I will give an overview of our recent studies on polariton quantum optics. I will show how polaritons can be produced in quantum entangled states, thanks to parametric scattering. By combining a time‐dependent density‐matrix model of the coherent polaritons, with a Boltzmann kinetics of the surrounding incoherent polariton gas in interaction with a thermal bath of phonons, the degree of entanglement can be quantified. It turns out that large entanglement is expected under realistic experimental conditions. I will further show how the polariton nonlinearity can be exploited to design quantum devices. As an example, I will model a system of coupled polariton modes, that operates as an efficient source of single photons by exploiting quantum interference between different pathways in the time evolution of the quantum field.
I will give an overview of our recent studies on polariton quantum optics. I will show how polaritons can be produced in quantum entangled states, thanks to parametric scattering. By combining a time‐dependent density‐matrix model of the coherent polaritons, with a Boltzmann kinetics of the surrounding incoherent polariton gas in interaction with a thermal bath of phonons, the degree of entanglement can be quantified. It turns out that large entanglement is expected under realistic experimental conditions. I will further show how the polariton nonlinearity can be exploited to design quantum devices. As an example, I will model a system of coupled polariton modes, that operates as an efficient source of single photons by exploiting quantum interference between different pathways in the time evolution of the quantum field.
We show theoretically that polariton pairs with a high degree of polarization entanglement can be produced through parametric scattering. In our proposed scheme, varying the polarizations of the two pumps, we have a complete control over the symmetries of the produced state (i.e. singlet or triplet output). Our microscopic model shows how a tomographic reconstruction, based on two-time correlation functions, can provide a quantitative assessment of the level of entanglement produced under realistic experimental conditions. Our study provides a suggestive perspective towards hybrid all-optical quantum devices where quantum information can be efficiently generated and controlled within the same structure. This result puts forward the robustness of pair correlations in solid-state devices, even when noise dominates one-body correlations.
Summary form only given. Semiconductor microcavities can be engineered to produce strong coupling between photons and quantum well excitons leading to the formation of new eigenstates called polaritons. Due to their peculiar dispersion, enabling non-equilibrium Bose-Einstein condensation [1], and their straightforward experimental accessibility, microcavity polaritons have gained considerable interest in recent years. Polariton states can be selectively populated by choosing the excitation angle and the wavelength of the excitation laser. After a lifetime of a few picoseconds the polariton decays and a photon is emitted from the cavity, carrying the complete information on the underlying polariton state. Several theoretical works [2-3] suggest that polariton-polariton interactions can be a source of non-classical photon correlations, producing photon pairs that are entangled in either the energy, the polarization, or the momentum degree of freedom. One appeal of such a system is that many parameters such as the polariton-polariton interaction strength are tunable [4] and multipartite entangled states can be created in a single device [5]. However, due to the lack of high-quality samples, experimental realisations of these schemes have been elusive so far. In this contribution, we will discuss some of our experimental work on the statistics of polariton-polariton parametric scattering. Excited polaritons can scatter in a four-wave mixing process into signal and idler states lying on a figure-eight shaped curve in k-space, where phase-matching is fulfilled. We make use of a sample with ultra narrow exciton resonances, which allows for the generation and detection of correlated light. We present our measurements of the second order correlation functions using time-correlated photon counting. We discuss in detail, how polariton quantum correlations depend on a number of experimental parameters, such as phase space location and excitation density and find that the results can be understood in the framework of the well known Bogoliubov theory for 2D Bose gases utilised for microcavity polaritons [6]. We find bunching below the parametric threshold and a slow buildup of coherence above the threshold similar to the quantum statistics of a non-degenerate optical parametric oscillator, which is well known for its nonclassical properties [7]. We discuss in particular the influence of strong background processes on the observed photon statistics.
The main scope of this thesis is the investigation of quantum properties of nonlinear, dissipative, optical and condensed matter systems. Compared with previous studies on this subjects, we reached several milestone of general interests for the theoretical and experimental community. \nThe main experimental platform we refer to in this thesis is microcavity polaritons: polaritons arise from the coupling of light with excitons in a quantum well, i.e., a 2D semiconductor material embedded in the microcavity. They have attracted the interest of the scientific community for their rich physics. Polaritons can form out-of-equilibrium Bose-Einstein condensates, they can experience superfluid phase transition and, due to their interaction via the excitonic component, they show a variety of phenomena which are typical of nonlinear physics, such as bistability and parametric scattering. \nIn particular, motivated by recent proposals about the generation of single photons from weakly nonlinear system, like the unconventional photon blockade, and by the production of squeezed states in polariton systems, we studied the possibility of obtaining antibunched light and other quantum states (like squeezed states and entangled states), when the nonlinear system shows some peculiar features: in particular we consider polariton soliton and cascade systems. \nFinally, when considering nonlinear incoherent processes, we investigate the effect of the PT-symmetry and the PT-symmetry breaking on the quantum state of the emitted light. \nHence, this research work has two main objectives: first, from the fundamental physics point of view, to understand the interplay between nonlinear phenomena and quantum optics, second to provide useful tools for future technological applications. Specifically, for the latter case, we demonstrate a theorem that links the measurement of intensity correlations with the occurrence of certain phase transitions, and we propose a setup to maximize the non-classicality of the light emitted by weakly non-linear systems via a cascade configuration. \n \n \n
It has been proposed that parametric scattering of microcavity polaritons can be a source of non-classical photon correlations, producing photon pairs that are entangled in either the energy [1], or the polarization degree of freedom [2]. In order to establish, whether a particular scheme is practically feasible, one has to take into account not only the parametric processes in question, but the polaritons' interaction with their environment. It is clear that any such interaction is going to reduce the quality of the entanglement produced by the desired parametric process.
Creating sufficiently strong entanglement between exciton-polaritons is a step of utmost importance towards the exploitation of these systems in quantum information processing with continuous variables. The generation of entanglement relies on the strength of the nonlinearity, which is weak for semiconductor microcavities. Recently, a way to essentially amplify the nonlinearity strength in these systems using two coherent laser fields was suggested, leading in theory to the creation of a fair amount of entanglement between exciton-polaritons in coupled cavities and networks in general. Throughout this process, the Josephson coupling and the enhanced nonlinearity in the two coupled cavities are held constant, with the former always larger. In this work we show that the entanglement generated with the above procedure can be substantially enhanced with the appropriate on-off switching of Josephson coupling between the cavities. Furthermore, we show that if we consider a time-dependent enhanced nonlinearity, through the modulation of the corresponding coherent laser fields, and allow it to attain larger values than the Josephson coupling, then we can generate larger values of entanglement using shortcuts to adiabaticity, a method developed to accelerate quantum adiabatic dynamics. The suggested methodologies are not restricted to exciton-polaritons but are expected to find applications in a wide spectrum of physical contexts, where nonlinear interacting bosons are encountered.
Using a high-quality GaAs planar microcavity, we optically generate polariton pairs, and verify their correlations by means of time-resolved single-photon detection. We find that correlations between the different modes are consistently lower than identical mode correlations, which is attributed to the presence of uncorrelated background. We discuss a model to quantify the effects of such a background on the observed correlations. Using spectral and temporal filtering, the background can be suppressed and a change in photon statistics towards non-classical correlations is observed. These results improve our understanding of the statistics of polariton-polariton scattering and background mechanisms, and pave the way to the generation of entangled polariton pairs.