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arXiv:2109.13758 · IEEE Transactions on Applied Superconductivity

Flux Pumping of Cooper Pairs Through a Josephson Energy-Suppression Pump

Angelo Greco, Luca Fasolo, Vito Marino, Emanuele Enrico

Published Dec 14, 2021• 1 citation
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Abstract

In this paper, we propose a novel kind of Josephson Energy Suppression Pump (JESP) controlled by a fully magnetic flux drive. The device presented here is composed of two superconducting loops interrupted at one side by superconducting nanowires which are joined together by a superconducting island. The phase difference developed at the edges of the nanowires by means of the magnetic flux threading the loops can collapse their Cooper condensates, leading to complete suppression of the Josephson energies. This mechanism allows to greatly reduce the leakage current when performing Cooper pair pumping by a pure magnetic pumping cycle without involving any gate modulation. The pumping capability of the JESP is studied through a master equation approach in the non-adiabatic case.

DOI
10.1109/tasc.2021.3135231

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In this paper, we propose a novel kind of Josephson Energy Suppression Pump (JESP) controlled by a fully magnetic flux drive. The device presented here is composed of two superconducting loops interrupted at one side by superconducting nanowires which are joined together by a superconducting island. The phase difference developed at the edges of the nanowires by means of the magnetic flux threading the loops can collapse their Cooper condensates, leading to complete suppression of the Josephson energies. This mechanism allows to greatly reduce the leakage current when performing Cooper pair pumping by a pure magnetic pumping cycle without involving any gate modulation. The pumping capability of the JESP is studied through a master equation approach in the non-adiabatic case.
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We introduce and discuss a scheme for Cooper-pair pumping. The scheme relies on the coherent transfer of a superposition of charge states across a superconducting island and is realized by adiabatic manipulation of magnetic fluxes. Differently from previous implementations, it does not require any modulation of electrostatic potentials. We find a peculiar dependence of the pumped charge on the superconducting phase bias across the pump and that an arbitrarily large amount of charge can be pumped in a single cycle when the phase bias is $\ensuremath{\pi}$. We explain these features and their relation to the adiabatic theorem.
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We have carried out first measurements on a novel type of a charge pump nicknamed the Cooper pair "sluice". The device is an adiabatic flux-assisted Cooper pair pump consisting of a mesoscopic superconducting island and of two SQUID loops. The operation is based on the periodic manipulation of a single voltage gate and two input coils controlling the magnetic flux through the SQUID loops. Based on theoretical estimates the device is capable of producing as high as 0.1 nA currents at better than 10-accuracy and looks thus like a promising candidate for the current source in completing the quantum measurement triangle. In spite of several nonidealities in our preliminary measurement setup, we have succeeded in verifying the pumping behavior of the sluice. The pumped currents were quite high, several hundred pA, but the accuracy needs to be increased. The results obtained so far suggest various ways of improving the measurement setup
We describe a protocol for the universal control of non-ideal πππ-periodic superconducting qubits. Our proposal relies on a πππ-SQUID: a superconducting loop formed by two πππ-periodic circuit elements, with an external magnetic flux threading the circuit. The system exhibits an extensive sweet spot around half-flux where residual 2π2π2π-periodic Cooper pair tunneling is highly suppressed. We demonstrate that universal single-qubit operations can be realised by tuning the flux adiabatically and diabatically within this broad sweet spot. We also assess how residual 2π2π2π-periodicity in πππ-SQUIDs impacts holonomic phase gates.
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