Quantum Computer Learns to Police Itself in Landmark Error-Control Experiment

Quantum computers promise to solve problems that would stump even the most powerful supercomputers on Earth, from designing better batteries and cleaner industrial catalysts to simulating molecules and materials too complex for conventional machines. But the same quantum properties that give these devices their extraordinary potential also make them extraordinarily fragile. Now, researchers from Rutgers University, IBM and a network of collaborating institutions have demonstrated a critical capability on the road to practical quantum computing: a quantum computer that can repeatedly check what is happening inside it and respond while it is running, resetting parts of the system to bring chaotic behavior under control. The experiment, published in Nature Physics, is among the largest demonstrations to date of real-time quantum feedback control, a capability widely considered essential for the fault-tolerant quantum computers of the future.

To understand why this matters, it helps to consider how quantum computers differ from the machines on our desks. Conventional computers store information in bits, each of which is either a 0 or a 1. Quantum computers use qubits, which can exist in a superposition, meaning they hold multiple possibilities between 0 and 1 until they are measured, at which point each produces one result or the other. This strange property, governed by the unusual rules that govern atoms and other extremely small particles, is what gives quantum computers their potential power. It is also what makes the information they carry so sensitive. Heat, stray electromagnetic signals, imperfect control pulses and tiny flaws in the hardware can all alter a qubit’s state, and over time the resulting errors can accumulate until a calculation can no longer be trusted.

The challenge of keeping a quantum computer on track is compounded by two fundamental restrictions that do not apply to ordinary computing. Unlike ordinary computer data, quantum information cannot simply be copied for safekeeping. And measuring it can change or destroy the very state being used. A fault-tolerant machine must therefore repeatedly detect and correct errors without losing the information it is meant to protect. Scaling quantum computers will require them to identify and correct errors while they are working, a capability known as error correction, which will allow for the construction of a fault-tolerant quantum computer. Such a device would be able to keep computing correctly even though its individual quantum bits are constantly making small mistakes. No one has yet built one.

“To build a fault-tolerant quantum computer, we have to make these checks and corrections not just once or twice, but a huge number of times during a calculation,” said Jedediah Pixley, a professor in the Department of Physics and Astronomy at the Rutgers School of Arts and Sciences and Director of the Center for Materials Theory, who is one of the senior authors of the study and a member of the Flatiron Institute, the scientific research division of the Simons Foundation in New York. “This experiment is a landmark on that road. It says, ‘OK, we’ve taken another important step to get there.'”

The experiment was conducted on a 156-qubit IBM Quantum Heron processor, one of the most powerful quantum processors in the world, with the researchers selecting a connected chain of as many as 100 qubits. The team programmed the system to alternate between two competing tasks. The first acted like a scrambler: it mixed information among neighboring qubits, spreading chaos through the system in much the same way that repeatedly shuffling a deck of cards makes the original order increasingly difficult to recognize. The second task served as a form of control: the computer checked individual qubits and reset them when necessary. Repeated often enough, those resets pushed the system toward a simple, orderly state selected by the researchers.

Overall, the team performed nearly 5,000 operations that linked pairs of qubits, along with nearly 5,000 checks and resets. According to the researchers, it was the largest successful demonstration of this approach to date. “The challenge was not simply to measure a qubit, but to do it repeatedly while the rest of the processor continued to operate,” said Maika Takita, principal research scientist for quantum error correction and dynamic circuits experiments at IBM, and another senior author of the study. “This experiment shows that present-day hardware can coordinate quantum operations, measurements and resets thousands of times across a large system.”

The researchers controlled how often the computer performed each task, and by changing that balance they could see whether scrambling or resetting would take over. When scrambling occurred more than half the time, chaos won and the system remained chaotic. When checking and resetting occurred more often, control won and the researchers could guide the qubits toward the orderly state they wanted. At roughly a fifty-fifty split, a slight change in which task occurred more often suddenly changed the entire system. Physicists call such a dramatic shift a phase transition, like water freezing into ice. Here, the qubits shifted together from chaotic behavior to a state the researchers could control, confirming a theoretical prediction that order can emerge from disorder in this setting.

“Small changes can lead to large and nearly impossible-to-predict consequences, like the classic idea that a butterfly flapping its wings in Africa could eventually contribute to a hurricane hitting Louisiana,” said Justin Wilson, an associate professor of physics at Louisiana State University and a study coauthor. “But this out-of-control behavior can be brought under control by randomly intervening, and there is a sharply defined rate at which that happens. It wasn’t obvious that this transition between chaos and control would survive in quantum physics, but our results show that it does.”

The project itself began in an unusually informal setting: a friendly bet at a 2021 birthday party for Pixley’s wife, while he was still at Rutgers and living in Highland Park, New Jersey. His longtime collaborator Sriram Ganeshan wagered that a puzzling quantum effect had a counterpart in ordinary physics, leading the group to develop a new theory about controlling quantum chaos. A chance conversation with IBM physicist Barbara Jones then helped turn that theory into an experiment. Pixley lost the bet and paid up with two scoops of blueberry ice cream, after Ganeshan and others joined him at the Aspen Center for Physics in Aspen, Colorado. What began as a party wager ultimately produced a theoretical framework robust enough to be tested on one of the world’s most advanced quantum processors.

The IBM processor allowed the researchers to test their theory on systems of up to 100 qubits, far beyond the roughly two dozen they could simulate on conventional computers, and the results closely matched their predictions. “What was striking was that the transition became more clearly defined as we studied larger systems,” said Haining Pan, a Rutgers postdoctoral researcher at the time of the study and a co-first author. “The agreement between the experiment and the theory showed that this was not an accidental feature of a small circuit. It was collective behavior emerging across the quantum system.” In addition to Rutgers and IBM, researchers affiliated with the City College of New York, the CUNY Graduate Center, Iowa State University, Ames National Laboratory, Pennsylvania State University and Louisiana State University contributed to the study. As the field moves toward machines that can correct their own mistakes in real time, this experiment offers both a technical milestone and a demonstration that theoretical predictions about quantum chaos can be verified at scales where collective behavior, not just individual qubits, takes center stage.

Subject of Research: Real-time feedback control and error correction in quantum computers

Article Title: Scientists move a step toward quantum computers that can correct their own mistakes

Article References: Scientists move a step toward quantum computers that can correct their own mistakes. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum computing, error correction, fault tolerance, qubits, quantum chaos, phase transition, IBM Quantum Heron, feedback control, Rutgers University, Nature Physics, quantum information, scrambling