Nanomechanical qubit could be highly stable

A quantum bit based on a vibrating carbon nanotube and a pair of quantum dots could be unusually resistant to noise. Although the new nanomechanical qubit is currently at the proposal stage, calculations by Fabio Pistolesi of the French National Centre for Scientific Research (CNRS) at the University of Bordeaux and colleagues in the US and Spain indicate that its so-called “decoherence time” – a measure of how long fragile quantum information can survive in a noisy environment – would be remarkably long, making it an attractive platform for quantum computing.

Quantum computers can, in principle, solve certain problems much faster than classical computers because they exploit a quantum system’s ability to be in a superposition of two or more states (as opposed to classical bits that have only 0 and 1 states). Promising candidates for such quantum bits, or qubits, include superconducting circuits, trapped ions, defects in solid materials and “artificial atoms” known 

as quantum dots.

New qubit platform

In the proposed nanomechanical qubit, a suspended carbon nanotube acts as a resonator, and its vibrations couple to a double quantum dot that forms within the nanotube itself. This double quantum dot has discrete electronic states, and the coupling between them allows the resonator to become strongly anharmonic – that is, the frequency of its oscillations depend strongly on their amplitude. In such an oscillator, even the tiniest change in the resonator’s amplitude is easily detected. This amplitude, Pistolesi explains, can then be used to store quantum information.

“Essentially the minimum possible oscillating amplitude (the quantum ground state) corresponds to the 0 of the qubit, while the next smallest amplitude (the first excited state) corresponds to 1,” he says. “These two states could easily be read out by a microwave signal. The fact that the frequency of the oscillator changes when its amplitude changes allows us to detect and manipulate the qubit.”

The qubits in such a platform would stay coherent for a long time, Pistolesi tells Physics World,because the information is stored in their mechanical oscillation amplitude and the oscillator can perform millions of oscillations before it starts to become damped.

Generating anharmonicity

While researchers knew that qubits based on such a mechanical oscillator would have a long coherence time, they were not sure how to introduce enough anharmonicity into them to make them controllable. Pistolesi and colleagues had previously found strong anharmonicity in a similar system (a carbon nanotube coupled to a single electron transistor). They undertook their present study to find out if they could also generate such anharmonicity in an oscillating nanotube coupled to a double quantum dot.

As well being a promising qubit platform, the oscillators could also be used as highly accurate quantum sensors thanks to their sensitivity to classical forces. Such devices might be employed to detect faint changes in acceleration, gravity, magnetic moments and electric fields.

The researchers, who report their work in Physical Review X, now plan to fabricate the qubit they have proposed and test its performance experimentally.

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