# Implementation of a canonical phase measurement with quantum feedback

## TL;DR

This paper demonstrates an optimal single-shot phase measurement for a photon using quantum feedback, surpassing traditional methods and enabling advanced quantum computing applications.

## Contribution

It introduces a novel implementation of a canonical phase measurement with quantum feedback, achieving optimal single-shot phase estimation.

## Key findings

- Surpasses heterodyne detection in phase measurement accuracy
- Validates quantum feedback as a tool to enhance detector precision
- Enables measurement of new physical observables in quantum optics

## Abstract

Much of modern metrology and communication technology encodes information in electromagnetic waves, typically as an amplitude or phase. While current hardware can perform near-ideal measurements of photon number or field amplitude, to date no device exists that can even in principle perform an ideal phase measurement. In this work, we implement a single-shot canonical phase measurement on a one-photon wave packet, which surpasses the current standard of heterodyne detection and is optimal for single-shot phase estimation. By applying quantum feedback to a Josephson parametric amplifier, our system adaptively changes its measurement basis during photon arrival and allows us to validate the detector's performance by tracking the quantum state of the photon source. These results provide an important capability for optical quantum computing, and demonstrate that quantum feedback can both enhance the precision of a detector and enable it to measure new classes of physical observables.