Tuned quasiparticle injection and extraction in non-equlibrium superconductors
Quasiparticle (qp) poisoning is an ever-present issue that appears in the majority of devices with superconducting components. Even at temperatures well below the transition to the normal state, the bare operation of the devices or external radiation can produce undesired excitations in the superconductor. Here , we present a controlled method for injection and extraction of qps which can help study their transport dynamics as well as show how to reduce them in a controlled manner. We use a superconductor--insulator--normal-metal--insulator--superconductor (SINIS) single-electron transistor (SET) as a turnstile for single electrons which is a robust single-electron current emitter and a candidate for a current standard. In this platform, excitations are injected one-by-one from the periodically gate-driven normal-metal island into both superconducting leads. Which can be probed by means of measuring the heat injected into the leads with an in-situ thermometer. Moreover, the measured current is closer to the desired value of one electron per driving cycle when the leads are absent of non-equilibrium excitations acting also as a probe for the qp density. We use this property for showing qp extraction with biased SIS junctions and for demonstrating the tunability of the qp extraction power. As a result, the turnstile current improves by varying the SIS junction bias, approaching its ideal value. The extraction method proves to be able to extract also the quiescent background excitations in a wide range of operation frequencies. Additionally, the probing method allows us to quantitatively asses the present qp density and the amount of extracted excitations. At the highest injection rate of 240 MHz we obtain a reduction of the density by an order of magnitude in the most efficient extraction regime.
 M. Marı́n-Suárez, J. T. Peltonen, and J. P. Pekola. Active quasiparticle suppression in a non-equilibrium superconductor. Nano Letters, 20(7):5065–5071.