ICFO engineers control motion at quantum limit

Researchers at the Institut de Ciències Fotòniques (ICFO) in Barcelona, in collaboration with Universite de Bordeaux, University of Chicago, and Argonne National Laboratory, have achieved control of mechanical motion at the zero-point motion scale using a carbon nanotube device. This advancement leverages tunable nonlinear electromechanics, with the nanotube serving as a mechanical oscillator coupled to a double-quantum-dot electronic two-level system. This setup has resulted in a mechanical anharmonicity three orders of magnitude greater than previous efforts, facilitating precise control and readout of nanomechanical systems.

The system's design features a suspended carbon nanotube that hosts a double-quantum dot, forming a tunable electronic two-level system. Researchers, led by C. B. Møller and R. Tormo-Queralt, demonstrated a purely quadratic cavity-based continuous readout of mechanical motion. This is enabled by a double-quantum dot symmetry, which can be adjusted via gate tuning to introduce significant linear transduction. The quadratic dependence of the readout allows direct access to the oscillator's energy and its averaged phonon number.

The team engineered a mechanical Kerr nonlinearity, creating a frequency shift dependent on oscillation amplitude, crucial for advanced control schemes. The design emphasizes ultrastrong coupling, maintaining experimental conditions within the dispersive regime. The system's Hamiltonian includes mechanical and cavity modes, alongside Pauli operators for the two-level system, enabling a substantial Kerr nonlinearity even at the zero-point motion scale.

Achieving manipulation of mechanical systems at this quantum limit relies on dispersive coupling. The configuration involves a carbon nanotube mechanical oscillator interacting with an electronic two-level system and a readout cavity. This setup provides a purely quadratic cavity-based continuous readout of mechanical motion, facilitated by a double-quantum dot symmetry that can be broken by gate tuning.

Researchers established a specific parameter set, including a bare mechanical frequency, electromechanical coupling strength, and an electronically tunable two-level system frequency, to operate within the far-detuned dispersive regime while meeting the ultrastrong coupling criterion. This ensures a significant mechanical Kerr nonlinearity at the quantum limit of motion. The electronic two-level system frequency is set higher than both mechanical and cavity frequencies, enabling a large Kerr nonlinearity and a purely quadratic readout while preserving the dispersive approximation's validity.

Conventional mechanical resonators often require large displacements for signal detection, introducing noise and limiting sensitivity. The quadratic dependence is crucial as it provides an absolute measure of displacement in zero-point motion units, potentially transforming nanoscale displacement sensors and advancing complex quantum devices.