Nowadays, researchers can control the quantum state of various real or artificial atoms. Is it possible to extend this capability to a much larger object, such as a mechanical resonator? The generation of quantum states of motion in a “macroscopic” object will answer fundamental questions and open the way to new applications in quantum information technologies. To achieve this ambitious goal, one promising strategy is to couple the mechanical resonator to an atomic-like system.

Together with colleagues at Institut Néel (Grenoble) and Lumin (Orsay), the team pioneered a device based on a vibrating microwire made of galium arsenide, which embeds a single quantum dot made of indium arsenide. A quantum dot is a semiconductor nanostructure that behaves as an atom. Indeed, it features discrete energy levels, as well as remarkable optical properties at liquid-helium temperature. Moreover, the bandgap energy of a semiconductor material is extremely sensitive to deformation of the crystal lattice. This simple mechanism strongly couples the emission wavelength of the quantum dot to the displacement of the microwire. Early studies focused on the fundamental, sub-megahertz resonance of the microwire, which behaves as a classical oscillator. Entering the quantum regime calls for a massive increase in the mechanical frequency, in order to minimize thermal noise. In addition, this will enable the all-optical control and detection of mechanical motion.

The team developed a new device in order to explore the high-frequency mechanical resonances of the microwire (Fig. a). A set of on-chip electrodes applies an oscillating electrostatic force to the microwire. Mechanical motion is detected thanks to a few quantum dots, whose luminescence is excited by a laser. When the wire vibrates, the optical emission line of each quantum dot is spectrally broadened (Fig. b). Experiments already revealed a flexural resonance with a frequency as high as 190 MHz, one thousand times larger than the one of the fundamental mode. Moreover, the coupling strength to this high-order vibration mode reaches a record value.

These findings pave the way towards the optical generation of quantum state of motion and the realization of coherent opto-mechanical interfaces. These are the objectives of the ANR project “AQOUSTIQS”, which gathers the above-mentioned partners and will be launched early 2025.

 
Figure: (a) Schematics of the device. On-chip electrodes drive the mechanical vibration of a conical microwire that embeds a few QDs near its base.
(b) Detection of the flexural resonance F7: when the wire is at rest, each quantum dot features a narrow optical emission line (the color codes the light intensity). The excitation of a mechanical resonance (here close to 189.5 MHz) leads to a spectral broadening. Because of the inhomogeneity of mechanical strain in the wire cross-section, this spectral broadening is different for each quantum dot.
(c) Mode shape of the flexural resonance F7:  the color codes the local mechanical strain (red: extension, blue: compression).