Quantum computers are constantly hampered by cosmic rays


IBM is one of many companies working on quantum computers

Nearly a fifth of all difficult-to-correct errors in quantum computers are caused by powerful particles from space hitting the machines. While these cosmic ray-induced errors have long been predicted by scientists, this first precise measurement of how often they occur could help to error proof future devices.

Ordinary computers are also susceptible to cosmic ray errors, but the delicate components that power quantum computers, known as qubits, are more at risk because their fragile quantum states are easily disturbed by outside forces. While some quantum computers employ error-correction techniques, these struggle to keep up when errors occur across multiple qubits at the same time, as they can’t recover from one error before the next occurs. Without knowing the exact source of these multi-qubit errors, it can be hard to protect against them.

To investigate, Patrick Harrington at the Massachusetts Institute of Technology and his colleagues set up an array of 10 superconducting qubits inside an extremely cold fridge with radiation detectors close by, and monitored them for 11 days. “We were able to see in time that the detectors went off right at the same moment that qubit errors occurred,” says Harrington. “Given the unlikelihood of that happening randomly, we were able to create a strong correlation between cosmic rays and the qubit errors.” In total, 17 per cent of the errors were down to cosmic rays.

The team found that rearranging the layout of superconducting components within the individual qubits helped them recover from errors more quickly, which could be useful for future quantum computing designs.

In a separate work, Xue-Gang Li at the Beijing Academy of Quantum Information Sciences in China and colleagues placed cosmic ray detectors near an array of 31 superconducting qubits, and also identified errors occurring at the exact same time as cosmic rays were spotted. They used qubits made from tantalum, and found that these recovered from errors much faster than qubits made from aluminium, like those used in Google’s Sycamore processor.

“These papers both also include some tantalising hints as to what might solve the problem,” says Matt McEwen at Google. However, there is still some way to go until cosmic ray-resistant quantum computers can be built. The qubits used in both experiments took 10 to 100 microseconds to recover, but standard error-correction techniques would need them to recover in much less than a single microsecond, says McEwen.

While errors are undesirable for quantum computation, the sensitivity of the machines to cosmic rays suggests they could be used as sensors. “For quantum computation, we want qubits that are bad detectors, in a sense, but seeing what our pinch points are now does suggest potential avenues of using quantum sensing in a qubit for particle detection,” says Harrington. “That’s actually something that may be useful for running quantum error correction, where having sensors nearby can flag or suggest that extra errors have occurred.”

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