# Nondestructive photon counting in waveguide QED

## TL;DR

This paper introduces a method to perform nondestructive photon counting in waveguide QED by entangling photons with atomic arrays, enabling high-fidelity, scalable, and robust photon number measurements.

## Contribution

The authors propose a novel scheme that maps photon counting to atom counting in waveguide QED, allowing nondestructive, high-fidelity measurements scalable with atom number.

## Key findings

- Fidelity increases with atom number
- Scheme is robust to disorder and finite Purcell factors
- Can implement nondestructive photon-number measurement

## Abstract

Number-resolving single-photon detectors represent a key technology for a host of quantum optics protocols, but despite significant efforts, state-of-the-art devices are limited to few photons. In contrast, state-dependent atom counting in arrays can be done with extremely high fidelity up to hundreds of atoms. We show that in waveguide QED, the problem of photon counting can be reduced to atom counting, by entangling the photonic state with an atomic array in the collective number basis. This is possible as the incoming photons couple to collective atomic states and can be achieved by engineering a second decay channel of an excited atom to a metastable state. Our scheme is robust to disorder and finite Purcell factors, and its fidelity increases with atom number. Analyzing the state of the re-emitted photons, we further show that if the initial atomic state is a symmetric Dicke state, dissipation engineering can be used to implement a nondestructive photon-number measurement, in which the incident state is scattered into the waveguide unchanged. Our results generalize to related platforms, including superconducting qubits.