PNNL

Abstract

Naturally occurring background radiation is a potential source of correlated decoherence events in quantum circuits that will challenge error correction schemes. To characterize the radiation environment in an unshielded laboratory, we have performed broadband, spectroscopic measurements of background events in silicon substrates located inside a millikelvin refrigerator, an environment representative of some quantum circuits. We measure the background spectra in silicon substrates of two thicknesses, 500 µm and 1500 µm, and obtain the average event rate and the average power deposition. The values are 0.023 events per second in a 25 mm^2 area, with 4.9 keV/s deposited in the substrate. We find that the spectrum of background events is nearly featureless, and its intensity decreases by a factor of 400 000 between 100 keV and 3 MeV for silicon substrates 500 µm thick. We find the cryogenic measurements to be in good agreement with predictions based on measurements of the terrestrial gamma-ray flux outside the refrigerator, published models of cosmic ray fluxes, and calculations of radiation propagation to and subsequent interaction with the silicon targets. No free parameters are required to predict the background spectra in the silicon substrates, and the good agreement between measurements and predictions allows confident assessment of the relative contributions of terrestrial and cosmic background sources. Our spectroscopic measurements are performed with superconducting microresonators located on micromachined silicon islands that define the interaction volume with background radiation. Microresonators transduce deposited energy to a readily detectable electrical signal. Microresonator readout closely resembles dispersive qubit readout, so similar devices—with or without micromachined islands—are suitable for integration with superconducting quantum circuits as detectors for background events. These results suggest several paths to reducing the impact of background radiation on quantum circuits.