Interference of nonequilibrium quasiparticles in a superconductor

We have observed an interference of nonequilibrium quasiparticles, injected from a copper electrode into an aluminium loop through a tunnel barrier. At temperatures below 1 K the tunnel current at fixed voltage bias is periodically modulated by external magnetic field. The amplitude of the modulation reaches maximum at a bias slightly below the gap energy, and decreases with the further increase of the bias voltage. For a given voltage bias the amplitude of the current oscillations decreases with increase of the temperature and the loop circumference.