26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Experimental Demonstration of Quantum State Smoothing in an Optical Parametric Oscillator

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Quantum Science and Technology (QST)

Description

Tracking the dynamics of a continuously monitored quantum system is usually performed using quantum filtering, which estimates the present state from measurement records available up to that time. When real-time estimation is not required, later measurement records can also improve the retrospective estimate of a quantum state. This approach, known as quantum state smoothing, can recover information lost through unobserved environmental channels.

We experimentally demonstrate quantum state smoothing using the intracavity field of an optical parametric oscillator. The output field is divided between two homodyne detectors, representing an observed channel and a hidden environmental channel. The observed record is processed using both filtering and smoothing, while the combined records provide an experimentally verifiable reference state. This enables direct comparison of the estimated state trajectories, purities, and deviations from the reference state.

We show that smoothing provides a more accurate estimate than filtering over a broad range of measurement efficiencies and homodyne angles. The smoothed states exhibit higher purity and a smaller trace-squared deviation from the reference states, confirming improved estimation accuracy. These results establish quantum state smoothing as a practical post-processing tool for tracking open quantum systems and recovering useful quantum resources.