Three separate teams around the world have passed the 99 percent accuracy threshold for silicon-based quantum computing, placing error-free quantum operations within tantalizing grasp. In Australia, a team led by physicist Andrea Morello of the University of New South Wales achieved 99.95 percent accuracy with one-qubit operations, and 99.37 percent for two-qubit operations in a three-qubit system.

A visualization of UNSW’s three-qubit system, which can perform quantum logic operations with over 99% accuracy. (Quantum operation fidelities above 99% were obtained in a three-qubit silicon quantum processor. The first two qubits (Q1, Q2) are the nuclear spins of individually-implanted phosphorus atoms (red spheres). The third qubit (Q3) is the spin of an electron that wraps around both nuclei (shiny ellipse).)

Credit: Tony Melov / UNSW

In the
Netherlands, a team led by physicist Seigo Tarucha of Delft University of
Technology achieved 99.87 percent accuracy for one-qubit operations, and 99.65
percent for two-qubit operations in quantum dots. Finally, in Japan, a team led
by physicist Akito Noiri of RIKEN achieved 99.84 percent accuracy for one-qubit
operations and 99.51 percent for two-qubit operations, also in quantum dots. All
three teams have published their results in the journal *Nature *today*.*

"Today's publication in

Natureshows our operations were 99 percent error-free," Morello says. "When the errors are so rare, it becomes possible to detect them and correct them when they occur. This shows that it is possible to build quantum computers that have enough scale, and enough power, to handle meaningful computation."

The three qubits can be prepared in a quantum entangled state, which unlocks the exponential power of quantum computers. (Nuclear spins are exceptionally good qubits, because of their exceptional isolation from the environment. This same feature, however, makes it difficult for them to interact and perform quantum logic operations. The team’s breakthrough consists in using a common electron to mediate the interaction, leading to high-fidelity universal quantum logic operations. Furthermore, the electron itself is a high-quality qubit, and can be placed in a fully quantum-entangled state with the two nuclei.)

Credit: Tony Melov / UNSW

Quantum
computing relies on quantum mechanics as the basis for operations. Information
is encoded in qubits, or quantum bits, the quantum computing equivalent of
binary bits, the basic units of information. However, where bits process
information in one of two states – a 1 or a 0 – a qubit can be in the state of
a 1, a 0, or both simultaneously.

The latter
state – 1 and 0 at the same time – is known as superposition. Maintaining
the qubits' superposition enables quantum computers to
solve complex mathematical problems by running calculations based on the probability of an
object's state before it is measured. This endeavor is highly prone to
error, however, and improving the fidelity of quantum operations has been the
subject of intense study.

The three-qubit system paves the way to scaling up the quantum processor in the future, because the electron can be easily entangled with other electrons or moved across the chip. (The three-qubit entangled state of nuclei and electron paves the way to scaling up the quantum processor in the future. The electron can be easily entangled with other electrons, or physically moved across the chip. In this way, the UNSW team will be able to manufacture and operate large arrays of qubits capable of robust and useful computations.)

Credit: Tony Melov / UNSW

In 2014,
Morello and his colleagues were able to demonstrate a whopping 35-second
lifespan for quantum information in a silicon substrate. Their qubits were
based on the spin states of nuclei, which, isolated from their environment,
enabled the setting of a new time benchmark.
But that very isolation proved a problem, too: it made it harder for the qubits
to communicate with each other, which is necessary for performing quantum
computation.

To resolve
this issue, Morello and team introduced an electron to their system of two
phosphorus nuclei via ion implantation into the silicon, one of the fundamental
processes for making microchips. This is how they created their three-qubit
system, and it worked.

L-R Asaad, Morello, Madzik (composite image): Serwan Asaad, Andrea Morello, and Mateusz Mądzik are lead authors of the UNSW paper which demonstrated 99 percent error-free quantum operations.

Credit: Kearon de Clouet / UNSW

"If you have two nuclei that are connected to the same electron, you can make them do a quantum operation," said physicist Mateusz Mądzik of UNSW. "While you don't operate the electron, those nuclei safely store their quantum information. But now you have the option of making them talk to each other via the electron, to realise universal quantum operations that can be adapted to any computational problem."

The other
two teams took a different approach. They created quantum dots of silicon and
silicon-germanium alloy, and installed a two-electron qubit gate; that is, a
circuit of multiple qubits. Then, they tweaked the voltage applied to their
respective systems, using a protocol called gate
set tomography to characterize their systems. Both teams found that
they too had achieved higher than 99 percent fidelity in their systems.

"The presented result makes spin qubits, for the first time, competitive against superconducting circuits and ion traps in terms of universal quantum control performance," says Tarucha. "This study demonstrates that silicon quantum computers are promising candidates, along with superconductivity and ion traps, for research and development toward the realization of large-scale quantum computers."

Any one of
these papers alone would be a significant achievement. The fact that all three
teams have reached the same milestone independently suggests that quantum
computing will now be surging ahead.

"You typically need error rates below 1 percent, to apply quantum error correction protocols," Morello says. "Having now achieved this goal, we can start designing silicon quantum processors that scale up and operate reliably for useful calculations."

The three
papers have been published in *Nature*. They can be found here, here and here.