Physicists invent the ‘flip-flop’ qubit – an entirely new form of quantum computing

A completely new design for quantum computing could make the technology much easier and cheaper to achieve, according to new research.

The invention involves a radically new kind of qubit, the quantum mechanical version of a classical computer’s ‘bit’.

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 The new idea means manufacturing quantum chips on an industrial scale might be much cheaper and easier than we thought possible. This is because it allows entanglement between qubits to be maintained at the perfect distance for them to be built on chips. The research has been published in Nature Communications.

In computing, information is stored in bits of either ‘1’ or ‘0’, but in quantum computing, the qubit can be any combination of the superposition of these two values. This relies on the quantum mechanical principle of entanglement, which means two atoms can be linked such that anything happening to one affects the other. It also means a qubit can store much more information than a bit, which is what makes the idea of quantum computing so appealing. 

There have been many attempts to make qubits, including trapped ions and superconducting materials. Many of these use either the atoms’ electron or nucleus property of spin. This spin can be controlled by a magnetic field, giving them the name spin qubits. 

But the problem with spin qubits is keeping a hold of entanglement. In order to create chips, the qubits need to be spaced at a distance of only 10-20 nanometres, or 50 atoms apart, and this does not work on a computer chip.

“If they’re too close, or too far apart, the ‘entanglement’ between quantum bits – which is what makes quantum computers so special – doesn’t occur,” said Guilherme Tosi, from the University of New South Wales in Australia, who invented the new qubit.

Flip-flops

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The new qubits are called ‘flip-flop qubits’ and they use the electron and the nucleus of a phosphorus atom. The difference is they are controlled by an electrical signal instead of a magnetic one. Electric signals are much easier to distribute, meaning the qubits can sit at exactly the correct distance apart.

The qubit works by pulling the electron away from the nucleus using an electric field, creating an electric dipole – with a positive charge on one side and a negative on the other. 

“This is the crucial point,” said Andrea Morello, leader of the team behind the design. “These electric dipoles interact with each other over fairly large distances, a good fraction of a micron, or 1,000 nanometres.”

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“This means we can now place the single-atom qubits much further apart than previously thought possible. So there is plenty of space to intersperse the key classical components such as interconnects, control electrodes and readout devices, while retaining the precise atom-like nature of the quantum bit.” 

“It’s a brilliant design, and like many such conceptual leaps, it’s amazing no-one had thought of it before,” said Morello.  “What Guilherme and the team have invented is a new way to define a ‘spin qubit’ that uses both the electron and the nucleus of the atom.”

Morello added that Tosi’s concept is as significant as a seminal 1998 paper in Nature, by Bruce Kane. That paper found a new architecture that could make a silicon-based quantum computer a reality, triggering Australia’s race to build a quantum computer. 

“Like Kane’s paper, this is a theory, a proposal – the qubit has yet to be built,” said Morello. “We have some preliminary experimental data that suggests it’s entirely feasible, so we’re working to fully demonstrate this. But I think this is as visionary as Kane’s original paper.”

Images: Tony Melov/UNSW

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