A team of scientists at a US lab is conducting experiments to understand how radiation from space and the environment affects quantum computing devices.
The experiment aims to reduce errors in quantum computers and enhance the detection capabilities of quantum sensors for elusive particles such as dark matter. The research is currently underway as part of ongoing efforts to improve the stability and accuracy of quantum technologies.
The experiments are being carried out at the Northwestern Experimental Underground Site (NEXUS), located about 350 feet underground at the Fermi National Accelerator Laboratory in the US. Scientists want to minimize interference from cosmic radiation and environmental noise, which can disrupt sensitive quantum systems and affect experimental results.
By conducting experiments deep underground, where thick layers of earth block most cosmic rays, researchers can create a quieter environment that allows them to study quantum devices more accurately.
A team used the facility to study how radiation influences superconducting qubits. These qubits are the basic units of information in quantum computers.
Their findings were recently published in the journal Nature Communications.
Why qubits are important
Quantum computers use qubits instead of the bits used in traditional computers. Classical bits can represent either 0 or 1. Qubits, however, can exist in multiple states simultaneously. It allows quantum computers to perform certain calculations much faster than ordinary machines.
Many scientists believe that superconducting qubits are among the most promising technologies for building large-scale quantum computers.
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However, qubits are extremely sensitive to their surroundings. Even tiny disturbances from radiation, electrical noise, or temperature changes can cause errors. These disturbances can also lead to a loss of information, a problem scientists call decoherence.
Understanding the sources of these disturbances is essential for making quantum computers more reliable.
Studying radiation effects
One major source of interference comes from high-energy particles such as cosmic rays and gamma rays. When these particles pass through a quantum chip, they can create bursts of electrical charge. These sudden charge changes can interfere with the information stored inside qubits.
Researchers can detect these events because the qubits used in the study are extremely sensitive to electrical fluctuations.
Daniel Bowring, a scientist at Fermilab and the study’s organizer, said the team wanted to understand whether these charge bursts affect multiple qubits simultaneously. “Scientists need to know if a single burst of charge can disturb several qubits as it moves through the chip,” Bowring said. “This is important for people who use quantum sensors to search for very faint signals from dark matter, and for researchers working on quantum error correction.”
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The new experiment builds on earlier work carried out in 2019 by researchers at the University of Wisconsin–Madison.
In that earlier study, scientists used the same type of chip containing four superconducting qubits. The experiment was conducted on the Earth’s surface. Researchers detected signals from both cosmic rays and gamma rays that created correlated electrical disturbances across multiple qubits.
To study the effect more clearly, scientists decided to move the experiment underground, where cosmic radiation is greatly reduced.
Experiments at the NEXUS underground lab
For the new study, the team used the same four-qubit chip and installed it inside the NEXUS laboratory at Fermilab.
The chip was placed inside a special cooling system known as a dilution refrigerator. This system keeps the qubits at extremely low temperatures to ensure they operate properly.
Researchers also surrounded the system with a lead shield to block gamma radiation from the environment.
They then ran the experiment under two different conditions. In one setup, the lead shield remained open. In the second setup, the shield was closed to reduce radiation exposure.
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By comparing the two sets of measurements, scientists hoped to see how much radiation was responsible for the charge bursts detected in the qubits.
Bowring said the team wanted to create the quietest possible environment for the experiment.
“Qubits can detect many kinds of very faint signals,” he said. “If we want to use them as particle detectors, we must be able to tell these signals apart.”
Scientists expected that closing the lead shield would greatly reduce the number of charge bursts recorded in the qubits.
The results did show a decrease in these events, but the reduction was smaller than researchers predicted. Even with the radiation shield closed, the scientists still detected some correlated charge noise affecting multiple qubits.
This surprising result suggests that another unknown source of interference may exist.
Grace Bratrud, a graduate researcher at Northwestern University and the lead author of the study, said the discovery raises new questions.
“The results suggest something other than known gamma radiation is producing these charge bursts inside the shield,” Bratrud said. “We still do not know what that source might be.”
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The research team is now planning several follow-up studies to identify the mysterious source of the extra charge bursts.
One possibility is that materials near the qubit chip may be emitting small amounts of radiation.
“Maybe there is some nearby material that releases gamma rays we did not expect,” Bratrud said. “We want to examine those materials carefully to see if they contain radioactive elements.”
Scientists also want to extend the experiment’s observation time. This will help them determine whether electrical charges become trapped in the chip’s material and release slowly over time.
In future experiments, researchers also plan to test a new type of qubit-based detector developed at SLAC National Accelerator Laboratory. This device is called a superconducting quasiparticle amplifying transmon (SQUAT).
By comparing results from the new sensor with the earlier qubit chip, scientists hope to understand how different devices respond to radiation and other environmental signals.
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Enectali Figueroa-Feliciano, a professor of physics and astronomy at Northwestern University and a co-author of the study, said the research could help design better quantum devices. “Scientists want to build systems where we can control how strongly qubits respond to their environment,” he said.
This control would allow engineers to design quantum computers that minimize environmental disturbances while also creating highly sensitive detectors for scientific research. The project was supported by the Quantum Science Center and involved scientists from several institutions worldwide, including Fermilab, Northwestern University, Stanford University and the University of Toronto.
Researchers believe the findings will play an important role in the future of quantum computing and in the search for dark matter in the universe.
