Quantum Non-demolition Detection of Single Microwave Photons in a Circuit

TL;DR

This paper demonstrates a quantum non-demolition (QND) detection scheme for microwave photons in superconducting circuits, enabling repeated, non-destructive photon number measurements with high selectivity, crucial for quantum information processing.

Contribution

It introduces a novel QND detector that maps photon number onto a qubit state using single-shot measurements, achieving 90% QND fidelity and high photon number selectivity.

Findings

Achieves 90% QND measurement fidelity.

Demonstrates selective sensitivity to specific photon number states.

Enables monitoring of photon-based quantum memories.

Abstract

Thorough control of quantum measurement is key to the development of quantum information technologies. Many measurements are destructive, removing more information from the system than they obtain. Quantum non-demolition (QND) measurements allow repeated measurements that give the same eigenvalue. They could be used for several quantum information processing tasks such as error correction, preparation by measurement, and one-way quantum computing. Achieving QND measurements of photons is especially challenging because the detector must be completely transparent to the photons while still acquiring information about them. Recent progress in manipulating microwave photons in superconducting circuits has increased demand for a QND detector which operates in the gigahertz frequency range. Here we demonstrate a QND detection scheme which measures the number of photons inside a high quality-factor microwave cavity on a chip. This scheme maps a photon number onto a qubit state in a single-shot via qubit-photon logic gates. We verify the operation of the device by analyzing the average correlations of repeated measurements, and show that it is 90% QND. It differs from previously reported detectors because its sensitivity is strongly selective to chosen photon number states. This scheme could be used to monitor the state of a photon-based memory in a quantum computer.