Resonant Cooper-Pair Tunneling: Counting Statistics and Frequency-Dependent Current Noise

Abstract

Resonant Cooper-pair tunneling, also known as Josephson quasiparticle tunneling, refers to transport cycles in superconducting single-electron transistors (SSET’s) which involve the transfer of both Cooper pairs and quasiparticles [1, 2, 3]. They have recently been the subject of renewed attention, both because of their unusual noise properties [4, 5, 6] and because of their utility in measuring the state of a charge superconducting qubit [7, 8]. In terms of noise properties, it has been shown that charge fluctuations associated with these processes can induce a population inversion in a coupled two-level system (i.e in terms of its charge noise, the transistor effectively has a negative temperature) [5, 6]. The shot-noise in the current through the transistor was also found to have remarkable properties [4, 5]. By tuning the strength of the Cooper pair tunneling relative to the quasiparticle tunneling, one could effectively tune the Fano factor determining the zero-frequency shot noise. It was possible to achieve a Fano factor greater than one, which was interpreted as a consequence of the effective charge associated with the transport cycle being greater than one. Perhaps more surprisingly, it was possible to reduce the Fano factor below 1/2, behaviour that was not fully explained. The finite frequency current noise also showed interesting behaviour [4]- in the regime where the Cooper-pair tunneling dominated the quasiparticle tunneling, a coherent peak in the current noise was predicted at the Josephson energy.