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Strain-mediated coupling in a quantum dot–mechanical oscillator hybrid system

We have realized an hybrid system made of a vibrating wire and a semiconducting quantum dot. The very large coupling between these two subsystems relies on mechanical strain, and is a first step towards the possibility of mapping the quantum properties of a single two-level system made by the quantum dot onto a mechanical oscillator

Owing to the recent progress in nanotechnology and in ultra-sensitive motion detection methods, it becomes now possible to associate quantum systems such as superconducting qubits, spins, atoms or quantum dots to mechanical oscillators. The issue of these hybrid systems is to transfer specifically quantum properties to a mechanical oscillator, opening the possibility of storing quantum information on mechanical degrees of freedom. The key point is to reach a sufficiently large coupling between the two subsystems, while still extracting efficiently the information. In collaboration with the NOF team of Néel Institute, we have coupled a semiconducting quantum dot to a oscillating wire: the quantum dot optical transition energy is modulated by the strain induced by the wire motion. This is the first time that strain coupling is used in an hybrid system. This type of coupling could also benefit to other material such as colored centers in diamond.

(a) Scanning electronic microscope image of the photonic “trumpet”. When the wire bends towards the right (b) or towards the left (c), the quantum dot, depicted as a yellow triangle, is then either under compressive or tensile strain, and its fundamental optical transition energy is either increased or decreased.

The mechanical oscillator is a GaAs conical wire of height 18 µm and diameter from 0.5 to 2 µm. The InAs quantum dot is located slightly above the base of the wire. This trumpet wave guide features record photon extraction efficiencies of up to 75%. Owing to its off-centered position within the nanowire circular cross-section, the quantum dot undergoes periodic strain as the wire oscillates along its fundamental flexural mode at around 500kHz. The alternatively tensile and compressive strain periodically alters significantly the quantum dot energy levels and therefore the spectral position of the photoluminescence lines. The corresponding coupling reaches almost the ultra-strong coupling regime in which it is in principle possible to separate the rest positions of the wire depending on whether the quantum dot is excited or not.

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