Glimpses of phase changes in quantum computers show researchers the tipping point

quantum computer

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Researchers at Duke University and the University of Maryland have used the frequency of measurements on a quantum computer to glimpse the quantum phenomena of phase changes — something akin to water turning into steam.

By measuring the number of operations that can be implemented on a quantum computing system without triggering the quantum state collapse, the researchers gained insight into how other systems — both natural and computational — reach their tipping points between phases. The results also provide guidance for: computer scientists work to implement quantum error correction that will eventually enable quantum computers to reach their full potential.

The results appeared online in the magazine on 3 June Nature physics

When water is brought to a boil, the movement of molecules evolves as the temperature changes until it reaches a temperature critical point when it starts to turn to steam. In a similar way, a quantum computing system can be increasingly manipulated in discrete time steps until its quantum state collapses into a single solution.

“There are deep connections between phases of matter and Quantum theorythat’s what’s so fascinating about it,” said Crystal Noel, assistant professor of electrical and electrical engineering computer engineering and physics at Duke. “The quantum computer system behaves in the same way as quantum systems found in nature — like liquid turning into steam — even though it’s digital.”

The power of quantum computers lies in the ability of their qubits to be a combination of both a 1 and a 0 at the same time, with system complexity growing exponentially as more qubits are added. This allows them to tackle a problem with enormous parallelism, such as trying to fit the pieces of a puzzle all at once instead of one at a time. However, the qubits must be able to maintain their quantum indecision until a solution is found.

One of the many challenges this poses is error correction. Some qubits will inevitably lose a piece of information and the system must be able to discover and fix these errors. But because quantum systems lose their “quantumness” when measured, keeping an eye out for errors is a tricky task. Even with extra qubits keeping an eye out, the more a quantum algorithm is examined for errors, the more likely it is to fail.

“Like water molecules about to become steam, there is a threshold of measurements that a quantum computer can withstand before it loses its quantum information,” Noel said. “And that number of readings is an analogy for how many errors the computer can withstand and still function correctly.”

In the new paper, Noel and her colleagues examine that transition threshold and the state of the system on both sides.

Working closely with Christopher Monroe, the Gilhuly Family Presidential Distinguished Professor of Engineering and Physics at Duke, Marko Cetina, Assistant Professor of Physics at Duke, and Michael Gullans and Alexey Gorshkov at the University of Maryland and the National Institute of Standards and Technology, the group co-designed software to run arbitrary quantum circuits tailored to the capabilities of their quantum system. The experiment was performed on one of the Duke Quantum Center’s ion trap quantum computers, one of the most powerful quantum computing systems in the world.

“The number of qubits in the system, the reliability of the operations, and the level of system automation combined at the same time is unique to this quantum computing system,” Noel said. “Other systems have been able to achieve each individually, but never all three at once in an academic system. That’s what allowed us to conduct these experiments.”

By averaging many random circuits, the team was able to see how the sample rate affected the qubits. As predicted, a critical point arose at which the system inevitably lost its coherence and quantum informationand by looking at how the system behaved on either side of that phase transition, researchers will be able to develop better approaches for error correction codes in the future.

The data also offers a unique look at how other phase changes occur in nature that researchers have never been able to see before.

“This demonstration is a perfect example of what we do uniquely at the Duke Quantum Center,” Monroe said. “While our quantum computers are made of atoms that are under excellent control with electromagnetic traps, lasers and optics, we can leverage these systems to do something completely different, in this case investigate the underlying quantum nature of phase transitions. This same quantum computer can also be applied in solving tricky models in fields ranging from chemical reactions, DNA sequencing and astrophysics, requiring expertise not only in atomic physics, but also in systems engineering, computer science and any field that defines the application which must be carried out.”

Error-free quantum computing becomes reality

More information:
Crystal Noel et al, Measurement-induced quantum phases realized in a quantum computer with trapped ions, Nature physics (2022). DOI: 10.1038/s41567-022-01619-7

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Duke University

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