Australian engineers have created a new quantum bit which remains in a stable superposition for 10 times longer than previously achieved, expanding the time during which useful calculations could be performed in a future silicon quantum computer.
The new bit, known as a ‘dressed qubit’, was developed by a team at the University of New South Wales (UNSW).
“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.
“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.”
The most recent research has implemented a new way to encode the information. They subjected the atom to a very strong, continuously oscillating electromagnetic field at microwave frequencies, and ‘redefined’ the quantum bit as the orientation of the spin with respect to the microwave field.
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 measured a dephasing time of 2.4 ms, a result that is 10 times 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’,” said 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,” said Morello.
He said, 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.
[Andrea Morello (top) and researcher Arne Laucht at work in the quantum lab. Image: UNSW]