Measurement-induced nuclear spin polarization

TL;DR

This paper introduces a measurement-based protocol for nuclear spin polarization in a spin-star system, achieving near-complete polarization of many nuclear spins efficiently and robustly, independent of magnetic field strength.

Contribution

It presents an analytical method to optimize measurement intervals for nuclear spin polarization, enabling high polarization levels with fewer measurements and robustness against certain interactions.

Findings

Hundreds to thousands of nuclear spins can be nearly fully polarized.

The protocol is insensitive to magnetic field variations.

It remains effective despite counterrotating interactions.

Abstract

We propose a nuclear-spin-polarization protocol in a general evolution-and-measurement framework. The protocol works in a spin-star configuration, where the central spin is coupled to the surrounding bath (nuclear) spins by flip-flop interaction of equal strength and is subject to a sequence of projective measurements on its ground state. Then a nondeterministic nuclear spin polarization could be implemented by entropy reduction through measurement. The optimized measurement-interval $\tau_{\rm opt}$ is analytically obtained in the near-resonant condition, which is relevant to the nuclear spins' polarization degree of the last-round measurement, the number of nuclear spins, and the coupling strength between the central spin and nuclear spins. Hundreds and even thousands of randomly aligned nuclear spins at the thermal state could be almost fully polarized with an optimized sequence of less than $20$ unequal-time-spacing measurements. In comparison to the conventional approaches, our protocol is not sensitive to the magnetic-field intensity, and it is robust against the extra counterrotating interaction in the near-resonant situation.