Experimental evidence for parity-based 2eperiodicity in a superconducting single-electron tunneling transistor

We present experimental current-voltage (I-V) measurements on an Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al single-electron tunneling transistor in the superconducting state. We observe a variety of features which result from Cooper-pair tunneling processes. At low bias voltages and low temperatures we find that the I-V curve is 2e periodic with respect to the gate-induced charge. Remarkably, this periodicity persists up to 300 mK, a behavior which we interpret as strong evidence of an incremental free energy which depends on whether the number of electrons on the superconducting island is even or odd.