Operating in a deep underground facility improves the locking of gradiometric fluxonium qubits at the sweet spots

TL;DR

This paper demonstrates that operating a gradiometric fluxonium qubit in a deep underground facility significantly enhances fluxon lifetime stability, reducing sensitivity to environmental disturbances and ionizing events.

Contribution

It introduces a method for flux-bias locking of gradiometric fluxonium using symmetric grAl loops and shows that underground operation extends fluxon lifetimes.

Findings

Flux-bias locking achieved at 0 or Φ₀/2 flux bias

Fluxon lifetime increases from hours to days underground

Suppression of magnetic field sensitivity by two orders of magnitude

Abstract

We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at $0$ or $\Phi_0/2$ effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding $100 \,\Phi_0$. However, even for states biased in the vicinity of $1 \,\Phi_0$, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.