A quantum bit with ten times better stability than previously achieved has been created

‘Dressed’ quantum bit has ten times better stability

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Australian engineers have created a new quantum bit that remains in a stable superposition ten times longer than previously achieved.

The development, described in the latest issue of the journal Nature Nanotechnology, would allow quantum computers to perform calculations for a much longer period of time than previously possible.

The team from the University of New South Wales (UNSW) in Australia achieved the improvement by creating what they describe as a dressed quantum bit, or qubit. Unlike a conventional qubit, which is essentially a single particle and its quantum states, the dressed qubit consists of a spin of an atom in silicon, which is merged with an electromagnetic field.

In a quantum bit, information is encoded into the quantum states instead of into ones and zeros, such as in conventional computers. The major advantage is that an atom can be in multiple quantum states at the same time, which allows it to store more information. However, the instability of these quantum states has been a major problem.

“We have now implemented a new way to encode the information: we have subjected the atom to a very strong, continuously oscillating electromagnetic field at microwave frequencies, and thus we have ‘redefined’ the quantum bit as the orientation of the spin with respect to the microwave field,” explained Arne Laucht, a research fellow at the School of Electrical Engineering & Telecommunications at UNSW, and lead author of the paper.

“This quantum bit is more versatile and more long-lived than the electron alone, and will allow us to build more reliable quantum computers.”

Since the electromagnetic field steadily oscillates at a very high frequency, any noise or disturbance at a different frequency results in a zero net effect.

In experiments the system maintained its quantum properties ten times longer than previously developed systems, for up to 2.4 milliseconds. That considerably expands the time in which calculations could be performed.

“This new ‘dressed qubit’ can be controlled in a variety of ways that would be impractical with an ‘undressed qubit’,” said Andrea Morello, leader of the research team and a programme manager in the Centre for Quantum Computation & Communication Technology at UNSW.

“For example, it can be controlled by simply modulating the frequency of the microwave field, just like in an FM radio. The ‘undressed qubit’ instead requires turning the amplitude of the control fields on and off, like an AM radio. In some sense, this is why the dressed qubit is more immune to noise: the quantum information is controlled by the frequency, which is rock-solid, whereas the amplitude can be more easily affected by external noise.”

As the device is built on standard silicon technology, the researchers believe it could be relatively easy to incorporate into commercial-scale systems in future.

Quantum technology promises to revolutionise computing and enable solving complex operations that current computers would take ages to tackle. That includes searching extremely large databases or modelling complex systems such as biological systems or chemical components for medical research.

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