Quasiparticle Spin Resonance and Coherence in Superconducting Aluminium

TL;DR

This paper demonstrates quasiparticle spin resonance in superconducting aluminium, measuring a spin coherence time of approximately 95 picoseconds, thus extending the understanding of spin dynamics in conventional superconductors.

Contribution

It provides the first measurement of quasiparticle spin coherence time in a BCS superconductor using resonance techniques.

Findings

Quasiparticle spin coherence time in aluminium is about 95 ps.

Resonance experiments can probe spin dynamics in superconductors.

Non-equilibrium magnetisation can be detected in conventional superconductors.

Abstract

Spin/magnetisation relaxation and coherence times, respectively T_1 and T_2, initially defined in the context of nuclear magnetic resonance (NMR), are general concepts applicable to a wide range of systems, including quantum bits [1-4]. At first glance, these ideas might seem to be irrelevant to conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, as the BCS superconducting ground state is a condensate of Cooper pairs of electrons with opposite spins (in a singlet state) [5]. It has recently been demonstrated, however, that a non-equilibrium magnetisation can appear in the quasiparticle (i.e. excitation) population of a conventional superconductor, with relaxation times on the order of several nanoseconds [6-10]. This raises the question of the spin coherence time of quasiparticles in superconductors and whether this can be measured through resonance experiments analogous to NMR and electron spin resonance (ESR). We have performed such measurements in aluminium and find a quasiparticle spin coherence time of 95+/-20ps.