Flux-noise-resilient transmon qubit via a doubly-connected gradiometric design

TL;DR

This paper introduces a doubly-connected gradiometric transmon qubit design that significantly enhances coherence times and stability by reducing flux noise sensitivity, without sacrificing tunability or adding measurement complexity.

Contribution

The paper presents a novel gradiometric transmon design with a nano-airbridge that improves coherence and stability, demonstrating practical flux-noise suppression in superconducting qubits.

Findings

Nearly threefold increase in Ramsey coherence time $T_2^*$

Energy relaxation times $T_1$ comparable to reference qubits

Enhanced long-term frequency stability without magnetic shielding

Abstract

Frequency-tunable superconducting transmon qubits are a cornerstone of scalable quantum processors, yet their performance is often degraded by sensitivity to low-frequency flux noise. Here we present a doubly-connected gradiometric transmon (the ``8-mon") that incorporates a nano-airbridge to link its two loops. This design preserves full electrical tunability and remains fully compatible with standard X-mon control and readout, requiring no additional measurement overhead. The airbridge interconnect eliminates dielectric loss, which enables the 8-mon to achieve both energy relaxation times $T_{\rm 1}$ comparable to reference X-mons and, in the small flux-bias regime, a nearly threefold enhancement in Ramsey coherence time $T_{\rm 2}^*$. This improved $T_{\rm 2}^*$ reaches the same order as $T_{\rm 1}$ without employing echo decoupling. The device also exhibits superior long-term frequency stability even without any magnetic field shielding. We develop a spatially correlated flux-noise model whose simulations quantitatively reproduce the experimental coherence trends, revealing the coexistence of short- and long-correlation-length magnetic noise in the superconducting chip environment. By unifying high tunability with intrinsic flux-noise suppression through a robust geometric design, the 8-mon provides a practical pathway toward more coherent and stable superconducting quantum processors.