Another milestone on the way to building useful and practical quantum computers has been passed. Researchers at the firm Quantinuum have used error-correction methods to make their Helios-1 quantum computer store information with an unprecedented degree of reliability, which is a key prerequisite for running complex calculations in the future.

Quantum computers hold immense potential for solving problems and making scientific discoveries that have, so far, been out of reach for even the best supercomputers. But they haven’t yet realised that potential, partly because all existing quantum computers make lots of errors. Consequently, researchers around the world are racing to perfect error-correction methods where a quantum computer’s physical building blocks, or qubits, are encoded into groups called “logical qubits”, which are a lot less likely to make errors when used to run calculations.

David Hayes at Quantinuum and his colleagues have now demonstrated the lowest error rate for logical qubits yet.

“This is definitely an impressive technical advance,” says Steven Flammia at Virginia Tech, who didn’t work on the project. “It’s super exciting,” says Hayes.

Helios-1 contains 98 qubits, each of which is based on an ion whose quantum state is controlled with lasers and electromagnetic fields. To turn these qubits logical, the team used a type of computer coding called “colour code”. When the qubits were then tested for their ability to store information, they made as few as five errors for every million uses of a qubit. This is an improvement on similar past experiments, where qubits would instead make hundreds of errors for every million uses.

The researchers also tested whether two of Helios-1’s logical qubits could be used for a simple, error-corrected mathematical operation. The error rate in this situation was roughly 10 times higher, or about seven errors for 100,000 qubit uses.

Hayes says Helios-1’s record-breaking performance partly follows from the Quantinuum team improving its control over the physical qubits. Specifically, whenever a qubit’s energy shifts in a deleterious way, which happens naturally but makes computations less accurate, it is now hit with a special laser pulse that corrects the shift. Then the colour code kicks in, leveraging the connections and correlations between the qubit and several others in the logical qubit to correct any remaining error.

The researchers have also shown that each logical qubit’s error rate decreases as more physical qubits are added to it. Specifically, the error rate diminished twelve-fold when they increased a logical qubit’s size from seven to 19 physical qubits, with the same diminishing being seen when the physical qubit count rose from 19 to 37. This result is important because quantum computers gain computational power as their qubit number increases, so only error-correction methods that also improve with increasing qubit counts stand a chance of being truly useful.

Murphy Niu at the University of California, Santa Barbara, says the new experiment is a major step towards quantum computing that is error-proof.

Flammia says this work sets the course for building what John Preskill at the California Institute of Technology has dubbed the “megaquop” machine, a computer capable of 1 million reliable quantum operations. This could make meaningful scientific discoveries, for example in materials science, and is thought to be the next important stage in the evolution of quantum computers.

Helios-1’s low error rates fit the bill for a megaquop machine, but what remains to be seen is whether these will stay so low once the logical qubits are used to execute more complex mathematical operations, says Flammia. This is an important missing ingredient as far as achieving fault-tolerant quantum computing goes, says Thomas Wong at Creighton University in Nebraska.

The question of the total number of qubits also looms large. “They only have roughly 100 physical qubits, but on the order of 100 logical qubits would be needed for known useful applications of megaquop machines,” he says.

Hayes says Quantinuum’s next quantum computer, Sol, will be nearly twice as large as Helios-1. In his view, the new experiment establishes the foundation for Sol and for other computers the company plans to build in the future. “We are doing all of our homework up front,” he says.

But competition within the quantum computing industry is stiff. So far, several research teams have made quantum computer prototypes with more than a thousand qubits and the firm QuEra, whose devices are based on extremely cold atoms, has announced that it will build a megaquop machine by 2028.