Homodyne monitoring of post-selected decay

Understand

We use homodyne detection to monitor the radiative decay of a superconducting qubit.

• According to the classical theory of conditional probabilities, the excited state population differs from an exponential decay law if it is conditioned upon a later projective qubit measurement.
• Quantum trajectory theory accounts for the expectation values of general observables, and we use experimental data to show how a homodyne detection signal is conditioned upon both the initial state and the finally projected state of a decaying qubit.
• We observe, in particular, how anomalous weak values occur in continuous weak measurement for certain pre- and post-selected states.