# Quantum back-action of variable-strength measurement

## TL;DR

This paper demonstrates how a superconducting qubit's state can be precisely monitored through variable-strength measurements, revealing the quantum back-action and enabling potential feedback control.

## Contribution

It experimentally shows that a partial measurement can keep a qubit in a pure state with evolution fully determined by measurement outcomes.

## Key findings

- Back-action depends on measured quantity
- Single measurement of both quadratures observed
- Quantum state evolution is stochastic but trackable

## Abstract

Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depends on the quantity which is measured. In a partial measurement performed by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrate this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures is observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.