Canonical phase measurement enabled by quantum feedback control

In addition to extracting information, measurements of quantum systems are a resource for enhancing control and precision. They can be used to alter what we are detecting and allow access to new observables. Current hardware can perform near-ideal measurements of photon number or field amplitude, the ability to perform an ideal phase measurement is still lacking, even in principle. 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 to suppress amplitude information. We validate the detector’s performance by tracking the quantum state of the photon source. These results demonstrate that quantum feedback can enable access to new classes of physical observables.