Mapping quantum state dynamics in spontaneous emission

TL;DR

This paper demonstrates how phase-sensitive homodyne detection and quantum state tomography can map and control the evolution of a quantum state during spontaneous emission, revealing measurement-dependent dynamics.

Contribution

It introduces a method to map quantum trajectories during spontaneous emission using homodyne detection and characterizes measurement-induced back-action on the quantum state.

Findings

Quantum trajectories depend on the measurement basis.

Continuous field detection enables control over quantum evolution.

Significant differences from quantum jump models were observed.

Abstract

The evolution of a quantum state undergoing radiative decay depends on how the emission is detected. We employ phase-sensitive amplification to perform homodyne detection of the spontaneous emission from a superconducting artificial atom. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-action of homodyne measurement. By tracking the diffusive quantum trajectories of the state as it decays, we characterize selective stochastic excitation induced by the choice of measurement basis. Our results demonstrate dramatic differences from the quantum jump evolution that is associated with photodetection and highlight how continuous field detection can be harnessed to control quantum evolution.