Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing

TL;DR

This paper demonstrates simultaneous monitoring of relaxation and dephasing in a superconducting qubit, enabling quantum state tomography and trajectory inference, revealing the interplay of decoherence channels and quantum measurement incompatibility.

Contribution

It introduces a method to retrieve information from both relaxation and dephasing channels in a superconducting qubit, allowing full quantum tomography and trajectory analysis.

Findings

Full quantum tomography of the qubit state achieved

Quantum trajectories conditioned on decoherence channels inferred

Observable effects of measurement incompatibility demonstrated

Abstract

Decoherence originates from the leakage of quantum information into external degrees of freedom. For a qubit the two main decoherence channels are relaxation and dephasing. Here, we report an experiment on a superconducting qubit where we retrieve part of the lost information in both of these channels. We demonstrate that raw averaging the corresponding measurement records provides a full quantum tomography of the qubit state where all three components of the effective spin-1/2 are simultaneously measured. From single realizations of the experiment, it is possible to infer the quantum trajectories followed by the qubit state conditioned on relaxation and/or dephasing channels. The incompatibility between these quantum measurements of the qubit leads to observable consequences in the statistics of quantum states. The high level of controllability of superconducting circuits enables us to explore many regimes from the Zeno effect to underdamped Rabi oscillations depending on the relative strengths of driving, dephasing and relaxation.