Entangling a polariton with one photon: effect of interactions at the single-particle level
Álvaro Cuevas, Blanca Silva, Juan Camilo López Carreño, Milena De Giorgi, Carlos Sánchez Muñoz, Antonio Fieramosca, D. G. Suárez-Forero, Filippo Cardano, Lorenzo Marrucci, Vittorianna Tasco, G. Biasiol, Elena del Valle, Lorenzo Dominici, Dario Ballarini, Giuseppe Gigli, Paolo Mataloni, Fabrice P. Laussy, Fabio Sciarrino, D. Sanvitto
Polaritons are quasi-particles originating from the coupling of light with matter and that demonstrated quantum phenomena at the many-particle mesoscopic level, such as BEC and superuidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behaviour remained fully accountable for by classical and mean-field theories. In this Article, we report the first experimental demonstration of a genuinely-quantum manifestation of microcavity polaritons, by swapping the entanglement between a polariton and an external photon from a photon pair generated by parametric down-conversion. Moreover we demonstrate how interactions manifest at a single polariton level by perturbing the entangled state with a rarefied polariton condensate. Our results open the page of quantum polaritonics and implement the first application of quantum spectroscopy.
Polaritons are quasi-particles originating from the coupling of light with matter and that demonstrated quantum phenomena at the many-particle mesoscopic level, such as BEC and superuidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behaviour remained fully accountable for by classical and mean-field theories. In this Article, we report the first experimental demonstration of a genuinely-quantum manifestation of microcavity polaritons, by swapping the entanglement between a polariton and an external photon from a photon pair generated by parametric down-conversion. Moreover we demonstrate how interactions manifest at a single polariton level by perturbing the entangled state with a rarefied polariton condensate. Our results open the page of quantum polaritonics and implement the first application of quantum spectroscopy.
Polaritons are quasi-particles that originate from the coupling of light with matter and that demonstrate quantum phenomena at the many-particle mesoscopic level, such as Bose-Einstein condensation and superfluidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behavior has remained fully accounted for by classical and mean-field theories. We report the first experimental demonstration of a genuinely quantum state of the microcavity polariton field, by swapping a photon for a polariton in a two-photon entangled state generated by parametric downconversion. When bringing this single-polariton quantum state in contact with a polariton condensate, we observe a disentangling with the external photon. This manifestation of a polariton quantum state involving a single quantum unlocks new possibilities for quantum information processing with interacting bosons.
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
Photon correlations are the basic conveyor of optical information in the quantum information age. By regulating a stream of photons and/or endowing them with quantum correlations such as entanglement, one can use them as qubits or to distribute cryptographic keys. Every time that a platform demonstrates its ability to generate photons that have such correlations, new prospects arise to extend the frontiers of quantum technology. One rising star to engineer quantum states in solid state optics are microcavity polaritons. These semiconductor heterostructures can trap both light (cavity photons) and matter (electron-hole pairs) to keep them under strong interaction for a long time. The resulting eigenstates, polaritons, have had resounding successes in the investigation of quantum phases such as Bose-Einstein condensates, superfluids and nonlinear quantum fluids. They also progress quickly to implement actual devices. Lately, their quantum character was consolidated by the observation of squeezing, indicating nonclassical features at a macroscopic level. Only missing to the polariton prize list is quantum correlation at the single particle level. In this Letter, we report the observation of single-photon antibunching from the emission of an out-of-equilibrium Bose-Eintein condensate. This is made possible thanks to generalized photon correlations that preserve the energy information as well as their time of detection. The success of this technique and its results open the era of quantum polaritonics in microcavities and demonstrate the concept of a new class of quantum correlations that can be pursued in a wide range of optical platforms.
Exciton polaritons are coupled states of matter and light, originated by the strong interaction between an optical mode and semiconductor excitons. This interaction can be obtained also at a single-particle level, in which case it has been shown that a quantum treatment is mandatory. In this work we study the light-matter entanglement of polaritons from a fully quantum formalism including pumping and dissipation. We find that the entanglement is completely destroyed if the exciton and photon are tuned at the resonance condition, even under very low pumping rates. Instead, the best condition for maximizing entanglement and purity of the steady state is when the exciton and photon are out of resonance and when incoherent pumping exactly compensates the dissipation rate. In the presence of multiple quantum dots coupled to the light mode, matter-light entanglement survives only at larger detuning for a higher number of quantum dots considered.
Entanglement generation by means of microcavity exciton-polaritons is an interesting application of the peculiar properties of these half-light/half-matter quasiparticles. In this paper, we theoretically investigate their luminescence dynamics and entanglement formation in single, and double cavities. We discuss the selection rules for polariton–polariton scattering in double cavities, and evaluate a number of possible parametric scattering schemes in terms of entanglement in the polarization degree of freedom, and identify the ones that are experimentally most promising.
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.