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How do you make a qubit 10 times as stable? Dress it up for work

No joke. Electromagnetic fields could help achieve dream of quantum computing

By Katyanna Quach, 18 Oct 2016

Dressing qubits in an electromagnetic field can make them 10 times more stable and able to perform more calculations over time in future quantum computers, according to new research in Nature Nanotechnology.

Qubits - or quantum bits - hold information in quantum computers just like bits do in conventional computers. Instead of two states, 0 and 1, however, a qubit can occupy an infinite number of states as its quantum states are entangled with one another.

The mishmash of quantum states interacting with its environment means that its state changes over time. A property known as quantum coherence erodes over time and the relation between quantum states and the information it holds vanishes, making quantum computers tricky to use.

Researchers from the University of New South Wales (UNSW) and Keio University have increased qubit stability by "dressing" it with electromagnetic fields.

The research improves upon previous work by the Centre for Quantum Computation & Communication Technology (CQC2T) at UNSW as it uses a varying magnetic field. The new coherence time of 2.4 milliseconds - 10 times longer than what has been previously achieved by researchers at CQC2T with a standard qubit.

Andrea Morello, leader of the research team, said: "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.

“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 high frequency of the varying magnetic field makes it harder for noise oscillating at a different frequency to disturb the quantum system. It can also be controlled better. Researchers can tune the system like an FM radio by changing the frequency of the field, says the researcher.

“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," Morello said.

Quantum computing is a competitive field with researchers racing ahead to improve the capabilities of quantum computers, promising to unleash the next generation of computers which are much faster and powerful. ®

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