First Measurement of Correlated Charge Noise in Superconducting Qubits at an Underground Facility

TL;DR

This study measures space- and time-correlated charge noise in superconducting qubits located deep underground, demonstrating significantly reduced charge jumps due to minimized ionizing radiation, and providing insights into environmental noise effects on quantum devices.

Contribution

First measurement of correlated charge noise in superconducting qubits in an underground, low-radiation environment, showing reduced charge jumps and environmental influence on qubit stability.

Findings

Charge jump rate reduced by nearly an order of magnitude underground.

Correlated charge jumps are influenced by ionizing radiation flux.

Over 22 hours of stable operation with no correlated jumps at >3mm scale.

Abstract

We measure space- and time-correlated charge jumps on a four-qubit device, operating 107 meters below the Earth's surface in a low-radiation, cryogenic facility designed for the characterization of low-threshold particle detectors. The rock overburden of this facility reduces the cosmic ray muon flux by over 99% compared to laboratories at sea level. Combined with 4$\pi$ coverage of a movable lead shield, this facility enables quantifiable control over the flux of ionizing radiation on the qubit device. Long-time-series charge tomography measurements on these weakly charge-sensitive qubits capture discontinuous jumps in the induced charge on the qubit islands, corresponding to the interaction of ionizing radiation with the qubit substrate. The rate of these charge jumps scales with the flux of ionizing radiation on the qubit package, as characterized by a series of independent measurements on another energy-resolving detector operating simultaneously in the same cryostat with the qubits. Using lead shielding, we achieve a minimum charge jump rate of 0.19$^{+0.04}_{-0.03}$ mHz, almost an order of magnitude lower than that measured in surface tests, but a factor of roughly eight higher than expected based on reduction of ambient gammas alone. We operate four qubits for over 22 consecutive hours with zero correlated charge jumps at length scales above three millimeters.