# Measurement of correlated charge noise in superconducting qubits at an underground facility

## TL;DR

Researchers measured how ionizing radiation affects charge noise in superconducting qubits using an underground facility with reduced cosmic rays.

## Contribution

They achieved a record low rate of correlated charge jumps in a four-qubit system using a low-radiation underground setup.

## Key findings

- Charge jumps in qubits correlated with ionizing radiation flux were measured.
- Lead shielding reduced charge jump rates significantly compared to surface-level measurements.
- Four qubits operated for over 22 hours with minimal correlated charge jumps beyond 3 mm separation.

## Abstract

The charge environment of superconducting qubits may be studied through the introduction of controlled, quantified amounts of ionizing radiation. 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 4π 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 \documentclass[12pt]{minimal}
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\begin{document}$$0.1{9}_{-0.03}^{+0.04}$$\end{document}0.19−0.03+0.04 mHz, almost an order of magnitude lower than that measured in surface tests, but a factor of roughly seven 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.

Ionizing radiation can cause simultaneous charge noise in multi-qubit superconducting devices. Here, the authors measure space- and time-correlated charge jumps in a four-qubit system in a low-radiation underground facility, achieving operation with minimal correlated events over 22 h at qubit separations beyond 3 mm.