Quantum Processor Shows Chaos-to-Order Transition Under Repeated Error Checks
Researchers in the United States have shown on a 156-qubit IBM Quantum Heron processor that repeated checks and resets can bring chaos under control, a mechanism needed for fault-tolerant quantum computers. The study, published in Nature Physics, partly demonstrates how real-time error correction could work. Fault tolerance means spotting and fixing errors while the computer is still running its calculations.
Why error correction matters
Today's quantum computers are small-scale, limited prototypes. Qubits, the basic units of quantum computers, can be 1, 0 or anywhere in between (a superposition) until they are measured. That is where the exponential gain in computing power comes from. But qubits are also very delicate and easily disturbed by heat or motion, which is why error correction is so important.
"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," says physicist Jedediah Pixley of Rutgers University. "This experiment is a landmark on that road."
Scrambling versus controlling
The team used a two-stage system built to both introduce chaos and then correct for it. They connected as many as 100 qubits at once and set up two alternating tasks: scrambling, which spreads chaos through the system, and controlling, which resets qubits where needed.
"The challenge was not simply to measure a qubit, but to do it repeatedly while the rest of the processor continued to operate," says IBM physicist Maika Takita. "This experiment shows that present-day hardware can coordinate quantum operations, measurements and resets thousands of times across a large system."
A sharp tipping point
The researchers shifted the balance between scrambling and resetting. Across nearly 5,000 qubit operations and almost 5,000 checks and resets, order returned when the checks and resets made up more than half of the operations overall. As more qubits were added, the tipping point became better defined in the data, evidence of a true quantum phase transition, like water turning to ice in a unified way.
"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," says Haining Pan of the University of Florida. Justin Wilson of Louisiana State University adds: "It wasn't obvious that this transition between chaos and control would survive in quantum physics, but our results show that it does."
The authors note that the checking-and-resetting scheme will need to be scaled up with more complex rules to be useful in real quantum computing. For now it works as a stress test of feedback loops that keep quantum systems running in an orderly way. More in our technology section.