Continuously Tunable Charge in Andreev Quantum Dots

Theoretische Physik, Schafmattstrasse 32, ETH-Zurich, CH-8093 Zu¨rich, Switzerland(Dated: February 5, 2008)We show that a quantum dot connected via tunnel barriers to superconducting leads traps acontinuously tunable and hence fractional charge. The fractional charge on the island is due toparticle-hole symmetry breaking and can be tuned via a gate potential acting on the dot or viachanges in the phase difference across the island. We determine the groundstate, equilibrium, andexcitation charges and show how to identify these quantities in an experiment.