Room temperature quantum computing on the way


Easy to make and stable enough at room temperature, are carbon nanosphere qubits the future of quantum computing? Belinda Smith reports.


Hooray for chemistry: naphthalene crystals, snapped in this polarised light micrograph, can be burnt to create carbon nanosphere qubits.
DR KEITH WHEELER / Getty Images

Quantum computing may be in reach "within a few years", thanks to tiny balls formed from burnt mothball ingredients.

A team from Switzerland, Australia and Germany showed carbon nanospheres produced from combusted naphthalene have quantum properties at room temperature – a far cry from other materials that need cooling to extremely low temperatures.

The work was published in Nature Communications.

Traditional computers, from your mobile phone through to the biggest supercomputers, use a binary system of bits – essentially, switches – that are either on or off and are represented as 1 and 0. They run through calculations one at a time.

The basic unit of quantum computing, a quantum bit or qubit, can also be on or off – or both at the same time. A qubit's information is denoted by its spin property: up, down or a superposition of both.

And this is where quantum computing gains its mind-boggling processing power. Instead of processing data sequentially, like a normal computer, they would be able to run millions of calculations at the same time. But stabilising the qubit is key, and is where a lot of quantum computing research stalls.

Qubits, in order to have those weird quantum properties, must be small – on the order of an atom or even an electron – so it's no surprise they're extremely delicate. The quantum information they hold is susceptible to external noise, such as vibrations from neighbouring atoms.

This is why some qubits, such as those made from silicon atoms, only work at super low temperatures. A warm atom is a jiggly atom – but cooling slows it almost to complete halt once you near absolute zero, or -273 ºC.

Of course, this requires some heavy-duty refrigeration – perhaps workable for large corporations such as IBM but not so much if quantum computing is to make it into homes.

Single qubits in the form of electrons in diamond have been manipulated at room temperature, but the problem with diamond is it's insulating – in other words, the electrons can't move so won't conduct electricity.

The technique involves extremely precise engineering too, where a single carbon atom must be removed from the diamond lattice and replaced with a nitrogen.

Yet other qubit materials need what's called "isotopic engineering", where they're peppered with particles fired from a nuclear reactor.

University of Sydney chemist Mohammad Choucair wondered: was there an easier way? Instead of trying to work with the stuff around a qubit, could he make a qubit that's stable at room temperature with any extra fiddling?

"I knew the key was in the material," he says. "I'm a chemist!"

Researchers have played around with carbon-based qubits for some years now, with graphene – a single-layer sheet of carbon atoms – and nanotubes made of rolled graphene promising candidates. But carbon nanospheres – tiny balls of carbon a few billionths of a metre wide – weren't so well-studied.

Choucair simply burnt naphthalene, the active ingredient in mothballs, in oxygen to create spheres of pure carbon, around 40 nanometres wide, filled with disordered carbon atoms.

Preparing conducting carbon nanospheres that operate as qubits at room temperature (right), simply by burning the active ingredient in mothballs, naphthalene (left).
Mohammad Choucair

"That's all you need to do," he says. "In fact, if you do more to them they don't work as well."

Their disordered interior is key to metallicity, or ability to conduct electricity – electrons can flow freely through the material.

Choucair and his colleagues – Bálint Náfrádi and László Forrócan from the Swiss Federal Institute of Technology in Lausanne and Klaus-Peter Dinse from the Free University of Berlin – zapped electrons in the carbon balls with magnetic fields at a balmy 27 ºC to set them spinning.

To be used in quantum computing, qubits must hold their spin longer than 100 nanoseconds. These carbon-based qubits retained their spin for 175 nanoseconds.

"It has all the good stuff," Choucair says. And now it's up to the engineers to build the computer, he adds. The next step is to incorporate what's called a quantum logic gate – essentially a switch, much like a transistor in a classical electrical circuit – to manipulate multi-qubit systems.

Still, the carbon-based qubits can be integrated into existing silicon technologies or be part of an all-carbon device – perhaps "within a few years", Choucair says.

Further reading:
Breakthrough for quantum computers
Will this quantum computer take down internet banking?
Triple entanglement paves way for quantum encryption

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.