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Coherent interactions between different quantum systems are essential for quantum information processing, which typically involves controlling individual quantum systems and transferring information between them.

Recent developments in the field of nano-optomechanics have proven phonons, the quanta of mechanical vibration in solids, to be a promising platform for quantum transduction. Coupling optical cavities to mechanical resonators allows the control over mechanical motion, paving the way for novel technologies with applications in communication, computing, and fundamental physics.

Embedding semiconductor quantum dots (QDs) – nanoscale structures that exhibit quantum confinement effects – into these nano-optomechanical systems creates a tripartite interaction between light, acoustic waves and excitons. In this hybrid interaction the intrinsic nonlinearity of an excitonic state in a QD can be exploited to create and manipulate non-classical states.

Tripartite in a zipper cavity

For his PhD research, Matteo Lodde explored the use of a zipper cavity to realize a design to facilitate the tripartite interaction.

A zipper cavity is an optomechanical cavity consisting of two double-clamped nanobeams with a one-dimensional photonic crystal cavity patterned on them.

The coupling between the mechanical resonator and the QD is achieved via the strain induced by the flexural vibrations of the nanobeams which modulates the energy levels of the QD.

Lodde numerically simulated and optimized the mechanical properties of the system as part his research. Significant effort was spent on engineering the strain profile to enhance the coupling rate between the exciton in the QD and the mechanical resonator.

For the in-plane flexural mode, the strain profile of the device was maximum in the area close to the clamping points of the nanobeams. These regions have been further optimized by tapering both ends of the device.

Measuring the QDs in these areas also ensured that the direct Jaynes-Cummings interaction between the QD and the optical field was negligible, which allowed for the identification of the effects mediated by the mechanical resonator.

Resonant driving

The presence of the optical cavity allowed the resonant driving of the mechanical mode with a modulated laser field, enhancing the effect of strain coupling.

By performing pump-probe measurements of the device, Lodde and his colleagues were able to prove a phonon population of ~ 10^8 at 10K. The strain coupling of single QD excitons were measured by performing a stroboscopic measurement of the photoluminescence of the dot.

In this experiment the semi-resonant pump laser used to excite electron-hole pairs in the QDs was synchronized with the laser used to drive the mechanical mode. By sweeping the phase difference between the two lasers, a coherent modulation of the exciton photoluminescence with an amplitude of 4.9 GHz was measured.

The strain coupling effect was confirmed by also showing that the modulation of the exciton energy only occurs when the mechanical cavity was resonantly driven and vanished when the modulation frequency was detuned by more than a mechanical linewidth.

From the amplitude of such modulation, Lodde was able to estimate a strain coupling rate per single phonon of 214 kHz, comparable to what has been observed in other devices which exploit external electrical driving of the mechanical resonator.

Since the proposed device does not require electrical contacts to be defined on or close to the chip, it may provide additional design flexibility in experiments involving phonons and two-level systems. Additionally, the device studied in Lodde’s thesis represents an additional contribution to the growing toolbox for the coherent control of phonons.

Title of PhD thesis: . Supervisors: Andrea Fiore and Ewold Verhagen.