Australian researchers have developed a new building block for a quantum computer, bringing the technology a tantalising step closer.
The University of New South Wales team, led by quantum physicist Michelle Simmons, has built the first two-qubit gate between atom qubits in silicon, they report in the journal Nature.
The quantum building block, which is capable of performing an operation of 0.8 nanoseconds, is around 200 times faster than existing spin-based two-qubit gates in silicon.
The work is the realisation of a theoretical paper they published 20 years ago where they said using atom qubits — or quantum bits, the quantum equivalent of a traditional bit or transistor — would be one of the best ways to build a quantum computer in silicon.
Quantum computing promises to allow us to process information much more efficiently, using fundamental physics at the sub-atomic scale, to solve problems in hours or minutes what would have taken conventional computers far longer.
Unlike traditional bits, which represent information as either a 1 or a 0, qubits can be 1 or 0, or both 1 and 0 at the same time. This is called a superposition, when the electron spin states on the qubit are a mixture of up and down at the same time.
It’s this quantum state, or coherence, that you want to hold for as long as possible when you’re building components for a quantum computer.
“The two things you need are superposition … and then entanglement where the state of one qubit depends on the other, and you create an entirely new state that doesn’t exist in the classical world,” Professor Simmons said.
To create the device, the researchers embedded two single atoms of phosphorus inside a silicon matrix in very close proximity to each other.
So what does this mean?
“It’s a very exciting result in a really difficult physical system to work with,” said quantum physicist Andrew White of the University of Queensland, who was not involved in the research.
There are many different ways you can entangle qubits, using different physical architectures such as trapped ions, superconducting circuits, single photons, neutral atoms, or silicon.
“What’s new about this is it was a different approach to making the gate,” Professor White said.
“An entangling gate is the heart of any quantum computer. Without one of these gates you don’t have a quantum computer.”
There are a whole lot of other things you need to do as well, but a qubit gate is one of the key things that makes a quantum computer entirely different from a classical computer.
Director of the Sydney Microsoft Quantum Laboratory and quantum physicist at the University of Sydney David Reilly, who was not involved in the study, was also excited by the research.
He said it builds upon previous work in 2005 using electron spins in a semiconductor that performed the same kind of coupling about 10 times faster in a gallium arsenide system.
And the University of New South Wales also demonstrated a two-qubit gate in silicon in 2015, when Professor Andrew Dzurak’s group used a metal oxide semiconductor device, as opposed to the atom qubits used by Professor Simmons’ team.
“The atom approach is following in the footsteps and showing that it too can demonstrate these types of results,” Professor Reilly said.
The search for the perfect qubit
With so many different approaches to quantum computing being pursued around the world, which is likely to win out?
“My personal opinion is that the world yet doesn’t have in its hand the perfect qubit that’s going to scale up into building a quantum computer,” Professor Reilly said.
“We’re still in the business to trying to find the best technology and results to date — including this one — don’t paint an obvious picture of scale up to the number of qubits you need to actually do something useful.”
Part of the problem is you need to make qubits with extremely high fidelity, or low margin for error.
“What appears to be a small error when you multiply that out for many billions of qubits it actually leads to an inability to build a machine that’s actually going to tackle real world problems,” Professor Reilly said.
Currently the fidelity of this two-qubit gate has been measured at around 90 per cent, which the team is currently working on improving through their manufacturing process.
But the benefit of working in silicon, Professor White said, is that provides a way to get a million qubits into a small physical footprint.
“That’s the reason why it’s exciting,” he said.
“It’s not going to bring us quantum computers next year, but it’s a great engineering feat.”
Professor Simmons said silicon brings together many benefits.
“Our bet 20 years ago was that we could pick something that had incredibly long coherence time, that would be fast, but it would also be manufactured well and that’s the kind of key result that is all of those combined,” she said.
“That’s what atom qubits will have the promise to do.”
“The atom itself is very small, and as a consequence it should have a very long coherent time,” she said.
“And if you can get them close enough together you should be able to have very fast operations between them.”
Now that they’ve made a two-qubit gate, the team’s next step will be to entangle information across 10 qubits, which they think they will be able to achieve in the next three to four years.
The line between easy and hard problems has changed because of quantum computing, said Professor Reilly, which is reason for a lot of excitement.
Quantum computers could help us tackle problems like finding the catalyst that will allow us to manufacture fertiliser more efficiently or better understand the world at the levels of atoms.
But Professor Reilly cautioned there was still more work we need to do to figure out what sort of problems we should be tackling with it.
“A quantum computer is not going to replace a classical computer. It’s going to be good for certain types of things, but it’s not a machine that just eats up big data,” he said.
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