Spin Environment of a Superconducting Qubit in High Magnetic Fields

TL;DR

This paper demonstrates a superconducting qubit that maintains coherence in high magnetic fields, revealing spin interactions and surface spin dynamics, and enabling hybrid quantum systems with spin environments.

Contribution

It introduces a resilient fluxonium qubit capable of operating beyond one Tesla, and explores spin-related loss mechanisms and surface spin behavior under high magnetic fields.

Findings

Identification of a paramagnetic spin-1/2 ensemble as a dominant loss mechanism.

Observation of suppressed flux noise indicating surface spin freezing.

Hyperpolarization of environmental TLSs unaffected by magnetic fields.

Abstract

Superconducting qubits equipped with quantum non-demolition readout and active feedback can be used as information engines to probe and manipulate microscopic degrees of freedom, whether intentionally designed or naturally occurring in their environment. In the case of spin systems, the required magnetic field bias presents a challenge for superconductors and Josephson junctions. Here we demonstrate a granular aluminum nanojunction fluxonium qubit (gralmonium) with spectrum and coherence resilient to fields beyond one Tesla. Sweeping the field reveals a paramagnetic spin-1/2 ensemble, which is the dominant gralmonium loss mechanism when the electron spin resonance matches the qubit. We also observe a suppression of MHz range fast flux noise in magnetic field, suggesting the freezing of surface spins. Using an active state stabilization sequence, the qubit hyperpolarizes long-lived two-level systems (TLSs) in its environment, previously speculated to be spins. Surprisingly, the coupling to these TLSs is unaffected by magnetic fields, leaving the question of their origin open. The robust operation of gralmoniums in Tesla fields offers new opportunities to explore unresolved questions in spin environment dynamics and facilitates hybrid architectures linking superconducting qubits with spin systems.