News in Science

New accuracy record set for quantum computing

Quantum record Australian scientists have developed the first silicon quantum technology capable of holding data with over 99 per cent accuracy.

The breakthrough, reported in two papers published in the journal Nature Nanotechnology, was achieved using two different types of silicon-based quantum bits or qubits, the basic information storing element in a quantum computer.

"We have demonstrated that with silicon qubit we can have the accuracy needed to build a real quantum computer," says Professor Andrew Dzurak of the University of New South Wales, who is one of the authors on both papers.

"That's the first time this has been done in silicon," he adds.

One method is based on advances in previous research using phosphorous atoms as qubits.

The team had previously only achieved 50 per cent accuracy using phosphorus atoms in silicon.

"In natural silicon each atom also has its own spin which affects the phosphorous atom, which is why the accuracy was only 50 per cent," says Dzurak.

"We solved the problem by removing all the silicon 29 isotopes that have magnetic spin leaving only silicon 28, which has no magnetic spin to influence the phosphorous, giving us an accuracy of 99.99 per cent."

The second method takes a new approach by turning a silicon transistor into an "artificial atom" qubit giving an accuracy of 99.6 per cent.

Transistors work by electrons flowing through an electronic gate that can be turned on or off, resulting in binary zeros and ones.

"What we've done is make a silicon transistor with just one electron trapped in that transistor," says Dzurak.

"This lets us use exactly the same sort of transistor that we use in computer chips and operate it as a qubit, opening the potential to mass-produce this technology using the same sort of equipment used for chip manufacturing."

The authors were also able to increase the time over which a silicon quantum system retains information, known as coherence time.

"In solid-state systems these times are typically measured in nano or micro seconds before the information gets lost," says Dzurak.

"We are getting a 30-second coherence time, which on the time-scale of doing calculations is an eternity."

Outstanding challenges

Silicon-based quantum devices have made major advances in the last five years, but there's still a long way to go to catch up with more developed technologies like super-conducting circuits and atomic ion traps, says Professor Michael Biercuk of the University of Sydney.

Biercuk, who was not involved in this research, has been part of a team working on quantum computers based on ion traps, which use a layer of charged beryllium atoms with interacting spins acting as qubits.

"The record for this kind of [atomic ion trap] quantum mechanical spin coherence is more like 15 minutes, and those are old results," says Biercuk.

"It will exciting to see how the [silicon] team works towards addressing the pretty significant outstanding challenges that have to be overcome in order to make these devices meet the potential many see in silicon-based quantum technologies."

"It's in the potential to scale to large systems that semiconductor devices have a real leg up on ion traps. The big challenge for ion traps is that we don't yet know how we can build large scale systems. Those problems are much better understood in silicon."

While ion traps have long coherence times and good quantum control, Dzurak points to the challenges faced in miniaturising the technology on to chips.

"The technology in building ion traps is very different from that in integrated circuitry that we use," says Dzurak.

"Silicon is what the industry is comfortable with, and has the longest solid-state coherence times so the information stays alive the longest and we can get the most accuracy.

"We can go better, these are only our first experiments."