Quantum correlation dynamics in photosynthetic processes assisted by molecular vibrations

During the long course of evolution, nature has learnt how to exploit quantum effects. In fact, recent experiments reveal the existence of quantum processes whose coherence extends over unexpectedly long time and space ranges. In particular, photosynthetic processes in light-harvesting complexes display a typical oscillatory dynamics ascribed to quantum coherence. Here, we consider the simple model where a dimer made of two chromophores is strongly coupled with a quasi-resonant vibrational mode. We observe the occurrence of wide oscillations of genuine quantum correlations, between electronic excitations and the environment, represented by vibrational bosonic modes. Such a quantum dynamics has been unveiled through the calculation of the negativity of entanglement and the discord, indicators widely used in quantum information for quantifying the resources needed to realize quantum technologies. We also discuss the possibility of approximating additional weakly-coupled off-resonant vibrational modes, simulating the disturbances induced by the rest of the environment, by a single vibrational mode. Within this approximation, one can show that the off-resonant bath behaves like a classical source of noise

Absorption in quantum electrodynamics cavities in terms of a quantum jump operator

We describe the absorption by the walls of a quantum electrodynamics cavity as a process during which the elementary excitations (photons) of an internal mode of the cavity exit by tunneling through the cavity walls. We estimate by classical methods the survival time of a photon inside the cavity and the quality factor of its mirrors

Quantum and classical correlations of multiply scattered light

Multiple scattering is a very common phenomenon since it occurs any time a wave meets a disordered medium. As almost any natural object has random structure in one form or another, the variety of the processes involving multiple scattering spans from electronic transport in solids to propagation of sound in a forest. In principle, multiple scattering is completely deterministic, and in the absence of absorption also reversible, which means that the information encoded into the incident wave can be perfectly recovered. However, in practice, due to its extreme complexity we often consider this process to be random, which leads to information loss. Within this approach correlations can be an important instrument of information recovery, because they directly quantify the amount of knowledge we get about the wave in a particular point from the measurement performed in a different point. In the first part of this thesis we study a novel type of mesoscopic correlations between the light intensities at the opposite sides of an opaque scattering slab. We study its dependence on the scattering medium properties and the incoming light beam parameters. In the last chapter of the first part we show how this correlation can be used to retrieve non-invasively the information about the shape of an object placed behind the scattering medium. In the second part we switch to the quantum aspects of the light propagation inside the scattering materials. We show that certain class of quantum correlations, quantum discord, can be present in the multimode output state of the scattered light even when the input light is in a thermal state, which is commonly considered classical. We propose a non-classicality measure based on the strength of this correlation, applying it to characterize the advantage due to the quantum measurement in discrimination of two coherent states in their mixture.Engineering and Physical Sciences Research Council (EPSRC

Brillouin cavity optomechanics: Single-quantum-level operations towards quantum memory applications

Cavity-enhanced Brillouin scattering interactions with gigahertz-frequency acoustic phonons offer a promising pathway towards the quantum coherent control of mechanical oscillators. In this thesis, I experimentally investigate single-quantum-level operations applied to thermal me- chanical oscillators by combining optical measurement techniques with Brillouin interactions in crystalline whispering-gallery-mode microresonator devices. These operations are explored for applications in quantum state engineering and optical quantum memories. Generating and characterising non-classical states of mechanical motion currently represents a key challenge in quantum cavity optomechanics, and the realisation of a quantum memory would enable the development of many quantum technologies. The advances reported here contribute to both of these active areas of research. In a series of three experiments, single- and multi-phonon addition and subtraction opera- tions applied to thermal mechanical states are explored. I present the first experimental inves- tigation of single-phonon addition and subtraction operations using a joint click-dyne detection scheme, where the effect of such operations are verified by observing a characteristic doubling of the mean occupation of the state. These techniques are then extended to multi-phonon subtraction. Here, the -parameterised Wigner function of the resulting non-Gaussian states are determined, advancing the state-of-the-art for optics-based mechanical state tomography. Finally, an interferometric detection scheme is employed that implements a superposition of phonon subtractions in two time bins, and the phase coherence between these two operations is demonstrated and studied. In this thesis, I also theoretically investigate the prospects of an optical quantum memory based on Brillouin cavity optomechanics. Using realistic parameters, I show that efficient storage and retrieval of single photons is feasible, and I identify two key applications: temporal multiplexing and temporal mode manipulation. The deleterious effect of thermal noise in such optomechanical quantum light storage is also considered. To conclude, an outlook towards some near-term and long-term experimental goals that can build upon on the achievements reported is presented.Open Acces