Quantum computers are notoriously fragile, but a new experiment shows one can keep an eye on itself while it works and step in to restore order. Physicists from Rutgers University, IBM and several other American institutions describe the feat in a paper published last week in Nature Physics.
The team ran its test on a 156-qubit IBM Heron processor, using a line of up to 100 linked qubits. Two kinds of steps competed with each other at random. One shuffled quantum information between neighbouring qubits, much like repeatedly mixing a deck of cards. The other picked out individual qubits mid-calculation, measured them and reset them to zero.
When the shuffling dominated, the system stayed chaotic. When measurements and resets came often enough, the team was able to steer the qubits to a simple state it had chosen in advance. Close to an even split, a tiny change in the balance flipped the entire chip from one behaviour to the other, a so-called phase transition that theory had predicted. In all, the runs combined close to 5,000 entangling gates with a similar number of measurement-and-reset steps.
Why does it matter? Any future fault-tolerant machine will have to check and repair itself constantly without wrecking the information it is processing. Co-author Jedediah Pixley called the experiment “a landmark on that road.” It is not yet error correction, though: resetting qubits to a known value is a different task from protecting unknown data.
Another point in its favour is scale. Conventional computers could simulate only around two dozen qubits of such a circuit, yet the 100-qubit results matched the predictions closely, and the transition grew sharper as the system got bigger.
The project traces back to a 2021 birthday party, where Pixley bet a colleague over a quantum puzzle. The stake turned out to be two scoops of blueberry ice cream, which Pixley ended up owing.
