Influence of nuclear spin polarization on spin echo signal of NV center qubit

TL;DR

This paper investigates how nuclear spin polarization affects the spin echo signals of NV center qubits, revealing a time-dependent phase shift that indicates environment-mediated qubit self-interaction, challenging the external noise assumption.

Contribution

It demonstrates the impact of nuclear polarization on NV center spin echo signals and introduces a method to distinguish Gaussian from non-Gaussian environments based on echo measurements.

Findings

Phase shift in echo signal depends on nuclear polarization and qubit state.

Gaussian approximation explains phase shift for weakly coupled nuclear spins.

Comparison of echo signals reveals environment's Gaussian or non-Gaussian nature.

Abstract

We consider the spin echo dynamics of a nitrogen-vacancy center qubit based the $S\!= \! 1$ ground state spin manifold, caused by a dynamically polarized nuclear environment. We show that the echo signal acquires then a nontrivially time-dependent phase shift. This effect should be observable for polarization $\approx \! 0.5$ of nuclei within $\sim \! 1$ nm from the qubit, and for the NV center initialized in a superposition of $m\! = \! 0$ and either $m\! =\! 1$ or $m\! =\! -1$ states. This phase shift is much smaller when the NV center is prepared in a superposition of $m\! = \! 1$ and $m\! =\! -1$ states, i.e. when the qubit couples to the spin environment in a way analogous to that of spin-$1/2$. For nuclear environment devoid of spins strongly coupled to the qubit, the phase shift is well described within Gaussian approximation, which provides an explanation for the dependence of the shift magnitude on the choice of states on which the qubit is based, and makes it clear that its presence is related to the linear response of the environment perturbed by an evolving qubit. Consequently, its observation signifies the presence environment-mediated self-interaction of the qubit, and hence, it invalidates the notion that the nuclear environment acts as a source of external noise driving the qubit. We also show how a careful comparison of the echo signal from qubits based on $m\! = \! 0,1$ and $m\! =\! \pm 1$ manifolds, can distinguish between effectively Gaussian and non-Gaussian environment.