Fermilab News Today
Multi-institutional researchers have taken their tests deep into the Illinois prairie to eliminate the unseen “noise” that plagues the world’s most powerful technologies. At Fermilab, 350 feet underground, these scientists are studying how background radiation affects superconducting qubits, the building blocks of quantum computers and next-generation particle detectors. Recently published in Nature Communications, their findings on “correlated charge noise” and its potential to corrupt quantum information are groundbreaking.
A Shield Against the Heavens
The Northwestern Experimental Underground Site, or NEXUS, is the site of the study. It is a facility built to offer an environment protected from the continuous barrage of cosmic rays that hit the Earth’s surface. This environmental isolation is not only desirable but also essential for quantum devices. A single ionizing particle, such a cosmic ray or a gamma ray, can cause a burst of electrical charge as it travels through a quantum device because superconducting qubits are so sensitive to their environment. A significant obstacle to the creation of fault-tolerant quantum computers is the possibility that these charge bursts could cause qubits to lose their “coherence,” resulting in computation mistakes.
Daniel Bowring, a scientist at Fermilab and the study’s principal organizer, stated that “understanding whether a charge burst could affect multiple qubits as the charge moves through the chip what researchers call correlated charge noise is crucial.” This association is especially troublesome since typical error-correction procedures could find it difficult to keep up if one event impacts several qubits at once.
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The Underground Advantage
Recent research builds on a 2019 study from the University of Wisconsin-Madison. In that previous work, researchers measured charge noise at the Earth’s surface using the same four-qubit device and found distinct signals from both cosmic and gamma radiation. To observe how the qubits performed in a “quieter” environment, researchers were able to isolate the variables by transporting the exact same chip to the NEXUS facility. This successfully blocked out the majority of cosmic rays.
The scientists put the qubit chip inside a dilution cooler with a thick lead barrier to further improve their readings. The scientists were able to identify the precise impacts of gamma radiation by taking measurements both with and without the lead shield. The objective was to see how the pace of charge bursts would change when the surroundings were made as silent as was practically practicable.
A Persistent Mystery
Although the researchers saw a reduction in charge bursts when the lead shield was closed, the outcomes were not precisely what they had expected. Surprisingly, associated charge noise continued even behind the lead shielding, and the noise reduction was smaller than anticipated. This implies that the interference is caused by something other than recognized external gamma radiation.
“That leads us to believe something else besides the known gamma radiation is causing charge bursts inside the shield,” stated Grace Bratrud, the primary author of the study and a graduate researcher at Northwestern University. The scientific community is currently engaged in a heated dispute regarding the origin of this residual noise. One hypothesis is that small amounts of radioactivity are being released by the materials used to build the experiment or those around the qubits.
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Dual Implications: Computing and Dark Matter
The NEXUS study’s findings have important ramifications for two quite distinct scientific domains. The information gives computer scientists a road map for creating noise reduction techniques to create more dependable, error-resistant quantum processors. Conversely, for particle physicists, the extraordinary sensitivity of these qubits is actually a feature rather than a bug .
Dark matter, the enigmatic material that makes up the majority of the universe’s mass but has never been directly discovered, can be found using qubits that are extremely sensitive to weak environmental signals. Although the specifically define dark matter, the general scientific community defines it as a non-luminous substance that interacts mostly through gravity.
“Qubits are sensitive to different types of faint signals,” Bowring pointed out. “If we want to use them as particle detectors, we need to be sure we can tell these signals apart from each other” . For the upcoming generation of physics investigations, the capacity to differentiate between a possible dark matter signal and a random background gamma ray is crucial.
The Path Ahead
To unravel the riddle of the extra charge bursts, the study team has already planned a number of follow-up investigations. To determine whether trapped charges within the chip’s substrate are released over extended periods of time, future tests will increase the observation times. Additionally, they intend to examine how various detection techniques handle varying energy levels using a more sophisticated sensor called a superconducting quasiparticle amplifying transmon (SQUAT), created by the SLAC National Accelerator Laboratory.
Reaching a level of “engineered response” is the ultimate objective. This degree of control will enable researchers to maximize environmental responses for the discovery of elusive particles while minimizing them for quantum computing, according to study coauthor Enectali Figueroa-Feliciano, a professor at Northwestern University.
Experts from the Illinois Institute of Technology, SLAC, Stanford, Tufts, and foreign partners from France and Canada collaborated extensively on the work, which was primarily supported by the Quantum Science Center. Researchers are searching for signals that will define the next century of science, not simply noise, as they continue to delve deeper into the NEXUS facility.
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