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Quantum: An artificial atom sets a micro-wire in motion

Researchers in our laboratory [collaboration] have succeeded in setting in motion an 18-µm-long mechanical oscillator by optically exciting an embedded artificial atom. This result is an important step towards the realization of interfaces that connect the quantum and classical worlds.

Published on 3 February 2021
Real or artificial atoms such as semiconductor quantum dots can now be prepared in a well-defined quantum state via optical excitation. This is not yet the case for a macroscopic mechanical system, due to its large number of degrees of freedom. Overcoming this limitation would open the way to extraordinarily precise force or position sensors as well as new functions for quantum information processing.

In this context, a promising strategy is to integrate an artificial atom (a quantum system with two energy levels) directly into a mechanical system in order to "imprint" its quantum state on the macroscopic device. The device proposed by Irig researchers and their colleagues is the result of several years of development. It consists of a conical micro-wire made of gallium arsenide, which embeds a single indium arsenide quantum dot, located close to the wire base, away 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 creates an electron-hole pair in the dot, which in turn becomes bigger. This modifies the stress field at the base of micro-wire thereby inducing its deflection. Repeating the optical excitation at the resonance frequency of the micro-wire makes it vibrate!

Detecting this vibration, however, represents a tremendous experimental challenge. Indeed, the vibration amplitude does not exceed 0.6 picometres (10-12 m), a thousand times smaller than the size of an atom! Even if the experiment is cooled down to very low temperature (20 K), this tiny displacement remains masked by the thermal fluctuations of the micro-wire position. The physicists took up the challenge thanks to an ultra-sensitive optical detection, and by repeating the measurement a large number of times under conditions of extreme stability. While significant 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 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), CEA-Irig, ENS Lyon, the University of Campinas (Brazil), and the University of Nottingham (UK). 

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