Elsa Mannila

Origins of quasiparticle excitations in aluminum

Quasiparticle excitations limit the performance of various superconducting devices. The quasiparticle density should be exponentially suppressed at low temperatures. However, commonly quasiparticle densities many orders of magnitude higher than in thermal equilibrium are observed. In addition to having a well shielded and filtered measurement setup, on-chip sources of quasiparticles may become important in devices incorporating, for instance, charge detectors. I will demonstrate how the superconducting aluminum island of a single-electron transistor is poisoned with quasiparticles through nonequilibrium phonons emitted from another superconducting island. In a device whose charge state is monitored with a detector with a normal metal island, we are able to keep the superconducting island free of quasiparticles for 97% of the time.