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Silicon quantum computers making progress –

In earlier quantum computers, circuits or superconducting ions in magnetic traps usually acted as basic units of computation. On the other hand, quantum computers based on the common chip material silicon were missing because they could not provide sufficiently reliable quantum parts and circuits. That has now changed: Three research groups are offering silicon-based quantum circuits that for the first time exceed the threshold of at least 99 percent reliability. These systems, which depend on the spin of electrons and foreign atoms in a semiconductor, still include only two quantum bits. However, the technology can be scaled up more easily than conventional quantum computers.

Quantum computers are considered the computers of the future because, thanks to quantum physical phenomena such as entanglement and superposition, they can solve many tasks more efficiently and quickly than conventional computers. The first quantum systems, such as Google’s “Sycamore” quantum computer and China’s “Jiuzhang” quantum computer, may already have demonstrated this quantum supremacy. Commercially used quantum computers already exist, including one from IBM. Until now, the size and performance of these quantum computers have been limited because their qubits, made up of virtual particles in superconducting coils or trapped ions, rapidly lose their coherence. Their quantum states usually remain stable only for about 100 microseconds, and the higher the number of qubits, the greater the disruptive effects. In addition, superconducting qubits are relatively large and thus difficult to fit on a chip – this limited the miniaturization of these previously very large computers.

Silicon and reliability problem

Silicon-based quantum computers could help. Since semiconductors are not only the common material for traditional computer technology, they are also well-suited to accommodate relatively stable and compact qubits. These usually consist of quantum dots – individual atoms or electrons whose spin direction can act as a digital zero or one. Such quantum dots can be created by cleverly combining silicon with foreign atoms such as germanium or phosphorous and using magnetic fields. The resulting bits are much smaller than those used previously and remain constant for up to 35 seconds. “In the quantum world, 35 seconds is half an eternity,” says Andrea Morello of the University of New South Wales. The problem, however, is that previous silicon-based quantum systems were not sufficiently reliable. Their results were well above the 99 percent accuracy required for such circuits.

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But that has now changed: Three research teams have successfully developed silicon-based quantum computers that exceed 99%. Their circuits, consisting of two qubits in the form of a CNOT gate, achieve an average reliability of 99.5 percent, and one qubit up to 99.95 percent, they report. Researchers led by Akito Nori of the RIKEN Research Center in Japan and a team led by Xiao Yue of Delft Technical University use spinning electrons in a silicon-germanium matrix as quantum dots. A third research team led by Morello created their own qubits using phosphorous impurities in silicon. The three groups used magnetic fields to control the behavior of the spins and thus the qubits.

Electron spins and foreign atoms

In order to turn quantum dots into an electric circuit, the two teams led by Nuri and Yue brought electrons used as qubits so close together that quantum mechanical wave functions overlap and their spins interact as a result. As a result, the spin electrons were entangled and thus fulfilled an important requirement for desired computations. The team led by Morello and first author Mateusz Madzik used a pair of phosphorous atoms that were paired together via a shared electron. In order to test their systems for their vulnerability to errors, research groups have asked them to implement several standardized algorithms — successfully. In all three cases, qubit gates proved to be reliable and operable.

According to the scientists, these advances prove that silicon-based quantum computers can also achieve the necessary fault tolerance. “The results presented make spin qubits competitive for the first time with superconducting circuits and ion traps in terms of their performance,” says Seigo Tarucha of the RIKEN Research Center. “This shows that silicon quantum computers are also a promising candidate for large-scale quantum computers.” In einem begleitenden Kommentar in „Nature” schreiben sie: „Die Resultate aller drei Gruppen bringen die siliziumbasierte Quanten-Informationsverarbeitung einen Schritt näher an eine praktisch nutzbare Quantencomputers-intarte Quantencomputers-Plat to have.”

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What: nature, doi: 10.1038/s41586-021-04182-y; doi: 10.1038/s41586-021-04273-w; doi: 10.1038/s41586-021-04292-7