Evolution of Nanowire Transmons and Their Quantum Coherence in Magnetic Field

TL;DR

This study explores nanowire transmons operating under magnetic fields, revealing their coherence properties, noise influences, and potential for high-field quantum computing applications.

Contribution

It demonstrates nanowire transmons' ability to operate at higher magnetic fields than standard transmons and analyzes their coherence and noise mechanisms.

Findings

Coherence up to 70 mT before superconducting gap closes

Charge noise coupling dominates qubit dephasing

Field-dependent relaxation and dephasing observed

Abstract

We present an experimental study of nanowire transmons at zero and applied in-plane magnetic field. With Josephson non-linearities provided by the nanowires, our qubits operate at higher magnetic fields than standard transmons. Nanowire transmons exhibit coherence up to 70 mT, where the induced superconducting gap in the nanowire closes. We demonstrate that on-chip charge noise coupling to the Josephson energy plays a dominant role in the qubit dephasing. This takes the form of strongly-coupled two-level systems switching on 100 ms timescales and a more weakly coupled background producing $1/f$ noise. Several observations, including the field dependence of qubit energy relaxation and dephasing, are not fully understood, inviting further experimental investigation and theory. Using nanowires with a thinner superconducting shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation.