Qubits, the fundamental unit of quantum information, form the building blocks of quantum computers. Because qubits are not limited to the two states, “0” and “1,” of an ordinary logic bit, but can simultaneously represent combinations of these states, they can carry out certain computations that would either be impossible or take millions of years to solve with even the most powerful computer in existence today. 

However, qubit circuits are extraordinarily fragile; even tiny changes in temperature can cause qubits to decohere or lose their quantum state. Superconducting qubits are often constructed from thin metal films deposited on a silicon wafer. Although stray sources of radiation from both Earth and space pass virtually unimpeded through the qubit film, they interact with the much thicker silicon base. Energy deposited in this interaction can change the state of individual qubits and destroy the delicate linkage, known as entanglement, between pairs of qubits.

Researchers at the National Institute of Standards and Technology (NIST)  have now made the first direct measurements of the damaging effects of two key sources of radiation: naturally-occurring gamma rays produced by radioactive elements in rocks and construction materials, and cosmic rays – energetic nuclei and other subatomic particles that continually rain down on Earth from space. 

Joseph Fowler and his colleagues conducted their study using silicon chips that resemble, both in size and composition, the chips used in superconducting qubit circuits. The chips were also cooled to the same ultra-low temperatures at which most qubit circuits operate.

Fowler and his colleagues employed a new type of superconducting energy-sensitive sensor, called a thermal kinetic inductance detector, or TKID, to measure both the rate at which gamma rays and cosmic rays struck the silicon and the energy they deposited. The NIST study is one of the first to use TKIDs to measure the energy of charged particles. 

The researchers placed the TKIDs on silicon wafers of two different thicknesses – 500 micrometers (millionth of a meter) and 1,500 micrometers. The measurements confirmed the accuracy of models that predict the effects of gamma rays and cosmic rays, enabling the team to assess the damaging effects of terrestrial gamma rays and cosmic rays over a wide range of energies.

The team found that the radiation environment measured in open air by large, commercial instruments exactly predicted the far lower rates of damaging events observed in their tiny, ultra-cold TKID sensors. In addition, cosmic ray particles – specifically, protons and neutrons – caused the most energetic disruptions in the silicon chips.

The study revealed that cosmic -ray particles protons and neutrons deposited the largest amounts of energy in the silicon chips. In addition, the study revealed that radiation pummeled the 1,500-micrometer thick chip more frequently and dumped two to three times more energy than in the 500-micrometer-thick chip.

Shrinking the thickness or size of the silicon could dramatically reduce disturbances in the qubit circuits, Fowler said. Thermally insulating the thin-film qubit circuit from sections of the underlying silicon chip could also help. That way, if the temperature of the chip temporarily increased due to bombardment by cosmic rays or gamma rays, less heat would be transferred to the circuit, minimizing disruptions.

Fowler and his colleagues, including researchers from the University of Colorado Boulder; the Pacific Northwest National Laboratory in Richland, Washington; and Centre College in Danville, Kentucky, reported their work online on Nov. 12 in PRX Quantum. 

Paper: J.W. Fowler, P. Szypryt, R. Bunker, E.R. Edwards, I.F. Florang, J. Gao, A. Giachero, S.F. Hoogerheide, B. Loer, H.P. Mumm, N. Nakamura, G.C. O’Neil, J.L. Orrell, E.M. Scott, J. Stevens, D.S. Swetz, B.A. VanDevender, M. Vissers, and J.N. Ullom. Spectroscopic measurements and models of energy deposition in the substrate of quantum circuits by natural ionizing radiation. PRX Quantum. Published online Nov. 12, 2024. DOI: https://doi.org/10.1103/PRXQuantum.5.040323