# Optical quantum nondemolition measurement of a solid-state spin without a cycling transition

## TL;DR

This paper demonstrates a method to achieve high-fidelity, quantum nondemolition spin measurement in solid-state defects by modifying the electromagnetic environment with an optical cavity, enabling broader quantum technology applications.

## Contribution

It introduces a cavity-enhanced technique to induce cycling transitions in solid-state defects, allowing single-shot spin readout without the need for natural cycling transitions.

## Key findings

- Achieved over 100-fold increase in optical cycling efficiency.
- Demonstrated 94.6% fidelity in spin state readout.
- Enabled probing of coherent spin dynamics.

## Abstract

Optically-interfaced spins in the solid state are a promising platform for quantum technologies. A crucial component of these systems is high-fidelity, projective measurement of the spin state. In previous work with laser-cooled atoms and ions, and solid-state defects, this has been accomplished using fluorescence on an optical cycling transition; however, cycling transitions are not ubiquitous. In this work, we demonstrate that modifying the electromagnetic environment using an optical cavity can induce a cycling transition in a solid-state atomic defect. By coupling a single Erbium ion defect to a telecom-wavelength silicon nanophotonic device, we enhance the cyclicity of its optical transition by a factor of more than 100, which enables single-shot quantum nondemolition readout of the ion's spin with 94.6% fidelity. We use this readout to probe coherent dynamics and relaxation of the spin. This approach will enable quantum technologies based on a much broader range of atomic defects.