Proposal for a nanomechanical qubit

TL;DR

This paper proposes a new mechanical quantum bit using a suspended carbon nanotube coupled to a double quantum dot, demonstrating potential for quantum computation and high-sensitivity sensing in the ultrastrong coupling regime.

Contribution

It introduces a method to realize a mechanical qubit via coupling nanotube modes to quantum dots, achieving anharmonicity and reduced dephasing, and discusses control and sensing applications.

Findings

Achieves anharmonicity in mechanical oscillators via quantum dot coupling.

Reduces dephasing from quantum dots by orders of magnitude.

Proposes protocols for qubit control, readout, and quantum sensing.

Abstract

Mechanical oscillators have been demonstrated with very high quality factors over a wide range of frequencies. These also couple to a wide variety of fields and forces, making them ideal as sensors. The realization of a mechanically-based quantum bit could therefore provide an important new platform for quantum computation and sensing. Here we show that by coupling one of the flexural modes of a suspended carbon nanotube to the charge states of a double quantum dot defined in the nanotube, it is possible to induce sufficient anharmonicity in the mechanical oscillator so that the coupled system can be used as a mechanical quantum bit. This can however only be achieved when the device enters the ultrastrong coupling regime. We discuss the conditions for the anharmonicity to appear, and we show that the Hamiltonian can be mapped onto an anharmonic oscillator, allowing us to work out the energy level structure and how decoherence from the quantum dot and the mechanical oscillator are inherited by the qubit. Remarkably, the dephasing due to the quantum dot is expected to be reduced by several orders of magnitude in the coupled system. We outline qubit control, readout protocols, the realization of a CNOT gate by coupling two qubits to microwave cavity, and finally how the qubit can be used as a static force quantum sensor.