Controlling and manipulating quanta of coherent acoustic<br> vibrations-phonons-in integrated circuits has recently drawn a lot of<br> attention, since phonons can function as unique links between<br> radiofrequency and optical signals, allow access to quantum regimes and<br> offer advanced signal processing capabilities. Recent approaches based<br> on optomechanical resonators have achieved impressive quality factors<br> allowing for storage of optical signals. However, so far these<br> techniques have been limited in bandwidth and are incompatible with<br> multi-wavelength operation. In this work, we experimentally demonstrate<br> a coherent buffer in an integrated planar optical waveguide by<br> transferring the optical information coherently to an acoustic<br> hypersound wave. Optical information is extracted using the reverse<br> process. These hypersound phonons have similar wavelengths as the<br> optical photons but travel at five orders of magnitude lower velocity.<br> We demonstrate the storage of phase and amplitude of optical information<br> with gigahertz bandwidth and show operation at separate wavelengths with<br> negligible cross-talk.

Inelastic scattering processes such as Brillouin scattering can often function in cascaded regimes and this is likely to occur in certain integrated opto-acoustic devices. We develop a Hamiltonian formalism for cascaded Brillouin scattering valid for both quantum and classical regimes. By regarding Brillouin scattering as the interaction of a single acoustic envelope and a single optical envelope that covers all Stokes and anti-Stokes orders, we obtain a compact model that is well suited for numerical implementation, extension to include other optical nonlinearities or short pulses, and application in the quantum-optics domain. We then theoretically analyze intra-mode forward Brillouin scattering (FBS) for arbitrary waveguides with and without optical dispersion. In the absence of optical dispersion, we find an exact analytical solution. With a perturbative approach, we furthermore solve the case of weak optical dispersion. Our work leads to several key results on intra-mode FBS. For negligible dispersion, we show that cascaded intra-mode FBS results in a pure phase modulation and discuss how this necessitates specific experimental methods for the observation of fiber-based and integrated FBS. Further, we discuss how the descriptions that have been established in these two classes of waveguides connect to each other and to the broader context of cavity opto-mechanics and Raman scattering. Finally, we draw an unexpected striking similarity between FBS and discrete diffraction phenomena in waveguide arrays, which makes FBS an interesting candidate for future research in quantum-optics.

The measurement of quantum signals that travel through long distances is fundamentally and technologically interesting. We present quantum-limited coherent measurements of optical signals that are sent from a satellite in geostationary Earth orbit to an optical ground station. We bound the excess noise that the quantum states could have acquired after having propagated 38,600 km through Earth's gravitational potential, as well as its turbulent atmosphere. Our results indicate that quantum communication is feasible, in principle, in such a scenario, highlighting the possibility of a global quantum key distribution network for secure communication. (C) 2017 Optical Society of America