# Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency

## TL;DR

This study demonstrates that a fixed-frequency concentric transmon qubit maintains quantum coherence up to 40 mT magnetic fields, with minimal dephasing impact, enabling potential applications in quantum sensing and hybrid systems.

## Contribution

It provides the first measurements of transmon qubit coherence in high magnetic fields without optimized design, and introduces an analytic formula for field-dependent qubit energies considering fabrication imperfections.

## Key findings

- Quantum coherence persists up to 40 mT magnetic field.
- Dephasing rate remains unaffected across a broad frequency range.
- Analytic model for field-dependent qubit energies considering second junction effects.

## Abstract

We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field compatibility. We demonstrate quantum coherence of the qubit up to field values of $B={40}\,\mathrm{mT}$, even without an optimized chip design or material combination of the qubit. The dephasing rate $\Gamma_\varphi$ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.