# Bayesian Quantum Measurement of a Single-Cooper-Pair Qubit

## Abstract

An interesting and important question is what happens to a single-Cooper-pair qubit during the measurement of its state, for example, by a single-electron transistor. If the measurement is sufficiently fast (to neglect other evolutions) and complete (so that the result is unambiguous), then the answer to this question is given by the standard collapse postulate of “orthodox” quantum mechanics [1]. However, this case is hardly realizable in practice. The reason is realistically weak coupling between the qubit and the detector and also finite noise of the detector (that requires some time before the signal-to-noise ratio becomes sufficiently large). In this case the measurement should be considered as a continuous process rather than instantaneous event and the simple collapse postulate cannot be applied. So, what happens to qubit state during such realistic measurement process? This question can be answered using recently developed Bayesian formalism [2] which essentially describes how the collapse process develops in time.

### Keywords

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