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The Phosphorous Atom Quantum Computing Machine

An Australian team unveils the fundamental building block of a scalable quantum computer that could be embedded in today’s silicon chips.

Back in the late 90s, a physicist in Australia put forward a design for a quantum computer. Bruce Kane suggested that phosphorus atoms embedded in silicon would be the ideal way to store and manipulate quantum information.

His idea was that the nucleus of the phosphorus atom could store a single qubit for long periods of time in the way it spins. A magnetic field could easily address this qubit using well-known techniques from nuclear magnetic resonance spectroscopy. That would allow single-qubit manipulations but not two-qubit operations, because nuclear spins do not interact significantly of each other.

For that, he suggested transferring the spin to an electron orbiting the phosphorus atom, which would interact much more easily with an electron orbiting a nearby phosphorus atom. Two-qubit operations would then be possible by manipulating the two electrons with electric fields.

The big advantage of the Kane quantum computer that excited many physicists at the time was that it was scalable. Since each atom could be addressed individually using standard electronic circuitry, it is straightforward to increase the size of the computer by adding more atoms and their associated electronic paraphernalia and then to connect it to a conventional computer.

Building a Kane quantum computer has become almost an obsession in Australia, where some 100 researchers have been working on the problem for over a decade.

They’ve made breakthroughs such as being able to implant phosphorus atoms at precise locations in silicon using a scanning tunnelling microscope. They’ve also been able to address the nuclear spins of these phosphorus atoms using powerful magnetic fields.

But the big unsolved challenge has been to find a way to address the spin of an individual electron orbiting a phosphorus atom and to read out its value.

Today, Jarryd Pla at the University of New South Wales in Sydney, and a few pals, say they’ve conquered this task the first time.

These guys implanted a single phosphorus atom in a silicon nanostructure and placed it in a powerful magnetic field at a temperature close to absolute zero. They were then able to flip the state of an electron orbiting the phosphorus atom by zapping it with microwaves.

The final step, a significant challenge in itself, was to read out the state of the electron using a process known as spin-to-charge conversion.

The end result is a device that can store and manipulate a qubit and has the potential to perform two-qubit logic operations with atoms nearby; in other words the fundamental building block of a scalable quantum computer.

“These results indicate that the electron spin of a single phosphorus atom in silicon is an excellent platform on which to build a scalable quantum computer,” say the team.

That looks to be a big advance for Australia’s effort to make a scalable quantum computer.

However, some stiff competition has emerged in the 15 years since Kane published his original design. In particular, physicists have found a straightforward way to store and process quantum information in nitrogen vacancy defects in diamond.

Then there is D-Wave Systems, which already manufactures a scalable quantum computer working in an entirely different way that it has famously sold to companies such as Lockheed Martin and Google.

The big advantage of the Australian design is its compatibility with the existing silicon-based chip-making industry. In theory, it will be straightforward to incorporate this technology into future chips.

Whether that’s what will happen in practice is hard to tell. Being first to market is a big advantage in the high-tech world and the Australian design is still years away from emerging from the labs.

There are plenty of hurdles to come that could down any of these emerging technologies. This race is far from over.

Ref: A single-Atom Electron Spin Qubit in Silicon

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