Long-lived coherences in strongly interacting spin ensembles

TL;DR

This study demonstrates that simple CPMG-like pulse sequences can significantly enhance and characterize the coherence of dense NV center spin ensembles in diamond, revealing the role of pulse errors and effective fields in coherence preservation.

Contribution

It shows how a basic pulse sequence can extend coherence and extract spin interaction details in dense NV ensembles, simplifying quantum control and characterization.

Findings

Counter-intuitive role of pulse errors in coherence preservation

Emergent effective field scales linearly with rotation offset

Quantitative extraction of spin densities from coherence data

Abstract

Periodic driving has emerged as a powerful tool to control, engineer, and characterize many-body quantum systems. However, the required pulse sequences are often complex, long, or require the ability to control the individual degrees of freedom. In this work, we study how a simple Carr-Purcell Meiboom-Gill (CPMG)-like pulse sequence can be leveraged to enhance the coherence of a large ensemble of spin qubits and serve as an important characterization tool. We implement the periodic drive on an ensemble of dense nitrogen-vacancy (NV) centers in diamond and examine the effect of pulse rotation offset as a control parameter on the dynamics. We use a single diamond sample prepared with several spots of varying NV density, which, in turn, varies the NV-NV dipolar interaction strength. Counter-intuitively, we find that rotation offsets deviating from the ideal {\pi}-pulse in the CPMG sequence (often classified as pulse errors) play a critical role in preserving coherence even at nominally zero rotation offset. The cause of the coherence preservation is an emergent effective field that scales linearly with the magnitude of the rotation offset. In addition to extending coherence, we compare the rotation offset dependence of coherence to numerical simulations to measure the disorder and dipolar contributions to the Hamiltonian to quantitatively extract the densities of the constituent spin species within the diamond.