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Quantum bits are noisy, and techniques are needed to handle this noise of quantum computers are to be useful.
One possibility is not to work with single qubits, but to do identical operations on bunches of linked physical qubits, combing their noisy outputs to create a single logical qubit that will be less noisy.
“Such fault tolerance enables reliable computation with noisy components and is the key to realising quantum computers,” according to QuTech, a Netherlands-based quantum component research company spun out from university TU Delft and national research lab TNO.
The qubits in question are formed by 13C nuclear spins (orange – diags left and above) in diamond at 10K, with information spread over five entangled (purple) spin qubits.
Electron spin in a nitrogen-vacancy centre (also purple) is used as an extra helper qubit to detect errors and, as an error could also occur on the helper qubit, a second helper qubit (green, made using 14N nuclear spin) identifies such errors.
A custom encoding system was created for the processor, which QuTech claims can deliver the correct result despite any single error anywhere in the system – according to the company, the University of Southern California has determined that such a seven qubit system is the smallest configuration that allows fault-tolerant error-correction.
The qubits were assembled inside QuTech’s 29 qubit quantum processor, in which the qubits exist in ultra-pure diamond grown by Element Six, that can sustain coherent quantum states “for minutes”, said QuTech.
“Because of the long coherence times of our qubits,” said project supervisor Tim Taminiau, “we could process the outcomes of the measurements on-the-fly while the processor was running. Very few systems around the world currently have this capability.”
That said, “there is still a long way to go for large-scale fault-tolerant computation,” said fellow supervisor Barbara Terhal. “Truly suppressing errors will require further improvements in the number of qubits and the quality of operations.”
Beyond the demonstration, the researchers aim to take a modular approach, linking multiple processors similar to the proof-of-concept into a larger one through optical quantum interconnects, all managed by conventional control electronics.
“Making many copies of small processors and connecting them together into one large quantum computer, is a flexible way to address the challenge of scaling up the number of qubits,” said Taminiau, who added that QuTech already has the interconnect technology and is working on other necessary parts.
The work is published at ‘Fault-tolerant operation of a logical qubit in a diamond quantum processor‘ in Nature (payment required for full access.
Images courtesy of QuTech
