A quantum computer can monitor what is happening inside it and respond while it runs, repeatedly resetting parts of the system to bring chaotic behavior under control. Researchers from Rutgers University, IBM and collaborating institutions have shown this in a study published in Nature Physics.
The experiment is among the largest demonstrations to date of real-time quantum feedback control, a capability considered essential for future fault-tolerant quantum computers, which will keep computing correctly even though their individual quantum bits, or qubits, constantly make 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 Rutgers physics professor and one of the senior authors. Maika Takita of IBM added that the experiment shows present-day hardware can coordinate operations, measurements and resets thousands of times across a large system.
The processor and the experiment
Image: IBM
The study ran on an IBM Quantum Heron processor with 156 qubits, using a connected chain of up to 100. The system alternated between two competing tasks: one mixed information among neighboring qubits, spreading chaos through the system like a deck of cards shuffled over and over; the other checked individual qubits and reset them when needed. In total the team performed nearly 5,000 operations linking pairs of qubits, along with nearly 5,000 checks and resets.
The balance between the two tasks decided the outcome. When scrambling happened more than half the time, the system stayed chaotic; when checking and resetting prevailed, the researchers could steer the qubits toward the orderly state they chose. Near a 50-50 balance, a small shift abruptly changed the whole system, a phase transition like water freezing into ice.
Justin Wilson of Louisiana State University compared the effect to the butterfly effect: small changes can have large, hard-to-predict consequences, yet this out-of-control behavior can be brought under control by intervening at random, at a sharply defined rate.
From a bet to a theory
The project began in 2021, at a birthday party, with a bet between Pixley and collaborator Sriram Ganeshan that a puzzling quantum effect had a counterpart in ordinary physics. The group developed a new theory of controlling quantum chaos, which IBM physicist Barbara Jones helped turn into an experiment. The processor allowed the theory to be tested on systems of up to 100 qubits, far beyond the roughly two dozen that conventional computers can simulate, and the results matched the predictions.
The study addresses a core difficulty: quantum information cannot be copied like ordinary data, and measuring it can alter it. A fault-tolerant machine must therefore detect and correct errors repeatedly without losing that information. The experiment does not yet produce such a computer, but it shows the path runs through this kind of control.
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