Cooper-pair transistor as a minimal topological quantum circuit

TL;DR

This paper demonstrates that a simple Cooper-pair transistor can host a nontrivial topological phase characterized by a Chern number, leading to robust quantized current responses suitable for topological quantum computing.

Contribution

It reveals a previously unrecognized topological property of the Cooper-pair transistor, expanding the potential for minimal topological quantum circuits.

Findings

The Cooper-pair transistor exhibits a nontrivial Chern number.

The quantized current response is robust against charge noise and quasiparticle poisoning.

All bands have the same Chern number due to band crossings with higher topological charges.

Abstract

The outlook of protected quantum computing spurred enormous progress in the search for topological materials, sustaining a continued race to find the most experimentally feasible platform. Here, we show that one of the simplest quantum circuits, the Cooper-pair transistor, exhibits a nontrivial Chern number which has not yet been discussed, in spite of the exhaustive existing literature. Surprisingly, the resulting quantized current response is robust with respect to a large number of external perturbations, most notably low-frequency charge noise and quasiparticle poisoning. Moreover, the fact that the higher bands experience crossings with higher topological charges leads to all the bands having the same Chern number, such that there is no restriction to stay close to the ground state. Remaining small perturbations are investigated based on a generic Master equation approach. Finally, we discuss a feasible protocol to measure the quantized current.