Thermally-Polarized Solid-State Spin Sensor

TL;DR

This paper introduces a novel, non-optical solid-state spin sensor that uses thermal polarization and microwave readout, enabling broad applicability and high sensitivity in magnetic field detection.

Contribution

It presents a new non-optical method for preparing and reading out spin states in solid-state sensors, expanding the potential for quantum sensing technologies.

Findings

Achieved a broadband magnetic sensitivity of 9.7 pT/√Hz.

Demonstrated a fully non-optical sensor architecture applicable to various defects.

Used microwave cavity readout with Cr³⁺ in sapphire for high efficiency.

Abstract

Quantum sensors based on spin defect ensembles have seen rapid development in recent years, with a wide array of target applications. Historically, these sensors have used optical methods to prepare or read out quantum states. However, these methods are limited to optically-polarizable spin defects, and the spin ensemble size is typically limited by the available optical power or acceptable optical heat load. We demonstrate a solid-state sensor employing a non-optical state preparation technique, which harnesses thermal population imbalances induced by the defect's zero-field splitting. Readout is performed using the recently-demonstrated microwave cavity readout technique, resulting in a sensor architecture that is entirely non-optical and broadly applicable to all solid-state paramagnetic defects with a zero-field splitting. The implementation in this work uses Cr$^{3+}$ defects in a sapphire (Al$_2$O$_3$) crystal and a simple microwave architecture where the host crystal also serves as the high quality-factor resonator. This approach yields a near-unity filling factor and high single-spin-photon coupling, producing a magnetometer with a broadband sensitivity of 9.7 pT/$\sqrt{\text{Hz}}$.