Operational meanings of a generalized conditional expectation in quantum metrology

Mankei Tsang

Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551

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A unifying formalism of generalized conditional expectations (GCEs) for quantum mechanics has recently emerged, but its physical implications regarding the retrodiction of a quantum observable remain controversial. To address the controversy, here I offer operational meanings for a version of the GCEs in the context of quantum parameter estimation. When a quantum sensor is corrupted by decoherence, the GCE is found to relate the operator-valued optimal estimators before and after the decoherence. Furthermore, the error increase, or regret, caused by the decoherence is shown to be equal to a divergence between the two estimators. The real weak value as a special case of the GCE plays the same role in suboptimal estimation – its divergence from the optimal estimator is precisely the regret for not using the optimal measurement. For an application of the GCE, I show that it enables the use of dynamic programming for designing a controller that minimizes the estimation error. For the frequentist setting, I show that the GCE leads to a quantum Rao-Blackwell theorem, which offers significant implications for quantum metrology and thermal-light sensing in particular. These results give the GCE and the associated divergence a natural, useful, and incontrovertible role in quantum decision and control theory.

The conditional expectation is an essential concept in probability theory and a basic tool in data analysis, as it allows one to infer hidden variables from observations in an optimal sense. Many have tried to generalize the concept for quantum mechanics, but it remains questionable whether such quantum conditional expectations are meaningful or useful. This work shows that a certain generalized conditional expectation can indeed be useful for quantum sensor design, thanks to its convenient mathematical properties.

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