Active spin lattice hyperpolarization: Application to hexagonal boron nitride color centers

TL;DR

This paper investigates the efficiency of active spin hyperpolarization methods in large spin systems, including realistic boron nitride color centers, using approximate and exact numerical techniques.

Contribution

It introduces an approximate Holstein-Primakoff method for large spin systems and applies it to hyperpolarization in boron nitride color centers, demonstrating high polarization potential.

Findings

Active driving effectively hyperpolarizes large spin baths.

Holstein-Primakoff approximation is validated for spin dynamics.

High nuclear spin polarization achievable in boron nitride centers.

Abstract

The active driving of the electron spin of a color center is known as a method for the hyperpolarization of the surrounding nuclear spin bath and to initialize a system with large number of spins. Here, we investigate the efficiency of this approach for various spin coupling schemes in a one-dimensional Heisenberg chain coupled to a central spin. To extend our study to the realistic systems with a large number of interacting spins, we employ an approximate method based on Holstein-Primakoff transformation. The validity of the method for describing spin polarization dynamics is benchmarked by the exact numerics for a small lattice, where the accuracy of the bosonic Holstein-Primakoff approximation approach is confirmed. We, thus, extend our analysis to larger spin systems where the exact numerics are out of reach. The results prove the efficiency of the active driving method when the central spin interaction with the spin bath is long range and the inter-spin interactions in the bath spins is large enough. The method is then applied to the realistic case of optically active negatively charged boron vacancy centers ($V_B$) in hexagonal boron nitride. Our results suggest that a high degree of hyperpolarization in the boron and nitrogen nuclear spin lattices is achievable even starting from a fully thermal bath. As an initialization, our work provides the first step toward the realization of a two-dimensional quantum simulator based on natural nuclear spins and it can prove useful for extending the coherence time of the $V_B$ centers.