Researchers from Rutgers, IBM and collaborating institutions have shown that a quantum computer can check what is happening inside it and respond while it is running, repeatedly resetting parts of the system to bring chaotic behavior under control.
The advance is a critical step toward building larger, more reliable quantum computers.
Today's quantum computers operate according to the unusual rules governing atoms and other extremely small particles. They could one day solve problems beyond the reach of conventional computers — designing better batteries and cleaner industrial catalysts, simulating molecules and materials too complex for today's computers, and running certain artificial intelligence programs.
In a study published in Nature Physics, researchers from Rutgers, International Business Machines Corp., and collaborating institutions showed that a quantum computer can repeatedly check and reset parts of itself while running. The experiment is among the largest demonstrations to date of real-time quantum feedback control, a capability considered essential for future quantum computers that will be fault tolerant.
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. A fault-tolerant quantum computer is a quantum computer that can keep computing correctly even though its individual quantum bits are constantly making small mistakes. No one has yet built such a device.
"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 the Director of the Center for Materials Theory.
"This experiment is a landmark on that road," said Pixley, who also is a member of the Flatiron Institute, the scientific research division of the Simons Foundation in New York, and one of the senior authors of the study. "It says, 'OK, we've taken another important step to get there.'"
The work also confirmed a prediction about quantum behavior. As researchers increased how often the computer checked and reset its fundamental information-storing units, known as qubits, the system abruptly shifted from chaos to a state they could control, showing how order can emerge from disorder.
Conventional computers store information in bits, each of which is either 0 or 1. Quantum computers use qubits, which can exist in a superposition. This means they hold multiple possibilities between 0 and 1 until they are measured, when each produces one result or the other.
These properties give quantum computers their potential power but make the information they carry extremely sensitive. Heat, stray electromagnetic signals, imperfect control pulses and tiny flaws in the hardware can alter a qubit's state. Over time, the resulting errors can accumulate until a calculation can no longer be trusted.
"The challenge was not simply to measure a qubit, but to do it repeatedly while the rest of the processor continued to operate," said another senior author of the study, Maika Takita, principal research scientist for quantum error correction and dynamic circuits experiments at IBM. "This experiment shows that present-day hardware can coordinate quantum operations, measurements and resets thousands of times across a large system."
Unlike ordinary computer data, quantum information cannot simply be copied. And measuring it can change or destroy the state being used. A fault-tolerant machine must repeatedly detect and correct errors without losing that information.
The researchers conducted the experiment on a 156-qubit IBM Quantum Heron processor, one of the most powerful quantum processors in the world, selecting a connected chain of as many as 100 qubits.
They 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. The process is similar to repeatedly shuffling a deck of cards. With every shuffle, the original order becomes more 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 researchers controlled how often the computer performed each task. 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 about 50-50, 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.
"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 began with a friendly bet at a 2021 birthday party for Pixley's wife while still at Rutgers and living in Highland Park, N.J. 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 the 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, Colo.
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.
