Once again the case of silicon

In a conventional digital computer, information is encoded in discrete bits which have the value of 0 or 1. In a quantum computer, information is encoded in qubits (quantum bits) which can exist as a superposition of 0 and of 1. A variety of physical systems – from superconducting circuits to ions held in traps – can be used to construct these qubits. But arguably the platform with the greatest potential to deliver large-scale quantum computing is the one that has delivered large-scale conventional computing: silicon.

Silicon qubits can be created using electron spins or confined holes in quantum dot structures, or with the nuclear spins of individual dopant atoms. A potential silicon-based quantum computing system can also be broken down into three distinct layers: a quantum processing unit (which uses an array of qubits), a quantum-classical interface, and a classical processing layer. As Fernando Gonzalez-Zalba and his colleagues explored in a review article in the December issue of Natural electronics¹all of these different layers can, in principle, be fabricated using the complementary metal-oxide-semiconductor (CMOS) technology that is currently at the heart of the electronics industry.

For now, demonstrations with silicon qubits are limited to small-scale devices, but are impressive nonetheless. Earlier this year, for example, reports of two-qubit gates with fidelities above 99% – and above the theoretical threshold required for fault-tolerant quantum computing – were published.^2,3,4which was closely followed by a report on controlling a six-qubit system⁵. A fully scaled quantum computer will require millions of embedded qubits, and so establishing the industrial manufacturing potential of such systems is a key step – and one explored in this issue of Natural electronics.

In the first of two papers on the subject, Dominik Zumbühl, Andreas Kuhlmann and their colleagues from the University of Basel and IBM Research in Zürich report the creation of hole spin qubits in field-effect transistor structures ( FinFET) similar to those used in advanced technologies. integrated circuits. (Also see the attached News & Views article on the work of Romain Maurand and Xavier Jehl at CEA-Grenoble.) temperatures above 4 K. These relatively high temperatures could allow integration of quantum hardware and electronics control on the same chip, an important requirement for scaling such systems.

In the second paper, Lieven Vandersypen, James Clarke and colleagues report the fabrication of spin qubits in a 300mm semiconductor fab using all-optical lithography and all-industrial processing. The approach offers high yields and the team – a collaboration between researchers from Delft University of Technology and Intel researchers – provides a powerful demonstration of how such silicon qubits can be created with advanced semiconductor manufacturing methods.