The ion-trap chip used to test a single qubit
Dr Jochen Wolf and Dr Tom Harty
A new record has been set for extremely precise control over qubits, the building blocks of quantum computers. This advance could lead to quantum computers that make fewer errors – if it can be replicated at a larger scale.
To manipulate or encode information into a qubit, a quantum computer can use a set-up called a single-qubit gate to change the quantum bit’s state, similar to the way one or more transistors manipulate a classical bit. Typically, this gate will fail at least once in every 1000 state changes – and sometimes even more frequently. Because computations usually require millions of operations and hundreds of qubit gates, these errors quickly accumulate and make computations unreliable.
Molly Smith, Aaron Leu and Mario Gely, all at the University of Oxford, and their colleagues have now made a single-qubit gate that only produces such errors once in about 10 million cases.
“The probability of being struck by lightning in a year is about three times higher than the probability that this qubit makes an error,” says Leu.
Leu and his colleagues made their qubit from a positively-charged calcium ion. They used electromagnetic forces to keep it trapped above a chip, which was equipped with tiny components that could emit well-controlled microwaves. These microwaves were key: by shooting them at the qubit, the researchers could create a gate that changed the qubit’s quantum state extremely reliably.
This work represents “a new world record” for researchers’ ability to control the state of a single qubit, says Daniel Slichter at the National Institute of Standards and Technology in Colorado. “Such a technical advance is impressive – many things have to work well to get to such high accuracies,” says Jonathan Home at ETH Zurich in Switzerland.
Slichter says that one factor in this achievement is the development of techniques for producing precisely calibrated microwaves. This technology has advanced significantly because it is crucial to conventional communication systems.
Although Gely’s team benefited from the quality of devices that could be bought off the shelf, the researchers had to painstakingly calibrate them. They also had to catalogue all the possible sources of error that showed up when the qubit and microwaves interacted, says Gely.
Yet, to implement any quantum computing program, the researchers will need not only more qubits, but also a different set of microwave controls that will dictate how those qubits interact with each other. The bigger and more complex this system becomes, the more room for error.
The researchers think they can scale up to two-qubit gates, while maintaining similarly low error rates, within a few years. In the meantime, Smith says their work characterising and eliminating errors may help everyone working on ion-based qubits.
Several companies are already making and commercialising such qubits, and they have previously been used to simulate poorly understood quantum matter. The new work “demonstrates that trapped-ion qubits are a leading platform for quantum computing today and will continue to be so in the future”, says Chris Langer at quantum computing firm Quantinuum, which makes qubits from laser-controlled ions.
“My personal belief is that this is an extremely compelling way to think about scaling a quantum computer,” says Slichter.
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