We propose a novel method for controlling quantum systems via feedback. Rather than modifying parameters of the controlled system, the feedback loop changes the way it interacts with environment (the unraveling of the quantum operation describing this interaction). As an example, a problem of controlling the state of two-mode atomic Bose–Einstein condensate, with one mode subject to interferometric phase-contrast imaging in the Mach–Zehnder interferometer is considered. Different unravelings of photodetection operation are implemented by varying the phase shift in the condensate-free arm of the interferometer. A master equation governing the evolution of the condensate is derived. Based on its steady-state solution, the Kullback–Leibler divergence between the atomic distributions with and without feedback is calculated. The results show that this divergence reaches maximum at non-trivial values of the parameters controlling unraveling.
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Funding
This work performed at the Institute of Automation and Electrometry, Siberian Branch, Russian Academy of Sciences was supported by the Ministry of Science and Higher Education of the Russian Federation (project no. AAAA-A21-121021800168-4).
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Tomilin, V.A., Il’ichov, L.V. Quantum Feedback Control Outside of the Controlled System. Jetp Lett. 116, 649–654 (2022). https://doi.org/10.1134/S0021364022602019
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DOI: https://doi.org/10.1134/S0021364022602019