Building a quantum computer has been called the ‘space race of the 21st century’ – a difficult and ambitious challenge with the potential to deliver revolutionary tools for tackling otherwise impossible calculations, such as the design of complex drugs and advanced materials, or the rapid search of massive, unsorted databases.
Its speed and power lie in the fact that quantum systems can host multiple ‘superpositions’ of different initial states, which in a computer are treated as inputs which, in turn, all get processed at the same time.
Quantum computing is one of the great challenges of the 21st century , manipulating nature at a subatomic level and pushing into the very edge of what is possible.
Professor Mark Hoffman, Dean of UNSW Engineering
“The greatest hurdle in using quantum objects for computing is to preserve their delicate superpositions long enough to allow us to perform useful calculations,” said Andrea Morello, leader of the research team and a Program Manager in the Centre for Quantum Computation & Communication Technology (CQC2T) at UNSW. “Our decade-long research program had already established the most long-lived quantum bit in the solid state, by encoding quantum information in the spin of a single phosphorus atom inside a silicon chip, placed in a static magnetic field.”
What Laucht and colleagues did was push this further: “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.”
The results are striking: 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. The researchers achieved an improvement by a factor of 10 in the time span during which a quantum superposition can be preserved.
Specifically, they measured a dephasing time of T2*=2.4 milliseconds – a result that is 10-fold better than the standard qubit, allowing many more operations to be performed within the time span during which the delicate quantum information is safely preserved.
“This new ‘dressed qubit’ can be controlled in a variety of ways that would be impractical with an ‘undressed qubit’,”, added Morello. “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”.
Since the device is built upon standard silicon technology, this result paves the way to the construction of powerful and reliable quantum processors based upon the same fabrication process already used for today’s computers.