Causal State Estimation and the Heisenberg Uncertainty Principle

• URL: http://arxiv.org/abs/2304.14476v3
• Date: Tue, 17 Oct 2023 18:50:44 GMT
• Title: Causal State Estimation and the Heisenberg Uncertainty Principle
• Abstract summary: We show that any causal estimate of linear observables of a linear, but not necessarily Markovian, system will satisfy the uncertainty principle. Results clarify the role of causal estimators to a large class of quantum systems.
• Score: 12.465699707229648
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: The observables of a noisy quantum system can be estimated by appropriately filtering the records of their continuous measurement. Such filtering is relevant for state estimation and measurement-based quantum feedback control. It is therefore imperative that the observables estimated through a causal filter satisfy the Heisenberg uncertainty principle. In the Markovian setting, prior work implicitly guarantees this requirement. We show that any causal estimate of linear observables of a linear, but not necessarily Markovian, system will satisfy the uncertainty principle. In particular, this is true irrespective of any feedback control of the system and of where in the feedback loop -- inside or outside -- the measurement record is accessed. Indeed, causal estimators using the in-loop measurement record can be as precise as those using the out-of-loop record. These results clarify the role of causal estimators to a large class of quantum systems, restores the equanimity of in-loop and out-of-loop measurements in their estimation and control, and simplifies future experiments on measurement-based quantum feedback control.

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