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Phys Rev Lett. 2013 Oct 4;111(14):147001. Epub 2013 Oct 3.

Excitation of single quasiparticles in a small superconducting Al island connected to normal-metal leads by tunnel junctions.

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Low Temperature Laboratory (OVLL), Aalto University School of Science, P.O. Box 13500, 00076 Aalto, Finland and Centre for Metrology and Accreditation (MIKES), P.O. Box 9, 02151 Espoo, Finland.


We investigate the dynamics of individual quasiparticle excitations on a small superconducting aluminum island connected to normal metallic leads by tunnel junctions. We find the island to be free of excitations within the measurement resolution. This allows us to show that the residual heating, which typically limits experiments on superconductors, has an ultralow value of less than 0.1 aW. By injecting electrons with a periodic gate voltage, we probe electron-phonon interaction and relaxation down to a single quasiparticle excitation pair, with a measured recombination rate of 16 kHz. Our experiment yields a strong test of BCS theory in aluminum as the results are consistent with it without free parameters.

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