Partial-measurement back-action and non-classical weak values in a superconducting circuit

TL;DR

This paper demonstrates how partial measurements in a superconducting circuit can be precisely controlled and used to observe non-classical weak values and violations of Leggett-Garg inequalities, advancing quantum measurement techniques.

Contribution

It introduces a method for indirect partial measurement of a transmon qubit with tunable strength and basis, linking measurement back-action to non-classical phenomena.

Findings

Controlled partial measurement affects qubit Rabi oscillations.

Non-classical weak values are observed in a solid-state system.

Violation of Leggett-Garg inequality confirmed.

Abstract

We realize indirect partial measurement of a transmon qubit in circuit quantum electrodynamics by interaction with an ancilla qubit and projective ancilla measurement with a dedicated readout resonator. Accurate control of the interaction and ancilla measurement basis allows tailoring the measurement strength and operator. The tradeoff between measurement strength and qubit back-action is characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. Combining partial and projective qubit measurements, we provide the solid-state demonstration of the correspondence between a non-classical weak value and the violation of a Leggett-Garg inequality.