  We present a superconducting device showing a clear parity effect in the
number of electrons, even when there is, on average, a single nonequilibrium
quasiparticle present and the parity of the island switches due to
quasiparticles tunneling in and out of the device at rates on the order of 100
Hz. We detect the switching by monitoring in real time the charge state of a
superconducting island connected to normal leads by tunnel junctions. The
quasiparticles are created by Cooper pairs breaking on the island at a rate of
a few kHz. We demonstrate that the pair breaking is caused by the backaction of
the single-electron transistor used as a charge detector. With sufficiently low
probing currents, our superconducting island is free of quasiparticles 97% of
the time.
