Recent years have witnessed impressive developments in the field of optomechanics with the demonstration of several milestones in sensing and mesoscopic quantum effects. However, typical schemes, based on light forces in optical cavities, are seriously limited by the smallness of the resulting interaction between single phonons and photons. In this seminar I will discuss an alternative approach, based on the optical excitation of quantum emitters embedded in nanomechanical resonators and their strong electron-phonon interactions. More specifically, I will illustrate it with theoretical analyses of schemes that: (i) provide direct evidence of nanomechanical energy quantization via the monitoring of phonon quantum jumps and (ii) allow to explore strong-coupling phenomena in quantum decoherence.

Typically, optomechanical schemes to achieve (i) rely on an effective interaction, quadratic in the mechanical displacement, that arises from a pair of mutually coupled optical cavity modes that couple linearly with opposite phases to the mechanical displacement. Importantly, such schemes are affected by a standard quantum limit [Miao et al. PRL '09] that usually imposes a strong optomechanical coupling condition. Here we analyse an alternative where the role of the two optical modes is played instead by a pair of coupled quantum emitters embedded in the resonator with a sub-wavelength separation. The strong coupling requirement is then circumvented by a destructive quantum interference in the spontaneous emission.

In turn, regarding (ii), we calculate the phonon-broadened fluorescence spectrum of a carbon-nanotube (CNT) localised-exciton transition under weak excitation for large suspended CNT length. We show that this regime, which is dominated by the divergent DOS of the flexural phonons, provides at low temperature an optical realisation of the "localised" phase of the subohmic spin-boson model that implies a collapse of the zero-phonon line.