Long-Lived Mechanically-Detected Molecular Spins for Quantum Sensing

TL;DR

This paper introduces SQUINT, a quantum sensing platform using molecular spins and mechanical readout to achieve high sensitivity and control at the molecular level, overcoming limitations of solid-state defect spins.

Contribution

It develops a nanoscale sensing method combining molecular electron spins, advanced decoupling sequences, and mechanical detection for enhanced quantum sensing capabilities.

Findings

Extended coherence times to ~400 μs in molecular droplets

Achieved frequency-selective detection of nanotesla AC fields

Demonstrated sensing and spectroscopy of local nuclear spins

Abstract

Quantum sensors based on individual spins provide unprecedented access to local magnetic fields in condensed matter, chemistry, and biology, with solid-state defect spins emerging as the leading platform. However, their molecular-sensing capabilities are limited by confinement to a host lattice, which prevents placement in close proximity to a target molecule. Molecular spins offer an alternative, enabling chemical tunability and flexible positioning relative to the target system. Here we present a nanoscale sensing platform that combines molecular electron spins, ultrasensitive mechanical readout, and Hamiltonian engineering. Using a modified XYXY dipolar decoupling sequence, we suppress electron-electron dipolar interactions across a broad distribution of control fields, extending coherence times to $\sim 400~\mu$s in an attoliter-scale droplet containing $\sim$100 trityl-OX063 radicals. Leveraging this sequence, we demonstrate frequency-selective detection of nanotesla-scale AC fields and perform sensing and spectroscopy of small, local nuclear-spin ensembles. Collectively, these results establish SQUINT (Spin-based QUantum Integrated Nanomechanical Transduction) as a framework for quantum sensing that affords molecular-level control over sensor properties and enables direct integration into complex molecular targets.