Observing quantum state diffusion by heterodyne detection of fluorescence

TL;DR

This paper demonstrates how heterodyne detection of fluorescence enables real-time observation of quantum state diffusion in a superconducting qubit, revealing the influence of measurement type on quantum trajectories and state evolution.

Contribution

It introduces heterodyne detection of fluorescence as a method to observe quantum state diffusion, providing experimental validation and insights into measurement-induced state dynamics.

Findings

Quantum trajectories follow a diffusion process influenced by measurement.

Qubit states occupy a deterministic surface in the Bloch sphere during evolution.

Monitoring fluorescence can generate superpositions and increase excitation probability.

Abstract

A qubit can relax by fluorescence, which prompts the release of a photon into its electromagnetic environment. By counting the emitted photons, discrete quantum jumps of the qubit state can be observed. The succession of states occupied by the qubit in a single experiment, its quantum trajectory, depends in fact on the kind of detector. How are the quantum trajectories modified if one measures continuously the amplitude of the fluorescence field instead? Using a superconducting parametric amplifier, we have performed heterodyne detection of the fluorescence of a superconducting qubit. For each realization of the measurement record, we can reconstruct a different quantum trajectory for the qubit. The observed evolution obeys quantum state diffusion, which is characteristic of quantum measurements subject to zero point fluctuations. Independent projective measurements of the qubit at various times provide a quantitative validation of the reconstructed trajectories. By exploring the statistics of quantum trajectories, we demonstrate that the qubit states span a deterministic surface in the Bloch sphere at each time in the evolution. Additionally, we show that when monitoring fluorescence, coherent superpositions are generated during the decay from excited to ground state. Counterintuitively, measuring light emitted during relaxation can give rise to trajectories with increased excitation probability.