# Quantum sensing and metrology with free electrons

## TL;DR

This paper introduces a new quantum sensing method using free electrons and light to achieve higher precision than traditional methods.

## Contribution

The novel approach uses free electrons and strong electron-light coupling to generate high-photon-number states for quantum sensing.

## Key findings

- Free electrons interacting with waveguided light can generate and detect high-photon-number states.
- Electron-beam splitters and two electron-waveguide interactions dramatically enhance optical-phase change sensitivity.
- The method achieves unprecedented sensitivity using currently available technology.

## Abstract

The quantum properties of matter and radiation can be leveraged to surpass classical limits of sensing and detection. Quantum optics does so by creating and measuring nonclassical light. However, better performance requires higher photon-number states, which are challenging to generate and detect. Here, we combine photons and free electrons to solve the problem of generating and detecting high-number states well beyond those reachable with light alone and further show that an unprecedented level of sensitivity and resolution is gained based on the measurement of free-electron currents after suitably designed electron–light interactions. Our enabling ingredient is the strong electron–light coupling produced by aloof electron reflection on an optical waveguide, leading to the emission or absorption of a high number of guided photons by every single electron. We establish through rigorous theory that, by combining electron-beam splitters with two electron–waveguide interactions, the sensitivity to detect optical-phase changes can be enhanced dramatically using currently attainable technology. These results inaugurate a disruptive quantum technology relying on free electrons and their strong interaction with waveguided light.

Despite the potential of free electrons for quantum technological applications, these are still limited by the weak electron-photon interactions. To address this challenge, the authors introduce electron-light couplers that produce high-photon-number state generation, enabling quantum sensing and metrology with unprecedented precision.