If you’ve ever tried to carry on a conversation in a noisy room, you can relate to the scientists and engineers trying to “hear” the signals from experimental quantum computers called qubits. These basic units of quantum computers are early in their development and remain temperamental, subject to all kinds of interference. Stray “noise” can masquerade as a functioning qubit or even render it useless.
That’s why physicist Christian Boutan and his colleagues at the Pacific Northwest National Laboratory (PNNL) were recently in party mode when they showed PNNL’s first functional superconducting qubit. It’s not much to look at. The housing — the size of a pack of chewing gum — is connected to wires that transmit signals to a nearby panel of custom radio-frequency receivers. Most importantly, though, it’s nestled in a shiny gold cocoon called a dilution fridge, and shielded from stray electrical signals. When the fridge is running, it is one of the coldest places on Earth, so very close to absolute zero, less than 6 millikelvin (about -460 degrees F).
The extreme cold and insulation transform the sensitive superconducting device into a functional qubit and slow down the movement of atoms that would destroy the qubit state. Next, the researchers listen for a characteristic signal, a blip, on their radio frequency receivers. The blip is related to radar signals used by the military to detect the presence of aircraft. Just as traditional radar systems emit radio waves and then listen for returning waves, PNNL’s physicists have used a low-temperature sensing technique to “hear” the presence of a qubit by emitting carefully crafted signals and decoding the returning message.
“You whisper to the qubit and listen to the resonator,” said Boutan, who built PNNL’s first qubit testbed. “If you hit the right frequency with a signal sent to the qubit, you see the peak of the resonator shift. The state of the qubit changes the resonator frequency. That’s the signal shift we’re listening to.”
This is not directly measuring the quantum signal, but looking for the trail it leaves behind. One of the many quirks of quantum computing is that scientists cannot directly measure the quantum state. Instead, they examine its impact on the strategically prepared environment around it. Therefore, PNNL’s expertise in radio frequency transmission and signal detection is essential, Boutan said. Any uncontrolled background noise can destroy qubit coherence.
All this special care is needed because the quantum signals the research team is trying to detect and record can quite easily be swamped by competing “noise” from various sources, including the materials in the qubit itself.
It’s still early days in quantum computing. Existing prototypes, such as those at PNNL’s physics lab, can be compared to the Macintosh PC when Apple founder Steve Jobs and his friends emerged from their garage. Only the investments and the stakes are a lot higher at this stage of the quantum computer age.
Scientists are mainly focused on the potential of quantum computers to solve pressing problems of energy production, use and sustainability. That’s why the US government’s investment alone exceeds $1 billion through the National Quantum Initiative and the Department of Energy’s National Quantum Information Science (QIS) Research Centers, which focus on advancing the science of quantum computing.
Contributing to three of the five QIS centers, PNNL is working on various aspects of quantum information science, including revealing and eliminating the sources of interference and noise that take qubits out of the usable state called “coherence” through computer codes. that take advantage of these quantum computers and improve the material design and construction of the qubits themselves. Boutan’s research on microwave quantum detection is supported by PNNL’s Laboratory Directed Research and Development program.
Caring for and feeding qubits
Superconducting qubits are made of exotic metals that react with oxygen in the atmosphere, forming metal oxides. You’ve seen this happen when iron turns to rust.
“It’s a materials problem,” said Brent VanDevender, a PNNL physicist who works on sources of interference in qubits. “We call them two-level systems. The term refers to any defects in your material, such as the oxides, that can mimic the qubit behavior and steal energy.”
PNNL materials scientist Peter Sushko and his colleagues are working on the challenge of stopping qubit “rust” with employees at Princeton University through their affiliation with the C2QA QIS Center. There, a team of researchers developed one of the most durable qubits reported to date. And yet, metal oxides quickly form on the exposed surface of these superconducting qubit devices.
In collaboration with their Princeton collaborators, Sushko and his team have proposed a protective coating that can interfere with oxygen in the air interacting with the surface of qubits and causing them to oxidize.
“Our goal is to remove disorder and be compatible with the underlying structure,” Sushko said. “We’re looking at a protective layer that comes on top in an orderly manner and prevents oxidation, minimizing the effects of disorder.”
This research builds on fundamental research by PNNL materials scientist Marvin Warner and colleagues. They have built up a wealth of knowledge on protecting sensitive superconducting metal-based devices by applying a microcoating that effectively protects the surface from damage that can affect performance.
“Controlling surface chemistry to protect emerging quantum properties of a material is an important approach to developing more stable and robust devices,” Warner said. “It perfectly capitalizes on the strength of PNNL as a chemistry laboratory.”
Soon, the team will build the proposed solution at Princeton University’s Quantum Device Nanofabrication Laboratory. Once built, it will undergo a series of tests. If successful, the qubit could be ready for rigorous testing of its longevity when faced with qubit-coherence-destroying bombardments from atmospheric radiation, also known as cosmic rays.
The number of places in the United States set up to study qubit fidelity in a well-shielded subterranean environment can be counted on one hand. Soon PNNL will be among them. Preparations to set up an underground qubit testing facility within PNNL’s Shallow Underground Laboratory are well underway. Decades of research into the effects of ionizing radiation have prepared scientists at PNNL to determine how well quantum devices can tolerate bombardment interference from natural radiation sources. Here, researchers and engineers are setting up a dilution refrigerator similar to the one in PNNL’s physics lab.
In an ultra-clean room with industry-leading ultra-pure materials synthesis and ultra-low background radiation detection, experimental qubits will be tested in a modified lead-shielded environment that reduces external gamma radiation by more than 99 percent.
Within the year, PNNL will be ready to complete the full cycle of qubit testing, from design and theory, to microfabrication, to environmental testing, to implementation with research partners.
“Fully functional quantum computers will only be useful if they become reliable,” Warner says. “With our research partners, we are preparing today to usher in that era today.”
Developing the next generation of quantum algorithms and materials
Quote: New superconducting qubit testbed benefits the development of quantum information science (2022, September 29) retrieved September 29, 2022 from https://phys.org/news/2022-09-superconducting-qubit-testbed-benefits-quantum.html
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