  We report on the realization of a single-electron source, where current is
transported through a single-level quantum dot (Q), tunnel-coupled to two
superconducting leads (S). When driven with an ac gate voltage, the experiment
demonstrates electron turnstile operation. Compared to the more conventional
superconductor - normal metal - superconductor turnstile, our SQS device
presents a number of novel properties, including higher immunity to the
unavoidable presence of non-equilibrium quasiparticles in superconducting
leads. In addition, we demonstrate its ability to deliver electrons with a very
narrow energy distribution.
