A quasiparticle-protected superconducting qubit
Abstract
Superconducting quantum circuits underpin large-scale quantum processors and are increasingly interfaced with optical systems, yet they are vulnerable to quasiparticle poisoning from high-energy radiation and optical fields. Gap engineering of Josephson junctions suppresses quasiparticle-induced relaxation, but quasiparticles near the junction can still shift the qubit frequency and cause phase errors. Here, we extend gap engineering beyond the junction using a flux-tunable transmon with a gold-on-niobium ground plane and island and strongly gap-engineered aluminum junctions designed to keep quasiparticles away from the junction leads. Compared with an all-aluminum transmon with nominally identical junctions, we demonstrate that the niobium device withstands more than an order of magnitude higher near-infrared optical power before excess relaxation appears. The Nb transmon shows no resolvable quasiparticle-induced frequency shift over the same power range. At the same time, we observe that dephasing and state-preparation-and-measurement infidelity increase with optical power even while relaxation remains protected. These results suggest that engineering the quasiparticle energy landscape beyond the junction can substantially increase qubit resilience, not only to radiation-induced error bursts, but also to nearby optical fields.
Related
- Multiple quasiparticle bound states in a local superconducting gap variation
- Characterization of Radiation-Induced Errors in Superconducting Qubits Protected with Various Gap-Engineering Strategies
- Second Chern Number Protected Gapless Vortex Modes in Superconducting Weyl Semimetals
- Topologically Protected Edge States in a Single Flux Qubit
- Dynamically protected erasure qubit via low-frequency charge driving
BibTeX
@misc{haug2026quasiparticleprotected,
title = {{A quasiparticle-protected superconducting qubit}},
author = {Trond Hjerpekjøn Haug and Johan Kolvik and Joey Frey and Paul Burger and Fabian Resare and Witlef Wieczorek and Simone Gasparinetti and Per Delsing and Raphaël Van Laer},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.12227},
eprint = {2610.12227},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.12227},
}
RIS
TY - UNPB
TI - A quasiparticle-protected superconducting qubit
AU - Trond Hjerpekjøn Haug
AU - Johan Kolvik
AU - Joey Frey
AU - Paul Burger
AU - Fabian Resare
AU - Witlef Wieczorek
AU - Simone Gasparinetti
AU - Per Delsing
AU - Raphaël Van Laer
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2610.12227
UR - https://arxiv.org/abs/2610.12227
AB - Superconducting quantum circuits underpin large-scale quantum processors and are increasingly interfaced with optical systems, yet they are vulnerable to quasiparticle poisoning from high-energy radiation and optical fields. Gap engineering of Josephson junctions suppresses quasiparticle-induced relaxation, but quasiparticles near the junction can still shift the qubit frequency and cause phase errors. Here, we extend gap engineering beyond the junction using a flux-tunable transmon with a gold-on-niobium ground plane and island and strongly gap-engineered aluminum junctions designed to keep quasiparticles away from the junction leads. Compared with an all-aluminum transmon with nominally identical junctions, we demonstrate that the niobium device withstands more than an order of magnitude higher near-infrared optical power before excess relaxation appears. The Nb transmon shows no resolvable quasiparticle-induced frequency shift over the same power range. At the same time, we observe that dephasing and state-preparation-and-measurement infidelity increase with optical power even while relaxation remains protected. These results suggest that engineering the quasiparticle energy landscape beyond the junction can substantially increase qubit resilience, not only to radiation-induced error bursts, but also to nearby optical fields.
ER -