Characterizing temperature and strain variations with qubit ensembles for their robust coherence protection

TL;DR

This paper introduces an unbalanced echo technique to mitigate interaction variations in nuclear spin ensembles, significantly enhancing coherence times for quantum memory and sensing applications, with experimental and theoretical validation.

Contribution

The paper presents a novel unbalanced echo method that refocuses interaction variations, improving coherence times in nuclear spin ensembles for quantum technologies.

Findings

20-fold increase in $T_2^*$ coherence time experimentally

First-principles prediction of interaction variations and their correlations

Achieves 400-fold coherence improvement under temperature inhomogeneity

Abstract

Solid-state spin defects, especially nuclear spins with potentially achievable long coherence times, are compelling candidates for quantum memories and sensors. However, their current performances are still limited by the decoherence due to the variation of their intrinsic quadrupole and hyperfine interactions. We propose an \textit{unbalanced echo} to overcome this challenge by using a second spin to refocus the variation of these interactions, which preserves the quantum information stored in the free evolution. The unbalanced echo can be used to probe the temperature and strain distribution in materials. Experimentally, we demonstrate a 20-fold $T_2^*$ coherence time increase in an ensemble of $\sim10^{10}$ nuclear spins in diamond. Theoretically, we develop first-principles methods to predict these interaction variations and reveal their correlation in large temperature and strain ranges. We numerically show that our method can refocus stronger noise variations than our current experiments and achieves a 400-fold coherence improvement for a 25~K temperature inhomogeneity.