Turnstile behaviour of the Cooper-pair pump

TL;DR

This study experimentally investigates the Cooper-pair pump's turnstile behavior with three Josephson junctions, demonstrating controlled current pumping with about 3% accuracy and exploring the energy minimization principle affecting quasiparticle configurations.

Contribution

The paper provides the first experimental demonstration of the Cooper-pair pump functioning as a turnstile device with detailed analysis of its accuracy and underlying energy minimization principles.

Findings

Achieved 3% accuracy in gate-induced current pumping.

Observed Landau-Zener transitions above 20 MHz.

Reproduced turnstile behavior through current rotation in the gate plane.

Abstract

We have experimentally studied the behaviour of the so-called Cooper pair pump (CPP) with three Josephson junctions, in the limit of small Josephson coupling EJ < EC. These experiments show that the CPP can be operated as a traditional turnstile device yielding a gate-induced current 2ef in the direction of the bias voltage, by applying an RF-signal with frequency f to the two gates in phase, while residing at the degeneracy node of the gate plane. Accuracy of the CPP during this kind of operation was about 3% and the fundamental Landau-Zener limit was observed to lie above 20 MHz. We have also measured the current pumped through the array by rotating around the degeneracy node in the gate plane. We show that this reproduces the turnstile-kind of behavior. To overcome the contradiction between the obtained e-periodic DC-modulation and a pure 2e-behaviour in the RF-measurements, we base our observations on a general principle that the system always minimises its energy. It suggests that if the excess quasiparticles in the system have a freedom to tunnel, they will organize themselves to the configuration yielding the highest current.