Scientists head underground to measure effects of gamma rays on superconducting qubits

Beneath Earth’s surface, shielded from the effects of most cosmic rays, is the Northwestern Experimental Underground Site, or NEXUS. Located about 350 feet underground at Fermi National Accelerator Laboratory, the research facility enables scientists to study the behavior of quantum devices in their quest to find evidence of dark matter.

It’s here that a multi-institutional team of scientists took measurements of correlated charge noise in a chip comprised of multiple superconducting qubits for the first time.

As reported in Nature Communications, their work will help inform the design of future quantum-based particle physics detectors, as well as develop noise reduction strategies to reduce qubit errors and decoherence.

“Understanding whether a charge burst could affect multiple qubits as the charge moves through the chip … is crucial to scientists who use quantum sensors …”

Superconducting qubits are a leading option for building quantum computers. However, they are sensitive to disturbances from their environment and can make errors. By understanding how electrical fluctuations called charge noise affect superconducting qubits, scientists can find ways to reduce these errors and improve quantum computers.

When an ionizing particle, like a cosmic ray or gamma ray passes through such a chip, it can create bursts of charge that can impact information stored in qubits. Scientists can directly measure these events because the qubits used in the study are incredibly sensitive to fluctuations in charge.

“Understanding whether a charge burst could affect multiple qubits as the charge moves through the chip — what researchers call correlated charge noise — is crucial to scientists who use quantum sensors to detect very faint signals that are possibly from dark matter, and to computer scientists, who are interested in correcting errors,” said Daniel Bowring, a scientist at Fermilab and organizer of this study.

This research is an extension of that conducted in 2019 by collaborators at the University of Wisconsin-Madison. In that study, scientists measured correlated charge noise on the Earth’s surface using the same chip comprised of four superconducting qubits. Among their findings were signals detected from both cosmic rays and gamma rays.

In the more recent study at Fermilab, scientists used the very same four-qubit chip and placed it in the underground NEXUS lab to block most cosmic rays. With a lead shield surrounding the dilution refrigerator that houses the qubit chip, they took measurements with the shield both open and closed to isolate and compare the effects of gamma radiation.

“Qubits are sensitive to different types of faint signals. If we want to use them as particle detectors, we need to be sure we can tell these signals apart from each other.”

“Qubits are sensitive to different types of faint signals. If we want to use them as particle detectors, we need to be sure we can tell these signals apart from each other,” said Bowring.

More specifically, they wanted to see what effect making things as quiet as possible would have on the rate of charge bursts on qubits and whether these effects were correlated in multiple qubits.

When comparing measurements, scientists expected to find a marked decrease in charge bursts with the shield closed. They did find a reduction, though overall less than expected. Interestingly, they found that, even with the shield closed, some correlated charge noise was present, indicating the presence of some other source of background interference.

“These comparisons will lead to new designs where we purposely engineer the amount of response to the environment,” said study coauthor Enectali Figueroa-Feliciano, a professor of physics and astronomy at Northwestern University. “Having that control will lead to quantum devices optimized to minimize their environmental response for use in quantum computing applications. It will also allow us to maximize it for quantum sensing applications.”

The study was conducted with support from the Quantum Science Center. Collaborating institutions include: Fermilab, Illinois Institute of Technology, Northwestern University, SLAC National Accelerator Laboratory, Stanford University, Tufts University, University of Wisconsin-Madison, University of Florida in Gainesville, Université Grenoble, University of Toronto, Université Paris-Saclay, and Wellesley College.

An accelerator technology test facility with a catchy name has achieved a major milestone at the U.S. Department of Energy’s Fermi National Accelerator Laboratory by successfully accelerating its first proton beams. The achievement marks an important step forward that will support research aimed at pushing the boundaries of stable, high-intensity beams in future particle accelerator programs.

“This marks a major advancement for our R&D program.”

“This marks a major advancement for our R&D program,” said Jonathan Jarvis, director of Fermilab’s Accelerator Research Division. “This new proton-beam capability lets us address the challenges we’ll face as we increase the beam power in Fermilab’s accelerators.”

The dedicated research and development accelerator complex is called FAST/IOTA — shorthand for Fermilab’s Accelerator Science and Technology facility and its Integrable Optics Test Accelerator ring. Particle accelerators empower scientists to study the universe at its smallest scales, and FAST/IOTA helps ensure the United States will continue to hold a preeminent position in high-energy particle physics.

FAST has two linear accelerators that are connected to the IOTA storage ring, one for electrons and now one for protons. The IOTA ring is equipped with unique magnets and other advanced technologies that enable researchers to study new concepts for cutting-edge accelerator systems and high-intensity beam physics.

Compared to Fermilab’s main accelerator complex, a large, well-established workhorse delivering high-energy protons for physics experiments, FAST/IOTA is designed to evaluate technologies that could improve the efficiency, reliability and performance of future particle accelerators.

“FAST/IOTA is a dedicated R&D accelerator complex that provides us the freedom to explore high-risk, high-reward ideas,” explained Jarvis. “These ideas could dramatically improve the way we design, build and operate particle accelerators.”

In accelerator physics, intensity refers to the number of particles packed into a beam and accelerated through the machine. With protons now circling the ring at around 7% of the speed of light, researchers can further investigate methods for mitigating beam instabilities, develop advanced control systems and discover ways to apply artificial intelligence to accelerator operations.

Fermilab is in the process of upgrading its high-power, proton-accelerator complex through the PIP-II project. This upgrade will enable higher-intensity beams for the lab’s neutrino science program, beginning with the Deep Underground Neutrino Experiment. FAST/IOTA will enhance this effort by providing advanced tools and understanding for navigating the challenges of such high-intensity operations.

By figuring out how to increase the number of particles in an accelerator beam and reduce beam losses, researchers can steer more particles toward a target. When more particles hit the target, they generate more secondary particles such as neutrinos. FAST/IOTA’s flexible, modular design makes it especially suited for developing new methods to increase and maintain beam intensity.

“Researchers can conduct tests and experiments without tying up the operations of a larger production facility,” said Trey Thompson, an engineering physicist on the FAST/IOTA team. “We can shut down, swap out part of the ring and install new experiments without the usual constraints.”

Before this upgrade, FAST/IOTA primarily worked with electron beams. For example, in 2022, Fermilab researchers published a landmark paper in Nature, reporting the first experimental demonstration of optical stochastic cooling — a breakthrough technique that cools a particle beam using its own emitted light. The team also conducted studies on single electrons, using the facility’s unique capabilities to track individual particles for hours at a time.

“Electrons were a perfect starting point and complement to our proton work,” said Jarvis. “They’re easy to work with, and they help us explore the advanced concepts and technologies that we are developing for our core proton program.”

With this upgrade, FAST/IOTA is also helping to pioneer the use of artificial intelligence in accelerator design and operations.

“About half of our program now focuses on enhancing accelerators with AI,” Jarvis said. “We’re building high-fidelity simulation environments that capture the complexity of the real machine and are fully integrated with its control systems. These virtual accelerators allow us to train AI systems to optimize performance and discover new configurations. The key is that we can then test those simulations in a real machine, at scale.”

AI is not only a research focus but also a practical tool in deploying the new proton systems.

“We used AI tools to help optimize the output of the proton source,” said Fermilab engineering physicist Chip Edstrom. “We’ve applied algorithms that treat the system as a black box — meaning we don’t need to understand every internal detail — and instead optimize parameters based on performance.”

This approach reflects Fermilab’s broader core value of ingenuity, which also makes FAST/IOTA an ideal environment for accelerator workforce development. Students and postdoctoral researchers gain “full-stack” experience, from design and construction to operation and data analysis for particle accelerators. At any given time, the program hosts multiple graduate students and postdoctoral researchers, exposing them to cutting-edge R&D spanning physics, engineering and AI.

“There aren’t many places where you can get hands-on experience with a facility of this scale,” Jarvis said. “It’s an incredible opportunity to train the next generation of accelerator scientists and technologists.”

And the impact of FAST/IOTA will extend well beyond Fermilab. “This is the realization of a vision that began over a decade ago,” Jarvis added. “We’ve built a uniquely capable facility that will help guide the operation and design of next-generation accelerators — from Fermilab’s own PIP-II project to machines around the world.”

Fermilab is a highly technical environment to work in. Can you explain the work you do at the lab?

Sounds like a very specialized focus! How did your career bring you to Fermilab? 

I have been at Fermilab for three years. My background is in cryogenics across various applications — from small cryocooler systems for space-based superconducting magnets to working at CERN on liquid argon detectors. I came to Fermilab to focus on large-scale cryogenic systems, particularly industrial liquid helium systems.

These cryogenic systems seem very complex. What’s the most challenging part of your work?

At IB1, we operate two large cryogenic liquefiers that produce liquid helium 24 hours a day for testing superconducting magnets and cavities. IB1 is unique because we operate both an old, manual liquefier and a newer, automated one. Making these two systems work together is a constant challenge. We consume all the liquid helium we produce, so efficiency and leak prevention are critical. The helium cycle is complex: we compress gaseous helium, liquefy it, store it in dewars, distribute it for tests, and then recover the boiled-off gas to start the cycle again.

What do you find to be the most rewarding aspect of working at Fermilab?

There are very few places in the world like Fermilab. Being surrounded by experts from so many different fields is incredibly enriching. It’s a privilege to work in such a collaborative and technically advanced environment.

Besides working with large cryogenic systems, what else do you enjoy doing?

I love kayaking! Illinois is a great place for it, with access to beautiful rivers and Lake Michigan. It’s a seasonal activity, so I try to squeeze in time to kayak two times a week.

Scientists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and several collaborating institutions are using a new type of quantum sensor called a superconducting microwire single-photon detector — or SMSPD — to improve particle detection efficiency and timing, characteristics essential for future accelerator-based experiments and dark matter detection experiments.  

The research is led by Fermilab, and collaborators include Caltech, NASA’s Jet Propulsion Laboratory and the University of Geneva.

This study builds on previous research conducted at Fermilab, which determined SMSPD sensors could efficiently detect individual high-energy charged particles like protons, electrons and pions.

The new study, conducted at CERN, takes the development of these high-efficiency sensors one step further by demonstrating improved particle detection efficiency and time resolution using sensors made from a thicker tungsten silicide film than that used previously. The thicker the wire, the better its ability to absorb energy from charged high-energy particles.

“This research is significant because it shows improvement from our initial measurements using SMSPDs for charged particle detection,” said Cristián Peña, a scientist at Fermilab who led the study.

“In addition, for the first time, we used SMSPDs to measure the detection efficiency of muons, potentially expanding their use for new avenues of exploration,” said Peña.

An international collaboration is investigating the feasibility of using muons in a future high-energy muon collider. Because of their unique properties and behaviors, scientists use these particles — 200 times heavier than electrons — to explore fundamental forces and particles. Future particle physics experiments will require more powerful, more intense colliders that produce millions of events per second. Within these events, detectors must be able to detect and track individual particles in both space and time with increasing precision. SMSPD sensors show great potential for this.

Compared to superconducting nanowire single photon detectors, or SNSPDs, the larger active area afforded by SMSPDs enables greater opportunity to track charged particles. This makes them ideal, not only for future accelerator-based experiments but for also seeking dark matter, and exploration of this new technology continues at a rapid pace.

In a separate study recently published in the Journal of Instrumentation, some of the same scientists involved in the research above conducted the first detailed temperature-dependent study of an SMSPD sensor array to use in low-background dark matter detection experiments.

“We are continuing to make strides in developing these sensors with greater precision and greater efficiency to meet the needs of next-generation particle accelerators,” said Si Xie, a scientist at Fermilab and joint appointee at Caltech. “We still have a lot of work to do, but this research shows we are progressing very well. We are excited to continue studying and improving these devices so they can help facilitate new physics discoveries.”