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New quantum bit developed by UNSW

An international research team, led by Professor Andrea Morello of the University of New South Wales (UNSW), has developed a new type of quantum bit, or “qubit,” called the “flip-flop” qubit. The flip-flop qubit is unique in that it combines the precision of single-atom qubits with the control of electrical signals, much like those found in traditional computer chips.

new qubit
The nuclear spin (“n”, in orange) and the electron spin (“e”, in blue) flip-flop with respect to each other while always pointing in opposite directions.

“Our group has had excellent qubits for a decade, but we wanted something that could be controlled electrically, for maximum ease of operation. So we had to invent something completely new,” said Professor Morello. The team at UNSW was the first to show that the spin of an electron and the nuclear spin of a single phosphorus atom in silicon could both be used as qubits, but they found that magnetic fields were difficult to control at the nanometer scale.

The team had a breakthrough when they realized that by defining the qubit as the combined orientation of the electron and nucleus, it could be controlled with electric fields alone. “This new qubit is called ‘flip-flop’ because it’s made out of two spins belonging to the same atom – the electron and the nuclear spin – with the condition that they always point in opposite directions,” said Dr Rostyslav Savytskyy, one of the lead authors of the paper published in Science Advances.

“Most importantly, such electron displacement is obtained simply by applying a voltage to a small metallic electrode, instead of irradiating the chip with an oscillating magnetic field. It’s a method that much more closely resembles the type of electrical signal normally routed within conventional silicon computer chips,” said Dr Tim Botzem, another lead experimental author.

flip flop qubit
Experimental measurement of the probability of finding the nuclear and the electron spins pointing ‘up’. The data shows clearly the flip-flopping dynamics, where the two spins swap orientation multiple times as the electrical driving signal is progressively applied.

The electrical control of the flip-flop qubit also has an important side effect. When an electron is displaced away from a nucleus, an electric dipole is formed. When two or more electric dipoles are placed close to each other, a strong electrical coupling is created, allowing for multi-qubit quantum logic operations. “This could be a game-changing development,” said Professor Morello.

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