A New Type of Qubit

Australian researchers have designed a new type of qubit – the building block of quantum computers – that they say will finally make it possible to manufacture a true, large-scale quantum computer.

Broadly speaking, there are currently a number of ways to make a quantum computer. Some take up less space, but tend to be incredibly complex. Others are simpler, but if you want it to scale up you’re going to need to knock down a few walls.

Some tried and true ways to capture a qubit are to use standard atom-taming technology such as ion traps and optical tweezers that can hold onto particles long enough for their quantum states to be analysed.

Others use circuits made of superconducting materials to detect quantum superpositions within the insanely slippery electrical currents.

The advantage of these kinds of systems is their basis in existing techniques and equipment, making them relatively affordable and easy to put together.

The cost is space – the technology might do for a relatively small number of qubits, but when you’re looking at hundreds or thousands of them linked into a computer, the scale quickly becomes unfeasible.

Thanks to coding information in both the nucleus and electron of an atom, the new silicon qubit, which is being called a ‘flip-flop qubit’, can be controlled by electric signals, instead of magnetic ones. That means it can maintain quantum entanglement across a larger distance than ever before, making it cheaper and easier to build into a scalable computer.

The Flip-Flop Qubit

“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,” says the researcher who came up with the new qubit, Guilherme Tosi, from the University of New South Wales in Australia.

The flip-flop qubit will sit in the sweet spot between those two extremes, offering true quantum entanglement across a distance of hundreds of nanometres.

In other words, this might be just what we’ve been waiting for to make silicon-based quantum computers scalable.

To be clear, so far we only have a blueprint of the device – it hasn’t been built as yet. But according to team leader, Andrea Morello, the development is as important for the field as the seminal 1998 paper in Nature by Bruce Kane, which kicked off the silicon quantum computing movement.

“Like Kane’s paper, this is a theory, a proposal – the qubit has yet to be built,” says 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.”

The flip-flop qubit works by coding information on both the electron AND nucleus of a phosphorus atom implanted inside a silicon chip, and connected with a pattern of electrodes. The whole thing is then chilled to near absolute zero and bathed in a magnetic field.

The qubit’s value is then determined by combinations of a binary property called spin – if the spin is ‘up’ for an electron while ‘down’ for the nucleus, the qubit represents an overall value of 1. Reversed, and it’s a 0.

That leaves the superposition of the spin-states to be used in quantum operations.

In flip-flop, researchers are able to control the qubit using an electric field instead of magnetic signals – which gives two advantages. It is easier to integrate with normal electronic circuits and, most importantly, it also means qubits can communicate over larger distances.

“To operate this qubit, you need to pull the electron a little bit away from the nucleus, using the electrodes at the top. By doing so, you also create an electric dipole,” says Tosi.

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

“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 easier to fabricate than atomic-scale devices, but still allows us to place a million qubits on a square millimetre.”

What this new flip-flop qubit means is a balance that could make future quantum computers small and potentially affordable.

“It’s a brilliant design, and like many such conceptual leaps, it’s amazing no-one had thought of it before,” says Morello.

The research has been published in Nature Communications.