How does a continuously measured qubit really evolve?

How does a continuously measured qubit really evolve?

Seminar author:Luis Pedro García-Pintos

Event date and time:10/13/2016 02:30:pm

Event location:GIQ - Seminar Room

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We focus on the question of what a quantum system really did during a period of time over which it was continuously monitored. First, we consider the simultaneous monitoring of two noncommuting observables—as recently implemented by the Siddiqi group at UC Berkeley—and show the manifest violation of standard inequalities for testing Leggett-Garg macrorealism. Despite this apparent lack of realism, we then construct a realistic, but epistemically restricted, model that exactly reproduces the quantum results. Second, for continuous measurements of a single observable it is widely recognized that the measurement output approximates the expectation value of the observable, hidden by additive white noise. Filtering the measurement readout can thus approximately uncover the dynamics of the expectation value during a single realization. However, using information from the entire output history yields a different, “smoothed”, observable estimate, which can be objectively closer to the experimental output than the usual expectation value.