# Prospect of a silicon-based quantum computer in our lifetime!

Researchers have made a crucial step towards realizing a solid-state quantum computer. They built a quantum logic gate on silicon and demonstrate the possibility to perform calculations between two quantum bits (‘qubits’ in short). The fact that the quantum bits use silicon-based technology is significant as silicon is a material we can readily fabricate and tailor, giving boost to the idea that realizing a working quantum computer is not that far-fetched, after all.

The remarkable advance was made by a group of scientists at the University of New South Wales (UNSW) in Sydney (Australia) and published yesterday in the journal *Nature*. “We’ve demonstrated a two-qubit logic gate – the central building block of a quantum computer – and, significantly, done it in silicon. Because we use essentially the same device technology as existing computer chips, we believe it will be much easier to manufacture a full-scale processor chip than for any of the leading designs, which rely on more exotic technologies,” says Andrew Dzurak, leader of the study.

In a classical computer, data is represented as binary bits, 0’s and 1’s, which are realized via transistors in integrated circuits – a modern smartphone or tablet can have up to one billion transistors with each transistor being less than 100 billionths of a metre in size. In the study conducted by the team at UNSW, two of these silicon-based ‘classical transistors’ were reconfigured to behave as quantum bits. “We’ve morphed those silicon transistors into quantum bits by ensuring that each has only one electron associated with it. We then store the binary code of 0 or 1 on the ‘spin’ of the electron, which is associated with the electron’s tiny magnetic field,” leading author of the study Dr Menno Veldhors added.

The advantage of a quantum computer over a classical one is based on the concept of ‘superposition’. While in classical computing each bit can only be either ‘0’ or ‘1’, in quantum computing a qubit |Ψ> can exist as a linear combination of both ‘0’ and ‘1’, specifically: |Ψ> = a|0> + b|1>, with the probability of the outcome for |0> being |a|^{2}, for |1> being |b|^{2}, and the total probability such that |a|^{2} + |b|^{2} = 1.

All right, but what are the actual benefits of this ‘exotic quantum superposition’? Well, for a start each qubit can, in principle, be in any superposition of the states |0> and |1> – via any of the infinite combinations of coefficient ‘a’ and ‘b’ – meaning that we have a much larger number of possible ways to store information. From this, it also derives that a quantum computer can process multiple operations at the same time* achieving true parallelism, in contrast with the insurmountable limit of serial processing (only one bit at a time) of a classical computer.

Up until now, it had not been possible to make two quantum bits interact with one another – thereby creating a logic gate – using silicon (it has however been achieved in other systems including: single photons, trapped ions, superconducting circuits, single colour centres in diamond and silicon, and superconducting quantum dots). The researchers at UNSW managed to do that. The implications of this achievement are remarkable: all the physical building blocks to realize a silicon-based quantum computer have now been successfully constructed. Engineers can finally begin the task of designing and assembling a functioning silicon-based quantum computer using a material, silicon, which we have become masters at fabricating and tailoring.

A full-scale quantum processor would find applications in many fields ranging from finance, to security, database searching and also healthcare, where it could help developing new medicines by dramatically accelerating the computer-assisted design of pharmaceutical compounds (or, for what matters, soil fertilizers, something that really matters to our future but it is rarely spoken of).

* The more curious reader can find additional information about the faster computational power of quantum computers by reading about *quantum computing amplitude amplifications* and *Grover’s algorithm*.

#### Carlo Bradac

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