Homodyne monitoring of post-selected decay

Abstract

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. Subject to homodyne detection, the density matrix evolves in a stochastic manner, but it is restricted to a specific surface in the Bloch sphere. We show that a similar restriction applies to the information associated with the post-selection, and thus bounds the predictions of the theory.

11 pages, 8 figures

References in corpus (7)

• Charge insensitive qubit design derived from the Cooper pair box
• Ultrasensitive Beam Deflection Measurement via Interferometric Weak Value Amplification
• A Straightforward Introduction to Continuous Quantum Measurement
• Mapping the optimal route between two quantum states
• Time-Symmetric Quantum Theory of Smoothing
• Prediction and retrodiction for a continuously monitored superconducting qubit
• Quantum smoothing for classical mixtures

Cited by in corpus (3)

• Prediction and retrodiction with continuously monitored Gaussian states
• Conditional past-future correlation induced by non-Markovian dephasing reservoirs
• Continuous monitoring measured signals bounded by past and future conditions in enlarged quantum systems