Engineering superconducting qubits to reduce quasiparticles and charge noise

TL;DR

This paper demonstrates methods to reduce quasiparticle-induced decoherence in superconducting qubits by device downsizing, shielding, and trapping, leading to record low charge-parity switching rates and improved stability.

Contribution

It introduces a scalable approach combining device design and electromagnetic environment shaping to mitigate quasiparticles in superconducting qubits.

Findings

Record low charge-parity switching rate (<1Hz) achieved.

Quasiparticle generation mainly caused by photon absorption at the junction.

Enhanced device stability against discrete charging events.

Abstract

Identifying, quantifying, and suppressing decoherence mechanisms in qubits are important steps towards the goal of engineering a quantum computer or simulator. Superconducting circuits offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs). Developing a quasiparticle mitigation strategy compatible with scalable, high-coherence devices is therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. Using a flip-chip design, we shape the electromagnetic environment of the qubit above the superconducting gap, inhibiting quasiparticle poisoning. Our findings support the hypothesis that quasiparticle generation is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption by the antenna-like qubit structure. We achieve record low charge-parity switching rate (<1Hz). Our aluminium devices also display improved stability with respect to discrete charging events.