&Cartridge; physics 14, p96
A hanging carbon nanotube coupled to a double quantum dot forms a mechanical oscillator that serves as a qubit.
Many of today’s quantum computers encode information in fragile quantum states that are difficult to maintain and scale. A qubit that instead alternates between two mechanical states could be used for quantum computing and to record the high quality and practical manipulability of mechanical oscillators. Fabio Pistolesi from the French National Center for Scientific Research (CNRS) and the University of Bordeaux and his colleagues are now proposing to implement such a qubit in the form of a carbon nanotube, the bending behavior of which is determined by the electronic states of two quantum dots  . The design could be used to develop quantum devices with large numbers of qubits and long qubit decoherence times.
A mechanical oscillator can be made into a qubit by introducing a controlled deviation from a simple harmonic oscillation. This deviation ensures that a driving force can only excite certain energy levels that correspond to the two states of the qubit. In the device proposed by Pistolesi and his colleagues, this deviation would be achieved by coupling the bending modes of a suspended carbon nanotube with two quantum dots – “artificial atoms” with discrete electronic states – in the nanotube itself. Theoretically, the researchers show that the qubit in a certain state (one of the two lowest oscillation amplitudes of the nanotube) could be produced by manipulating the electron localization of the quantum dots. The two states could easily be read by a microwave signal. The researchers also show how a logic gate could be implemented to entangle two qubits.
Their robust quantum states make mechanical oscillators a promising platform for quantum computers. However, due to their sensitivity to classical forces, according to the researchers, the oscillators also have the potential to detect weak changes in acceleration, gravity, magnetic moments and electrical forces.
Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.
- F. Pistolesi et al., “Proposal for a nanomechanical qubit”, Phys. Rev. X11, 031027 (2021).