desirable for research in atomic physics, in studying strongly interacting Rydberg gases, dipole interactions between Rydberg atoms, Rydberg atom blockade and so on. Creation of Rydberg states in ultracold systems could be of significance due to their long range interactions. In this presentation, Rydberg excitation of ytterbium that has been enabled using the techniques of laser cooling and trapping is discussed. The experimental setups for cooling the ytterbium atoms in a magneto-optical trap and further exposing them to the light required for the Rydberg transition is explained, followed by a brief account on the data that has been obtained so far.

The subject of this work is the cosmic muon tomography of geological structures, an area that was born of the encounter between two scientific disciplines: particle physics and geosciences. It was an internship within the framework of the DIAPHANE project which is committed to tomography of the Soufrière of Guadeloupe which is an active volcano. An internship supervised by Jacques Marteau at the Lyon nuclear physics institute. We use the attenuation of the flow of muons in the dome to characterize its spatial distribution of matter: the larger and / or dense the dome is, the lower the flow of muons would be and vice versa. For example, a cavity in a homogeneous medium is translated for the instrument by an increase in the flow of muons at the axes of observation which pass through the cavity. We looked at the distribution of energy deposited in each matrix of telescope to select the right events and also the distribution of the muon flux along different channels in 2 d and 3 d. Finally an image of tomography was obtained where one can distinguish the different densities and structures. We have also compared the results of two phases for one of the sites around the Soufriere and verified and confirmed the relevance between them with different analyzes.

Can the EPR paradox be resolved without recourse to non-locality?

Abstract

The completeness debate related to quantum mechanics traces its origin to the seminal paper by Einstein, Podolsky and Rosen (EPR) dating back to 1935. EPR proposed a thought experiment which led one to conclude that either quantum mechanics was incomplete and that a search for local hidden variable theories was justified or that quantum mechanics was complete and that "spooky" action-at-a-distance is an incontrovertible aspect of physical reality. These ideas had to await a simpler formulation by Bohm and a theorem by Bell before this debate could be addressed experimentally. Experimental resolution using entangled photons violating Bell's inequality, some as recent as 2015 closing various loopholes, have only confirmed that quantum mechanics is complete and that our ideas of local realism are untenable with physical reality.

While the spin angular momentum(SAM) associated with photons has been known since almost a century, the revelation that photons can also possess an orbital angular momentum(OAM) is a recent one. This opens up the possibility of creating locally correlated SAM/OAM photon states that are mathematically isomorphic with non-locally correlated entangled states. Measuring the SAM is straightforward as it is related to the polarization, whereas OAM being related to wavefront helicity needs a phase sensitive technique for its projection. We propose and demonstrate an interferometric device that acts as an OAM projector and use it to measure correlations between photon SAM and OAM. We create locally correlated SAM/OAM states and show that strong correlations for such states culminate in a violation of Bell's inequality thereby resolving the EPR debate without taking a recourse to non-locality.

Strongly correlated electron systems such as high-temperature superconductors and pseudo-gap states are a cornerstone of modern condensed matter research. A complementary approach to studying solid-state systems is to build an experimentally tunable quantum system governed by the Hubbard model, which is thought to qualitatively describe these systems. Ultracold fermionic quantum gases in optical lattices provide a clean and tunable implementation of the Hubbard model. At the same time, optical microscopy in these systems gives access to single-site observables and correlation functions, and provides dynamic control of the potential landscape at the single-site level. But so far ultracold atom experiments have not been able to reach the low-temperature regime of the Hubbard model, which becomes particularly interesting when doped. 

In this talk I will report on the observation of antiferromagnetic long-range order in a repulsively interacting Fermi gas of Li-6 atoms on a 2D square lattice containing about 80 sites. The ordered state is directly detected from a peak in the spin structure factor and a diverging correlation length of the spin correlation function. When doping away from half-filling into a numerically intractable regime, we find that long-rang order extends to doping concentrations of about 15%. I will also report on our most recent progress towards creating ultra-low entropy states using entropy redistribution, as well as the detection of spinon-holon string configurations for doped systems. These results open the path for a controlled study of the low-temperature phase diagram of the Hubbard model.

In this talk I will discuss how experimentally available bilayer lattice systems could be used to prepare quantum many-body states with exceptionally low entropy in one layer, by dynamically disentangling the two layers. In regimes where all single particle excitations are gapped in one layer, disentangling maps directly to effective cooling of that layer, by shuttling entropy into the other layer. For both bosonic and fermionic atoms, we study the corresponding dynamics showing that disentangling can be realised cleanly in ongoing experiments. The corresponding entanglement entropies are directly measurable with quantum gas microscopes, and as a tool for producing lower-entropy states, this technique opens a range of applications beginning with simplifying production of magnetically ordered states of bosons and fermions.

The strong interactions between Rydberg excitations can result in spatial correlations between the excitations. In this work, I investigate how the character of the Rydberg-Rydberg interactions affects the nature of the spatial correlations and the evolution of those correlations in time. I use direct imaging of the center-of-mass positions of the individual Rydberg atoms and pair-correlation analysis to observe the atom-pair kinetics due to binary dipolar forces. In the first experiment, atoms are excited to a Rydberg state that experiences a repulsive van der Waals interaction.  The Rydberg excitations are prepared with a well-defined initial separation, and the effect of van der Waals forces is observed by tracking the interatomic distance between the Rydberg atoms. The atom trajectories and thereby the interaction coefficient C6 are extracted from the pair correlation functions of the Rydberg atom positions. In the second experiment, the Rydberg atoms are prepared in a highly dipolar state by using adiabatic state transformation. The atom-pair kinetics that follow from the strong dipole-dipole interactions are observed. The pair correlation results provide the first direct visualization of the electric-dipole interaction and clearly exhibit its anisotropic nature. The observation also shows the dynamics reminiscent of disorder-induced heating in strongly coupled particle systems.

References

[1] N. Thaicharoen, A. Schwarzkopf, and G. Raithel, Phys. Rev. A 92, 040701(R) (2015)
[2] N. Thaicharoen, L.F. Gonçalves, G. Raithel, Phys. Rev. Lett. 116, 213002 (2016)
[3] L. F. Gonçalves, N. Thaicharoen, and G. Raithel, J. Phys. B: At. Mol. Opt. Phys. 49, 154005 (2016)

Optical parametric oscillators (OPOs) are among the most widely used sources of nonclassical light. When pumped at sufficiently high powers, parametric gain in these systems overcomes losses and there is oscillation. The bright beams of light that are emitted display nonclassical correlations which are typically characterized by spectral homodyne measurements. We have thoroughly investigated these systems over the past years and shown that spectral homodyne detection cannot provide the full information needed to reconstruct the quantum state. Resonator detection, on the other hand, does not suffer from the same shortcomings. In this talk, we will examine the conditions to obtain complete information about the spectral quantum state and also describe trends for use of these nonclassical light sources on silicon chips.

Spinor Bose-Einstein condensates exhibit both superfluid and magnetic order, and accommodate phases with rich symmetry properties and topological defects. Transitions between these phases can be induced by tuning external fields. This system has proved ideal for studying non-equilibrium quench dynamics in both experimental and theoretical studies. In this talk I will discuss simulations of order formation in a quasi-2D spin-1 ferromagnetic condensate following a quench from an unmagnetised phase to one of three ferromagnetic phases, each with distinct symmetry properties (easy-plane, easy-axis or isotropic). I will firstly review results on the scale invariant growth of order associated with topological defect annihilation. I will then discuss recent results on weak wave turbulence identified in the easy-plane phase. This turbulence drives energy from long wavelength spin waves to a short wavelength thermalised field. The shape of the cascade in momentum space moves to longer wavelengths in a scale invariant way across time scales much longer than the time scale of topological defect annihilation. This surprising result highlights the role that turbulence can play in conservative phase ordering, and suggests the presence of a second nonthermal fixed point in the non-equilibrium dynamics.

The work presented in this talk is part of the QUIPS experiment (Quantum
Information Processing Systems) in the research group of Prof. Dr. G. Birkl at TU
Darmstadt. In that experiment neutral 85Rb atoms are cooled in a magneto-optical
trap (MOT) and then loaded into a two-dimensional register of optical dipole traps in
order to implement a universal set of quantum gates needed for quantum
computation.
The talk gives a short summary of the current status of the experimental set-up. It
describes the difficulties of the two-photon excitation scheme and explains the
adjustments done in the last months in order to demonstrate coherent Rydberg
excitation in periodical optical dipole registers.

One path to understand the interplay of one- and many-body physics is to study a system
with an impurity. Here we follow it using two one-dimensional models: a weakly-interacting
Bose gas with an impurity, and a Fermi gas with a strongly-interacting impurity. For the
former we present a simple analytical approach that captures the ground state features. The
latter also admits analytical treatment as it can be mapped on the Heisenberg spin chain.
For the Bose gas we calculate properties in the thermodynamic limit, and the approach
to this limit. In the Fermi gas case we focus on small trapped systems and examine the
corresponding Heisenberg spin chain. These results lay the foundation for our future studies:
1) three-dimensional Bose polaron in cold atoms and nuclear physics,
2) formation and time dynamics of polaron quasiparticles.

Interdisciplinary research into the utility of magnetic molecules for quantum computing applications represents one of the frontiers of materials science. This lecture will describe recent results of continuous-wave (cw) and pulsed EPR studies on related families of lanthanide containing molecules that have attracted tremendous interest as potential hybrid electron-nuclear spin qubits. A molecular approach is attractive because it enables systematic control of the quantum states of the lanthanide (the qubit) via molecular geometry, and allows functionalization of the molecule in order to engineer interactions between qubits.

The first example involves a Ho^III (4f ¹⁰) ion encapsulated within a (W₅O₁₈)₂ cage. The Ho ion experiences a significant magnetic anisotropy due to crystal-field splitting of the spin-orbit coupled total angular momentum (J = L + S = 8) ground state, resulting in a pair of low-lying m_J = ±4 singlets that are further split by a strong hyperfine interaction with the I = ⁷/₂ nuclear spin [1]. A small departure from a square antiprismatic (D_4d symmetry) coordination geometry results in a Zeeman diagram (with B parallel to the molecular symmetry axis) with multiple avoided crossings between the 16 [(2I + 1) x 2] lowest-lying electron-nuclear sub-levels. Right at these avoided crossings, the EPR transition frequencies are insensitive to dipolar field fluctuations associated with the surrounding electron/nuclear spin bath, which represent the main source of decoherence. These so-called ‘atomic clock transitions’ (named after the principle which gives atomic clocks their exceptional phase stability) give rise to long coherence (T₂) times [2]. Formally forbidden Δm_I= ±1 hybrid electron/nuclear clock transitions are also observed upon application of a transverse field.

The second example involves a bis-phthalocyanine radical coupled to a Tb^III ion, revealing a highly anisotropic signal that is attributed to the radical, suggesting a significant coupling to the lanthanide spin [3]; the radical EPR spectrum would be expected to be essentially isotropic otherwise. This work is important given the recent demonstration that radical bearing ligands provide a means of addressing lanthanide qubits integrated into single-molecule devices.

[1]    S. Ghosh, S. Datta, L. Friend, S. Cardona-Serra, A. Gaita-Ariño, E. Coronado, S. Hill, Dalton Trans. 41, 13697 (2012).

[2]    M. Shiddiq, D. Komijani, Y. Duan, A. Gaita-Ariño, E. Coronado, S. Hill, Nature 531, 348-351 (2016).

[3]    D. Komijani, A. Ghirri, M. Affronte, M. Ruben, S. Hill, in preparation.

Cold atomic gases are a versatile platform for the study of quantum many-body phenomena. Especially atoms excited to highly-lying electronic states --- so-called Rydberg atoms --- offer rather intriguing possibilities for the exploration of strongly correlated dynamics with interacting spin systems.

In this talk I will show that the out-of-equilibrium behaviour of Rydberg gases is governed by emergent kinetic constraints, which are often used to mimic dynamical arrest or excluded volume effects in idealised models of glass forming substances. This leads to a remarkably rich physics including non-equilibrium phase transitions and localisation phenomena. Moreover, Rydberg gases offer intriguing opportunities for the systematic exploration of the role of competing quantum and classical dynamical effects on non-equilibrium phase transitions.

I will conclude by discussing how the above findings can be employed to gain a new perspective on the physics of Dynamic Nuclear Polarisation in interacting electronic and nuclear ensembles, which is an out-of-equilibrium method to drastically enhance the performance of Magnetic Resonance Imaging applications.

Dilute gases of ultracold atoms and molecules are at the heart of amazing progress over the past thirty years in atomic, molecular and optical physics from both experimental and theoretical points of view. The ultralow velocity of the particules allows for long observation times and induces an extreme sensitivity to weak interactions, thus unveiling properties usually hidden at room temperatures, and opening unique opportunities for controlling matter at the single quantum level.

In our group we are focusing our theoretical studies on two kinds of molecular systems: cold neutral dipolar molecules composed of two alkali-metal atoms [1,2] and one alkali atom and one alkaline-earth atom [3].Such systems exhibit a rich dynamics often assisted or controlled by light. They are suitable for studying anisotropic interactions between particles, with exciting prospects toward ultracold chemistry and quantum simulation.

I shall present an overview of our recent theoretical achievements in this domain, based on new accurate quantum chemistry computations  of potential energy surfaces of ground and excited molecular states and of relevant transition dipole moments. In particular, formation processes and optical shielding of collisions will be discussed.

[1]  M. Guo, B. Zhu, B. Lu, X. He, F. Wang, R. Vexiau, N. Bouloufa-Maafa, G. Quéméner, O. Dulieu, D. Wang, Phys. Rev. Lett., 116, 205303 (2016)
[2] D. Borsalino, R. Vexiau, M. Aymar, E. Luc-Koenig, O. Dulieu, N. Bouloufa-Maafa, J. Phys. B, 49, 055301 (2016)
[3] P. Zuchowski, R. Guérout and O. Dulieu, Phys. Rev. A 90, 012507 (2010).

Bose polaron is generally believed as a promising many-body system to host the novel Efimov correlations. Nevertheless, no signature of Efimov physics has been reported in the existing Bose polaron experiments up to date. In this talk, I will show that the Efimov physics can be directly observed in the spectroscopy measurement of Bose polarons with large mass imbalance. Taking the ⁶Li(impurity)- ¹³³Cs(bosons) system as an example, our calculation shows two visible Efimov branches in the spectral response of ⁶Li atoms, as well as the spectral broadening due to their hybridizations with the attractive polaron branch. These results suggest that the highly mass-imbalanced atomic mixtures can serve as an ideal platform for the observation of intriguing few-body correlations in a many-body environment. 

Motivated by a recent experiment [E. Nicklas et al Phys. Rev. Lett. (2015)], I consider the paradigmatic case of pre-thermal critical states of an interacting O(N) model, for studying universal real-time crossovers among the dynamical critical points of a macroscopic quantum system after a quench. These novel features of pre-thermal critical dynamics are extracted using a combination of renormalization group analysis and exactly solvable large N limit.

I will give an update of our work on catastrophes (caustics) that appear in the dynamics of two-mode fields such as a BEC in a double well, emphasizing scaling properties and taking the point of view that these structures constitute an example of universality in dynamics. Apart from plasmon dynamics, we will also consider catastrophes associated with pi-oscillations.  Time permitting, I will also briefly introduce some other topics we are working on such as using a BEC to make a position measurement of an impurity atom, and my take on the Abraham-Minkowski controversy.

Public Evening Lecture

at the occasion of the inauguration of the

Center for Quantum Dynamics of the Ruprecht-Karl University Heidelberg

“Time and Einstein in the 21st century”

Prof. Bill Phillips

Abstract: At the beginning of the 20th century Einstein changed the way we think about Nature. At the beginning of the 21st century Einstein's thinking is shaping one of the key scientific and technological wonders of contemporary life: atomic clocks, the best timekeepers ever made. Such super-accurate clocks are essential to industry, commerce, and science; they are the heart of the Satellite Navigation System that guides cars, airplanes, and hikers to their destinations. Today, atomic clocks are still being improved, using atoms cooled to incredibly low temperatures. Atomic gases reach temperatures less than a billionth of a degree above Absolute Zero,without freezing. Such atoms enable clocks accurate to better than a second in 80 million years as well as both using and testing some of Einstein's strangest predictions.

Programme

16:00 - 17:30 hours:
Poster presentation of the groups of the Center for Quantum Dynamics in the Foyer of the Kirchhoff Institute.