How to Prevent Stray Radiation from Disrupting Superconducting Qubits
It’s crucial to eliminate low-level radioactive sources from laboratories that experiment with superconducting qubits, as highlighted by two recent studies. Enhancing the coherence times of quantum devices is essential for moving towards practical quantum computing.
Two related articles in the journals PRX Quantum and Journal of Instrumentation detail the most significant sources of ionizing radiation that interfere with superconducting quantum computers and how to mitigate their effects. These insights pave the way for a deeper investigation into radiation-induced errors in protected underground research facilities.
A research team led by physicists from the Department of Energy’s Pacific Northwest National Laboratory, in collaboration with experts from MIT’s Lincoln Laboratory and the National Institute of Standards and Technology among other academic institutions, has published findings that aim to help the quantum computing field prepare for advancements in qubit technology.
Electronic Disturbances Complicate the Longevity of Qubits
“We are approaching a time when improvements in design and materials will lead to qubits that are stable enough, making stray radiation from the environment the primary obstacle to maintaining quantum coherence,” stated physicist Brent VanDevender, one of the lead researchers. VanDevender was among the first to recognize natural ionizing radiation as a destabilizing factor for operational qubits, the fundamental units of quantum computers.
Even minimal disruptions can induce errors that cause superconducting qubits to lose their quantum state, a phenomenon known as decoherence. The team discovered that cosmic radiation and naturally occurring isotopes that emit low-level ionizing radiation from common materials contribute equally to decoherence.
“After determining the impact of ionizing radiation on superconducting qubits, it became clear we needed to meticulously and quantitatively recognize radiation sources in our environment,” explained lead experimental physicist Ben Loer. “Our expertise in measuring ultra-low radiation levels in the lab led us to evaluate radiation sources within the experimental setups themselves, particularly cryostats where qubits are analyzed.”
“We discovered that many electrical connectors were heavily contaminated, making them sources of radiation,” VanDevender added.
Enhancing Conditions Underground
The combined findings of the two studies suggest effective strategies for protecting sensitive experimental equipment from radiation exposure.
In their publication in the Journal of Instrumentation, the research team describes measures implemented to significantly minimize atmospheric and isotope radiation exposure at a shielded underground qubit testing facility located on the Richland, Washington campus of PNNL, referred to as the Low Background Cryogenic Facility. This facility, situated within an ultra-clean underground lab, includes a cryostat, also known as a dilution refrigerator, designed to cool superconducting qubit devices to nearly absolute zero, which is vital for stabilizing these quantum computing devices. The research team reports that this lead-shielded cryostat could lower the error rate by a factor of 20 compared to the rates observed in standard unshielded above-ground facilities.
Additionally, the researchers note that implementing straightforward practices such as removing natural radiation sources from materials within the dilution refrigerator can significantly enhance the viability of quantum computing devices. These sources consist of metal isotopes—naturally occurring forms of elements that emit radiation in the form of alpha, beta, and gamma rays—which can disrupt quantum devices.
During their investigation into these radiation sources in the lab, they utilized specialized ultra-sensitive detection techniques to pinpoint contaminants in silicon, copper, and ceramic electronic elements like circuit boards and data-collecting cables, as well as the qubits themselves. To mitigate the influence of these devices, the team recommends replacing beryllium-copper alloys typically used in cables with materials like brass. Future research objectives include assessing the effectiveness of “radiation-hardened” qubits that are less affected by radiation and studying low-background materials.
Applying Insights from Sensitive Detection Technology
In the accompanying study published in PRX Quantum, the researchers directly measured ionizing radiation interactions on a superconducting sensor situated inside a cryocooler, a refrigerator that achieves ultra-cold cryogenic temperatures. They employed simple radiation detection circuits printed on silicon, similar to that used for qubits. In this study, they demonstrated that stray radiation interacting with a silicon circuit board—potentially causing decoherence in a qubit or other detrimental effects on circuit performance—matched closely with predicted rates and energy spectra.
The research team drew upon experience gained from designing and constructing double beta decay detectors, neutrino detectors, and dark matter detectors, which are similarly sensitive to low radiation levels. They identified two complementary methods to reduce the sensitivity of superconducting elements to stray radiation: isolating superconducting components on crystal “islands” and making the crystal substrate thinner.
“We’ve identified the key radiation sources, and we are eager to test how new devices will perform within our low-background facility,” commented Loer.
This research was funded by the Department of Energy’s Office of Science through its Nuclear Physics program’s Quantum Horizons initiative, as well as an Early Career Award to Ben Loer, along with the High Energy Physics program’s Quantum Information Science Enabled Discovery (QuantISED) program and internal investments from PNNL.
