Quantum computing has taken a step forward with the development of a programmable quantum processor made with silicon.

The team used microwave energy to align two electron particles suspended in silicon, then used them to perform a set of test calculations.

By using silicon, the scientists hope that quantum computers will be more easy to control and manufacture.

The research was published in the journal Nature.

The old adage of SchrÃ¶dinger’s Cat is often used to frame a basic concept of quantum theory.

We use it to explain the peculiar, but important, concept of superposition; where something can exist in multiple states at once.

For Schrodinger’s feline friend – the simultaneous states were dead and alive.

Superposition is what makes quantum computing so potentially powerful.

Standard computer processors rely on packets or bits of information, each one representing a single yes or no answer.

Quantum processors are different. They don’t work in the realm of yes or no, but in the almost surreal world of yes and no. This twin-state of quantum information is known as a qubit.

**Unstable liaisons**

To harness their power, you have to link multiple qubits together, a process called entanglement.

With each additional qubit added, the computation power of the processor is effectively doubled.

But generating and linking qubits, then instructing them to perform calculations in their entangled state is no easy task. They are incredibly sensitive to external forces, which can give rise to errors in the calculations and in the worst-case scenario make the entangled qubits fall apart.

As additional qubits are added, the adverse effects of these external forces mount.

One way to cope with this is to include additional qubits whose sole role is to vet and correct outputs for misleading or erroneous data.

This means that more powerful quantum computers – ones that will be useful for complex problem solving, like working out how proteins fold or modelling physical processes inside complex atoms – will need lots of qubits.

Dr Tom Watson, based at Delft University of Technology in the Netherlands, and one of the authors of the paper, told BBC News: “You have to think what it will take to do useful quantum computing. The numbers are not very well defined but it’s probably going to take thousands maybe millions of qubits, so you need to build your qubits in a way that can scale up to these numbers.”

In short, if quantum computers are going to take off, you need to come up with an easy way to manufacture large and stable qubit processors.

And Dr Watson and his colleagues thought there was an obvious solution.

**Tried and tested**

“As we’ve seen in the computer industry, silicon works quite well in terms of scaling up using the fabrication methods used”, he said.

The team of researchers, which also included scientists from the University of Wisconsin-Madison, turned to silicon to suspend single electron qubits whose spin was fixed by the use of microwave energy.

In the superposition state, the electron was spinning both up and down.

The team were then able to connect two qubits and programme them to perform trial calculations.

They could then cross-check the data generated by the quantum silicon processor with that generated by a standard computer running the same test calculations.

The data matched.

The team had successfully built a programmable two-qubit silicon-based processor.

Commenting on the study, Prof Winfried Hensinger, from the University of Sussex, said: “The team managed to make a two qubit quantum gate with a very respectable error rate. While the error rate is still much higher than in trapped ion or superconducting qubit quantum computers, the achievement is still remarkable, as isolating the qubits from noise is extremely hard.”

He added: “It remains to be seen whether error rates can be realised that are consistent with the concept of fault-tolerant quantum computing operation. However, without doubt this is a truly outstanding achievement.”

And in an accompanying paper, an international team, led by Prof Jason Petta from Princeton University, was able to transfer the state of the spin of an electron suspended in silicon onto a single photon of light.

According to Prof Hensinger, this is a “fantastic achievement” in the development of silicon-based quantum computers.

He explained: “If quantum gates in a solid state quantum computer can ever be realised with sufficiently low error rates, then this method could be used to connect different quantum computing modules which would allow for a fully modular quantum computer.”

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