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  • Featured in Physics
  • Open Access

Spectroscopic Measurements and Models of Energy Deposition in the Substrate of Quantum Circuits by Natural Ionizing Radiation

Joseph W. Fowler1,2,*, Paul Szypryt1,2, Raymond Bunker3, Ellen R. Edwards3, Ian Fogarty Florang2,1, Jiansong Gao1,†, Andrea Giachero2,1,4, Shannon F. Hoogerheide5, Ben Loer3 et al.

H. Pieter Mumm5, Nathan Nakamura1,2, Galen C. O’Neil1, John L. Orrell3, Elizabeth M. Scott6, Jason Stevens1,2, Daniel S. Swetz1, Brent A. VanDevender3, Michael Vissers1, and Joel N. Ullom1,2

  • *Contact author: joe.fowler@nist.gov
  • †Currently at Amazon Web Services Center for Quantum Computing, Pasadena, California, USA
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PRX Quantum 5, 040323 – Published 12 November, 2024

DOI: https://doi.org/10.1103/PRXQuantum.5.040323

Abstract

Naturally occurring background radiation is a potential source of correlated decoherence events in superconducting qubits that will challenge error-correction schemes. In order to characterize the radiation environment in an unshielded laboratory representative of superconducting qubits’ environments, we performed broadband, spectroscopic measurements of background radiation events inside a millikelvin refrigerator. The spectrometer was designed to mimic the size and composition of a quantum circuit. Specifically, we measured the background radiation spectra in silicon substrates of two thicknesses, 500 and 1500 µm, and one area, 25 mm2. The observed spectra span energies from a few kilo-electron-volts up to nearly 10 MeV, are nearly featureless, and decrease in intensity by a factor of 40 000 between 100 keV and 3 MeV for the 500-µm substrate. We integrate the spectra to obtain the average event rates and deposited power levels. These quantities correspond to a rate of 0.023 events per second and a power of 4.9 keV s−1, when counting events that deposit at least 40 keV for the 500-µm-thick substrate. We find that the cryogenic measurements are in good agreement with predictions based on simple measurements of the terrestrial gamma-ray flux outside the refrigerator, published models of cosmic-ray fluxes, a crude model of the cryostat, and radiation-transport simulations. This model requires no free parameters to predict the background radiation spectra in the silicon substrates. The agreement between measurements and predictions demonstrates that the model we present can be used to assess the relative contributions of terrestrial and cosmic-ray sources to background radiation interactions in silicon substrates of varying thickness. These spectroscopic measurements are performed with a novel combination of superconducting microresonators located on micromachined silicon islands that define the interaction volume with background radiation. The resonators transduce deposited energy to a readily detectable electrical signal. Microresonator readout closely resembles dispersive superconducting qubit readout, so similar devices—with or without micromachined islands—are suitable for integration with superconducting quantum circuits as detectors for background radiation events. For our specific laboratory conditions, we find that gamma-ray emissions from radioisotopes are responsible for the majority of events that deposit E<1MeV. We present results demonstrating that the background radiation spectrum contains relevant contributions from cosmic-ray particles other than muons, particularly a tail of multi-mega-electron-volt events due to protons and neutrons. These observations suggest several paths to reducing the impact of background radiation on quantum circuits, supported by an empirically validated model for generating reliable predictions of radiation interactions with silicon substrates.

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Quantifying the Background Radiation Hitting Superconducting Qubits

Published 12 November, 2024

Researchers have characterized the naturally occurring background radiation hitting a typical quantum circuit—a result that might help with the engineering of devices that are less vulnerable to radiation-induced decoherence.

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Qubits, the fundamental physical unit of quantum information, promise extraordinary computational power, but they are extraordinarily fragile. Rare but catastrophic “decoherence events” are known to happen without warning and can ruin hours-long calculations. Naturally occurring background radiation has long been suspected as the primary culprit. Such backgrounds consist of both gamma rays emitted by concrete and other building materials and cosmic rays, energetic particles from space. With a new type of superconducting energy detector, researchers have been able to directly measure the radiation spectrum generated in a qubitlike circuit by natural background radiation.

The new sensor is a thermal kinetic inductance detector, or TKID. It uses an unusual property of superconducting materials (in this case, titanium and titanium nitride) to monitor the temperature of a much thicker centimeter-scale silicon “island.” This thin-film superconductor on an isolated island makes TKIDs similar in size, composition, and temperature to qubit circuits. The TKIDs were operated for many days at a time, collecting tens of thousands of records of disruptive events. The spectrum of energies and the event rates they observed were consistent with the known terrestrial and cosmic sources of radiation.

This confirmation will help quantum information scientists decide on productive strategies to shield their circuits. We plan further development of TKID technology as a sensitive detector for charged particles that will be useful for many nuclear physics measurements. At the same time, we hope that the sensor design can be adapted to reside alongside qubits on the same silicon substrate, where TKIDs might serve as reliable indicators of when the delicate linkage in a quantum computation has been compromised.

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PRX Quantum 5, 040323– Published 12 November, 2024
  • Received 16 April 2024
  • Revised 9 August 2024
  • Accepted 1 October 2024
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