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Visualization
of an entangled three-qubit system. (Tony Melov/UNSA) |
In quantum computing, a key milestone has just been attained.
Three
different organizations from around the world have achieved 99 percent accuracy
in silicon-based quantum computing, putting error-free quantum operations
within reach.
A team
lead by University of New South Wales physicist Andrea Morello achieved 99.95
percent accuracy with one-qubit operations and 99.37 percent accuracy with
two-qubit operations in a three-qubit system in Australia.
In quantum
dots, a team led by Delft University of Technology physicist Seigo Tarucha
achieved 99.87 percent accuracy for one-qubit operations and 99.65 percent accuracy
for two-qubit operations in the Netherlands.
Finally,
in quantum dots, a team led by RIKEN scientist Akito Noiri achieved 99.84
percent accuracy for one-qubit operations and 99.51 percent accuracy for
two-qubit operations.
All
three teams published their findings in the journal Nature.
"Our processes were 99 percent error-free," Morello says.
"Because faults are so few, they can be detected and corrected when they occur. This demonstrates that quantum computers with sufficient scale and power to handle meaningful processing can be built."
The
operations of quantum computing are based on quantum mechanics. Qubits, or
quantum bits, are the quantum computing counterpart of binary bits, the
fundamental units of information.
Whereas
bits process information in one of two states –a 1 or a 0 – a qubit can be in
any of these states at the same time.
Superposition
refers to the state of having both 1 and 0 at the same time. Quantum computers
may solve complex mathematical problems by executing calculations based on the
probability of an object's condition before it is measured by maintaining the
qubits' superposition. However, this endeavour is prone to error, and enhancing
the fidelity of quantum processes has been a focus of research.
Morello
and his colleagues were able to show a quantum information lifespan of 35
seconds in a silicon substrate in 2014. Their qubits were based on nuclei's
spin states, which when isolated from their surroundings allowed for the
creation of a new time benchmark. But that isolation was also a problem: it
made it more difficult for the qubits to communicate with one another, which is
required for quantum processing.
To remedy
this problem, Morello and his colleagues used ion implantation into silicon, one
of the most basic microchip manufacturing methods, to insert an electron into
their system of two phosphorus nuclei. This is how they built their three-qubit
system, and it was successful.
"You can make two nuclei coupled to the same electron do a quantum activity," UNSW scientist Mateusz Mdzik explained.
"Those nuclei safely preserve their quantum information when you don't activate the electron. However, you can now make them communicate with each other via the electron, resulting in universal quantum operations that may be applied to any computer issue."
The other
two teams went in a different direction. They made silicon and
silicon-germanium alloy quantum dots and installed a two-electron qubit gate,
which is a circuit with multiple qubits. Then, using a protocol called gate set
tomography to characterise their systems, they modified the voltage provided to
their individual systems.
Both teams
discovered that their systems had achieved greater than 99 percent fidelity.
"The presented result puts spin qubits on par with superconducting circuits and ion traps in terms of universal quantum control performance for the first time," Tarucha says.
"This research shows that silicon quantum computers, together with superconductivity and ion traps, are excellent prospects for research and development toward the realisation of large-scale quantum computers."
Any one of
these papers would be a remarkable accomplishment on its own. The fact that all
three teams have independently hit the same milestone means that quantum
computing is about to take off.
"To use quantum error correction procedures, you normally need error rates below 1%," Morello says.
"Now that we've accomplished this, we can start constructing silicon quantum processors that can scale up and perform useful calculations."
References:
- Mądzik, M.T., Asaad, S., Youssry, A. et al. Precision tomography of a three-qubit donor quantum processor in silicon. Nature 601, 348–353 (2022). https://doi.org/10.1038/s41586-021-04292-7
- Xue, X., Russ, M., Samkharadze, N. et al. Quantum logic with spin qubits crossing the surface code threshold. Nature 601, 343–347 (2022). https://doi.org/10.1038/s41586-021-04273-w
- Noiri, A., Takeda, K., Nakajima, T. et al. Fast universal quantum gate above the fault-tolerance threshold in silicon. Nature 601, 338–342 (2022). https://doi.org/10.1038/s41586-021-04182-y
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