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

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, cryogenicfacility designed for the characterization of low-threshold particle detectors.The rock overburden of this facility reduces the cosmic ray muon flux by over99movable lead shield, this facility enables quantifiable control over the fluxof ionizing radiation on the qubit device. Long-time-series charge tomographymeasurements on these weakly charge-sensitive qubits capture discontinuousjumps in the induced charge on the qubit islands, corresponding to theinteraction of ionizing radiation with the qubit substrate. The rate of thesecharge jumps scales with the flux of ionizing radiation on the qubit package,as characterized by a series of independent measurements on anotherenergy-resolving detector operating simultaneously in the same cryostat withthe qubits. Using lead shielding, we achieve a minimum charge jump rate of0.19^+0.04_-0.03 mHz, almost an order of magnitude lower than thatmeasured in surface tests, but a factor of roughly eight higher than expectedbased on reduction of ambient gammas alone. We operate four qubits for over 22consecutive hours with zero correlated charge jumps at length scales abovethree millimeters.