Prediction and retrodiction for a continuously monitored superconducting qubit

TL;DR

This paper demonstrates how to use continuous measurement data from a superconducting qubit to predict and retrodict its quantum state, enhancing understanding of quantum measurement processes.

Contribution

It introduces a method to derive predictive and retrodictive density matrices from experimental data for a superconducting qubit.

Findings

Successfully derived density matrices from experimental data

Demonstrated prediction and retrodiction of measurement outcomes

Enhanced understanding of quantum measurement dynamics

Abstract

The quantum state of a superconducting transmon qubit inside a three-dimensional cavity is monitored by reflection of a microwave field on the cavity. The information inferred from the measurement record is incorporated in a density matrix $\rho_t$, which is conditioned on probe results until $t$, and in an auxiliary matrix $E_t$, which is conditioned on probe results obtained after $t$. Here, we obtain these matrices from experimental data and we illustrate their application to predict and retrodict the outcome of weak and strong qubit measurements.