  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.
