Radiative cooling of a spin ensemble

TL;DR

This paper demonstrates a novel method to control electron spin temperature independently from the lattice by engineering cavity fields, enabling hyperpolarisation and signal enhancement in spin resonance techniques.

Contribution

It introduces a universal approach to manipulate spin temperature via cavity coupling, surpassing traditional lattice-based relaxation limits.

Findings

Achieved over twofold increase in electron spin polarisation.

Controlled spin cooling below lattice temperature.

Demonstrated applicability to ESR and NMR signal enhancement.

Abstract

Physical systems reach thermal equilibrium through energy exchange with their environment, and for spins in solids the relevant environment is almost always the host lattice in which they sit. However, recent studies motivated by observations from Purcell showed how coupling to a cavity can become the dominant form of relaxation for spins, given suitably strong spin-cavity coupling. In this regime, the cavity electromagnetic field takes over from the lattice as the dominant environment, inviting the prospect of controlling the spin temperature independently from that of the lattice, by engineering a suitable cavity field. Here, we report on precisely such control over spin temperature, illustrating a novel and universal method of electron spin hyperpolarisation. By switching the cavity input between loads at different temperatures we can control the electron spin polarisation, cooling it below the lattice temperature. Our demonstration uses donor spins in silicon coupled to a superconducting micro-resonator and we observe an increase of spin polarisation of over a factor of two. This approach provides general route to signal enhancement in electron spin resonance, or indeed nuclear magnetic resonance through dynamical nuclear spin polarisation (DNP).