Number fluctuations of sparse quasiparticles in a superconductor

Understand

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium.

• The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles.
• We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to 25-55 per cubic micron.
• We show that this saturation is consistent with the measured saturation in the quasiparticle lifetime, which also explains similar observations in qubit decoherence times.