# Incoherent qubit control using the quantum Zeno effect

## TL;DR

This paper demonstrates a novel method of qubit control using the quantum Zeno effect through continuous measurement, enabling error detection and high-fidelity state tracking without direct Hamiltonian manipulation.

## Contribution

It introduces a measurement-only control technique employing a dynamically controllable observable and continuous monitoring, advancing quantum feedback and state purification methods.

## Key findings

- Successfully demonstrated state control via the quantum Zeno effect in circuit-QED.
- Achieved high-fidelity state tracking through post-selected measurement records.
- Provided a new tool for quantum feedback protocols and rapid state purification.

## Abstract

The quantum Zeno effect is the suppression of Hamiltonian evolution by repeated observation, resulting in the pinning of the state to an eigenstate of the measurement observable. Using measurement only, control of the state can be achieved if the observable is slowly varied such that the state tracks the now time-dependent eigenstate. We demonstrate this using a circuit-QED readout technique that couples to a dynamically controllable observable of a qubit. Continuous monitoring of the measurement record allows us to detect an escape from the eigenstate, thus serving as a built-in form of error detection. We show this by post-selecting on realizations with arbitrarily high fidelity with respect to the target state. Our dynamical measurement operator technique offers a new tool for numerous forms of quantum feedback protocols, including adaptive measurements and rapid state purification.