A rendering of the Oxford University team’s ion trap chip. Credit: Dr Jochen Wolf and Dr Tom Harty.
Physicists at the University of Oxford have set a new global benchmark for the accuracy of controlling a single quantum bit.
The team also achieved the lowest-ever error rate for a quantum logic operation – just 0.000015%, or one error in 6.7 million operations.
This record-breaking result represents nearly an order of magnitude improvement over the previous benchmark, set by the same research group a decade ago.
To put the result in perspective: a person is more likely to be struck by lightning in a given year (one in 1.2 million) than for one of Oxford’s quantum logic gates to make a mistake.
The findings are a major advance towards having robust and useful quantum computers.
Co-author on the paper, Professor David Lucas, from the University of Oxford’s Department of Physics, said: “As far as we are aware, this is the most accurate qubit operation ever recorded anywhere in the world.
“It is an important step toward building practical quantum computers that can tackle real-world problems.”
Photograph of the Oxford University team’s ion trap chip. Credit: Dr Jochen Wolf and Dr Tom Harty.
Millions of operations
To perform useful calculations on a quantum computer, millions of operations will need to be run across many qubits.
This means that if the error rate is too high, the final result of the calculation will be meaningless.
Although error correction can be used to fix mistakes, this comes at the cost of requiring many more qubits.
By reducing the error, the new method reduces the number of qubits required and consequently the cost and size of the quantum computer itself.
Co-lead author Molly Smith (Graduate Student, Department of Physics, University of Oxford), said: “By drastically reducing the chance of error, this work significantly reduces the infrastructure required for error correction, opening the way for future quantum computers to be smaller, faster, and more efficient.
“Precise control of qubits will also be useful for other quantum technologies such as clocks and quantum sensors.”
Unprecedented
This unprecedented level of precision was achieved using a trapped calcium ion as the qubit (quantum bit).
These are a natural choice to store quantum information due to their long lifetime and their robustness.
Unlike the conventional approach, which uses lasers, the Oxford team controlled the quantum state of the calcium ions using electronic (microwave) signals.
This method offers greater stability than laser control and also has other benefits for building a practical quantum computer.
For instance, electronic control is much cheaper and more robust than lasers, and easier to integrate in ion trapping chips.
Furthermore, the experiment was conducted at room temperature and without magnetic shielding, thus simplifying the technical requirements for a working quantum computer.
The previous best single-qubit error rate, also achieved by the Oxford team, in 2014, was one in one million.
The group’s expertise led to the launch of the spinout company Oxford Ionics in 2019, which has become an established leader in high-performance trapped-ion qubit platforms.
Whilst this record-breaking result marks a major milestone, the research team caution that it is part of a larger challenge; quantum computing requires both single- and two-qubit gates to function together.
Currently, two-qubit gates still have significantly higher error rates – around 1 in 2,000 in the best demonstrations to date – so reducing these will be crucial to building fully fault-tolerant quantum machines.
The findings are to be published in Physical Review Letters.
