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19. June 2019 14:00

Noninvasive measurement of dynamic correlations

Philipp Uhrich
Stellenbosch University, South Africa

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)