Superconducting Qubits Above 20 GHz Operating over 200 mK

TL;DR

This paper demonstrates superconducting qubits operating above 20 GHz at temperatures up to 250 mK, showing resilience to quasiparticle decoherence and thermal noise, which could facilitate scalable quantum computing and hybrid experiments.

Contribution

The authors develop niobium trilayer junction transmons with frequencies up to 24 GHz that maintain coherence at higher temperatures than previously possible.

Findings

Decoherence times of about 1 microsecond at high frequencies

Qubits operate effectively up to 250 mK without quasiparticle-induced relaxation

High-frequency qubits show potential for scalable quantum architectures

Abstract

Current state-of-the-art superconducting microwave qubits are cooled to extremely low temperatures to avoid sources of decoherence. Higher qubit operating temperatures would significantly increase the cooling power available, which is desirable for scaling up the number of qubits in quantum computing architectures and integrating qubits in experiments requiring increased heat dissipation. To operate superconducting qubits at higher temperatures, it is necessary to address both quasiparticle decoherence (which becomes significant for aluminum junctions above 160 mK) and dephasing from thermal microwave photons (which are problematic above 50 mK). Using low-loss niobium trilayer junctions, which have reduced sensitivity to quasiparticles due to niobium's higher superconducting transition temperature, we fabricate transmons with higher frequencies than previously studied, up to 24 GHz. We measure decoherence and dephasing times of about 1 us, corresponding to average qubit quality factors of approximately $10^5$, and find that decoherence is unaffected by quasiparticles up to 1 K. Without relaxation from quasiparticles, we are able to explore dephasing from purely thermal sources, finding that our qubits can operate up to approximately 250 mK while maintaining similar performance. The thermal resilience of these qubits creates new options for scaling up quantum processors, enables hybrid quantum experiments with high heat dissipation budgets, and introduces a material platform for even higher-frequency qubits.