V. F. Maisi, S. V. Lotkhov, A. Kemppinen, A. Heimes, J. T. Muhonen, J. P. Pekola
In superconductors, microscopic degrees of freedom freeze out at low temperatures, provided no energy exceeding the superconducting gap is available. Excitations, in form of quasiparticles, lead to dissipation and dephasing. This is the prime trait for superconducting electronics, e.g. in qubits, electron turnstiles and kinetic inductance detectors. Relaxation rates in a superconductor become exponentially slow at the same time as the excitations diminish at low temperatures. In this letter we experiment on a small superconducting island cooled down to have no excitations for most of the time under quiescent conditions. We demonstrate that with normal metallic tunnel probes one can study the excitation-relaxation dynamics, free of multi-electron tunneling. By injecting electrons to the island we measure the recombination dynamics down to a single quasiparticle pair.
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http://arxiv.org/abs/1212.2755
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