Today, real or "artificial" atoms such as semiconductor "quantum dots" can be prepared in a well-defined quantum state via optical excitation. This is not so for a macroscopic mechanical system, due to its large number of degrees of freedom. If this barrier could be overcome, the path would be open for extraordinarily precise force or position sensors, or even new functions in quantum information processing.
In this context, one promising strategy is to introduce an artificial atom (a quantum system with two energy levels) into the core of a mechanical system in order to "imprint" its quantum state on the macroscopic device.
Several years in the making, the device developed by the Irig researchers and their colleagues consists of a conical gallium arsenide semiconductor microwire, at the base of which is embedded an indium arsenide quantum dot that is offset from its symmetry axis. In this new experiment, the quantum dot is illuminated with laser pulses tuned to the transition between its two energy levels. Each absorbed photon causes an electron–hole pair to appear in the dot. This induces an increase in its volume and therefore a significant change in the stress field running through the microwire, inducing its deflection. Repeating the optical excitation at the resonance frequency of the microwire makes it vibrate!
Nevertheless, the detection of this vibration represents a formidable experimental challenge. Indeed, its amplitude does not exceed 0.6 picometres (10-12 m), i.e. one thousandth the size of an atom! Even though the experiment is cooled to a very low temperature (20 K), this tiny displacement is still masked by thermal fluctuations that affect the position of the microwire. The physicists rose to the challenge thanks to an ultra-sensitive optical detection process, which had to be repeated many times under extremely stable conditions.
While considerable progress is still needed to transfer quantum states from an artificial atom to a macroscopic mechanical oscillator, this proof of concept highlights the strong potential of the all-optical manipulation of hybrid systems combining a quantum dot and a semiconductor microwire.
This work was carried out in collaboration with the Institut Néel (CNRS/Université Grenoble Alpes/Grenoble INP), the CEA-Irig, the ENS Lyon, the University of Campinas (Brazil), and the University of Nottingham (United Kingdom).