Optimized Cooper pair pumps

Abstract

In adiabatic Cooper pair pumps, which are operated by means of gate voltage modulation only, the quantization of the pumped charge during a cycle is limited due to the quantum coherence of the macroscopic superconducting wave function. In this work, we show that it is possible to obtain very accurate pumps in the nonadiabatic regime by a suitable choice of the shape of the gate voltage pulses. We determine the shape of these pulses by applying quantum optimal control theory to this problem. In the optimal case, the error, with respect to the quantized value, can be of the order of 10−6e, i.e., reduced by up to 5 orders of magnitude with respect to the adiabatic pumping. In order to test the experimental feasibility of this approach, we consider the effect of charge noise and the deformations of the optimal pulse shapes on the accuracy of the pump. Charge noise is assumed to be induced by random background charges in the substrate, which is responsible for the observed 1/f noise. Inaccuracies in the pulse shaping are described by assuming a finite bandwidth for the pulse generator. In realistic cases, the error increases to 1 order of magnitude at most as compared to that of the optimal case. Our results are promising for the realization of accurate and fast superconducting pumps.

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