Sussex physicists keep a quantum bit stable for 12 minutes, smashing previous record

Physicists at the University of Sussex have kept a single quantum bit in a stable superposition for 12 minutes and 40 seconds, more than 20 times longer than the previous record, in a result that colleagues say removes one of the most stubborn practical obstacles to quantum computing.

A qubit, the quantum equivalent of a bit in an ordinary computer, can hold a mixture of 0 and 1 at once, but the mixture decays within seconds or fractions of a second as the qubit interacts with its surroundings. Most of the qubits in the machines now being built by companies and governments lose their state in well under a second; the best trapped-ion qubits previously managed about 35 seconds.

The Sussex team, led by Dr Priya Sundaram, used a single ytterbium ion held in a vacuum by electric fields and shielded by a new arrangement of magnetic coils that cancels fluctuations in the Earth’s field a thousand times more precisely than before. The result is published on Thursday in Physical Review Letters.

“The record is nice, but the point is what it allows,” Sundaram said. “Twelve minutes is long enough to move quantum information physically between machines, to store it while you wait for other operations to finish, and to run error-correction cycles thousands of times over. A quantum computer with a memory that forgets in a second is a very different engineering problem from one with a memory that holds for ten minutes.”

The experiment holds information rather than processing it, and the group acknowledges that operating on the qubit, rather than leaving it alone, introduces errors of its own. But Dr Felix Oduya of the National Physical Laboratory in Teddington, who reviewed the paper, said the shielding technique could be applied to existing ion-trap machines “without rebuilding them from scratch”.

Britain has invested about £2.5bn in quantum technologies since 2014, and the government’s national quantum strategy sets a target of a machine capable of a trillion operations by 2035. Most commercial efforts use superconducting circuits, which operate faster but lose coherence in microseconds. Trapped ions are slower but, as the Sussex result underlines, far more stable.

The group has already begun a follow-up aiming to hold two entangled ions for a similar period, which Sundaram described as “the next thing that matters”. A spin-out company based in Brighton, which holds a licence on the shielding design, said it expected to ship a commercial version to a European laboratory next year.