Breakthrough for silicon quantum computers – Silicon-based quantum bit circuits achieve more than 99 percent reliability for the first time –

A crucial hurdle on the way to silicon quantum computers has been cleared: for the first time, three teams have constructed quantum circuits in silicon that significantly exceed the limit of at least 99 percent reliability. As a result, silicon-based quantum computers are moving up to the level of the conventional quantum systems and could even surpass them in terms of scalability and stability in the future, as the scientists report in the specialist journal “Nature”.

Quantum computers are no longer a dream of the future: the first quantum systems have already proven that they can solve tasks faster than conventional computers. And companies like Google and IBM are already offering commercial use of their quantum computers. These systems use either quantum bits from superconducting quasiparticles or ions trapped in magnetic traps as computing units.

But this has disadvantages: The qubits are relatively large and susceptible to interference – their state usually only remains stable for around 100 microseconds. This increases the error rate and makes it more difficult to expand quantum computers to more qubits and higher powers – the previous record is 127 qubits.

The new quantum systems adorn the cover of the current “Nature” issue. © Nature

Silicon instead of superconductors or ions

Quantum computers based on silicon could help. Because the semiconductor from which classic computer chips are made enables particularly small, stable and well defined qubits. These quantum dots consist of individual atoms or electrons whose spin direction can serve as a digital zero or one. In recent years, it has already been possible to individually control such silicon quantum bits at practical temperatures and to keep them coherent for up to 35 seconds.

“In the quantum world, 35 seconds is half an eternity,” explains Andrea Morello of the University of New South Wales. The problem, however, is that previous silicon-based quantum systems are too error-prone. Individual silicon qubits achieve a reliability of 99.9 percent. However, if two of them are combined to form a logic gate, the error tolerance drops below the 99 percent threshold that is necessary for correct calculations.

Electron spins and foreign atoms

But that has now changed: three research groups have independently developed silicon-based quantum circuits that achieve more than 99 percent reliability.
Two of these systems use the spins of individual electrons in a silicon-germanium matrix as quantum dots. The qubits of the third system are generated by spins from phosphorus impurities in the silicon. All three teams used magnetic fields to control the behavior of the spins and thus the qubits.

The researchers combined two of their qubits into logical NOT and CNOT gates and then used standardized algorithms to test the systems’ performance and susceptibility to errors. The result: The two-qubit gates achieved a reliability of between 99.35 and 99.65 percent, as reported by the three teams. “When errors become so rare, we can detect and correct them as soon as they occur,” explains Morello, leader of one of the three research teams.

quantum circuit
Morello and his team used two phosphorus impurity atoms and an electron as the basis for their quantum circuit. © Tony Melov / UNSW

From the qubit to the logic gate

For their quantum circuits, Morelli and his team built on previous experiments using phosphorus atoms injected into silicon as qubits. “Two-qubit operations with this are not trivial, because these two atoms cannot be directly coupled with each other,” the researchers explain. In order to still construct a logical gate, they used an additional electron as a mediator. Both phosphorus qubits can be entangled with this electron and thus enable an interconnection.

The two teams led by Akito Nori from the RIKEN research center in Japan and Xiao Yue from the Technical University of Delft used a different technology. Their qubits consist of single electrons in a heterogeneous matrix of germanium-enriched silicon and pure silicon. For the logic gates, they brought the quantum dots so close together that their spins interacted and entanglement became possible.

In order to test their systems and their susceptibility to errors, the three research groups had their silicon quantum computers implement several standardized algorithms – with success. In all three cases, the qubit gates proved to be reliable and operable.

Silicon quantum computers are moving up

According to the scientists, these advances prove that silicon-based quantum computers can also achieve the necessary error tolerance. “The presented results make spin qubits competitive for the first time with superconducting circuits and ion traps in terms of their performance,” says Seigo Tarucha from the RIKEN research center. “This demonstrates that silicon quantum computers are also promising candidates for large-scale quantum computers.”

Ada Warren and Sophia Economou from Virginia Polytechnic Institute see it similarly. In an accompanying commentary in Nature, they write: “Results from all three groups bring silicon-based quantum information processing a step closer to being a practical quantum computing platform – a status that few other systems, including superconducting qubits and ion traps, have achieved to date.” .” (Nature, 2022; doi: 10.1038/s41586-021-04182-y; doi: 10.1038/s41586-021-04273-w; doi: 10.1038 / s41586-021-04292-7)

Quelle: RIKEN, University of New South Wales

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