Emitter ensembles interact collectively with the radiation field. In the case of a one-dimensional array of atoms near a nanofiber, this collective light-matter interaction does not only lead to an increased photon coupling to the guided modes within the fiber, but also to a drastic enhancement of the chirality in the photon emission. We show that near-perfect chirality is already achieved for moderately-sized ensembles, containing 10 to 15 atoms. This is of importance for developing an efficient interface between atoms and waveguide structures with unidirectional coupling, with applications in quantum computing and communication such as the development of non-reciprocal photon devices or quantum information transfer channels.

Open quantum systems with chiral interactions can be realized by coupling atoms to guided radiation modes in photonic waveguides or optical fibers. In their steady state these systems can feature intricate many-body phases such as entangled dark states, but their detection and characterization remains a challenge. Here we show how such collective phenomena can be uncovered through monitoring the record of photons emitted into the guided modes. This permits the identification of dark entangled states but furthermore offers novel capabilities for probing complex dynamical behavior, such as the coexistence of a dark entangled and a mixed phase. Our results are of direct relevance for current optical experiments, as they provide a framework for probing, characterizing and classifying classical and quantum dynamical features of chiral light-matter systems.

We report on the formation of a diverse family of transverse spatial polygon patterns in a microcavity polariton fluid under coherent driving by a blue-detuned pump. Patterns emerge spontaneously as a result of energy-degenerate polariton-polariton scattering from the pump state to interfering high order vortex and antivortex modes, breaking azimuthal symmetry. The interplay between a multimode parametric instability and intrinsic optical bistability leads to a sharp spike in the value of second order coherence g (2) (0) of the emitted light, which we attribute to the strongly superlinear kinetics of the underlying scattering processes driving the formation of patterns. We show numerically by means of a linear stability analysis how the growth of parametric instabilities in our system can lead to spontaneous symmetry breaking, predicting the formation and competition of different pattern states in good agreement with experimental observations. PACS numbers: Valid PACS appear here arXiv:1612.03048v2 [cond-mat.mes-hall]

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We study theoretically a laser-driven one-dimensional chain of atoms interfaced with the guided optical modes of a nanophotonic waveguide. The period of the chain and the orientation of the laser field can be chosen such that emission occurs predominantly into a single guided mode. We find that the fluorescence excitation line shape changes as the number of atoms is increased, eventually undergoing a splitting that provides evidence for the waveguide-mediated all-to-all interactions. Remarkably, in the regime of strong driving the light emitted into the waveguide is non-classical with a significant negativity of the associated Wigner function. We show that both the emission properties and the non-Gaussian character of the light are robust against voids in the atom chain, enabling the experimental study of these effects with present-day technology. Our results offer a route towards novel types of fiber-coupled quantum light sources and an interesting perspective for probing the physics of interacting atomic ensembles through light.

We investigate the creation and control of emergent collective behavior and quantum correlations using feedback in an emitter-waveguide system using a minimal model. Employing homodyne detection of photons emitted from a laser-driven emitter ensemble into the modes of a waveguide allows to generate intricate dynamical phases. In particular, we show the emergence of a time-crystal phase, the transition to which is controlled by the feedback strength. Feedback enables furthermore the control of many-body quantum correlations, which become manifest in spin squeezing in the emitter ensemble. Developing a theory for the dynamics of fluctuation operators we discuss how the feedback strength controls the squeezing and investigate its temporal dynamics and dependence on system size. The largely analytical results allow to quantify spin squeezing and fluctuations in the limit of large number of emitters, revealing critical scaling of the squeezing close to the transition to the time-crystal. Our study corroborates the potential of integrated emitter-waveguide systemswhich feature highly controllable photon emission channels -for the exploration of collective quantum phenomena and the generation of resources, such as squeezed states, for quantum enhanced metrology.

The orbital angular momentum and the polarisation of light are physical quantities widely investigated for classical and quantum information processing. In this work we propose to take advantage of strong light-matter coupling, circular-symmetric confinement, and transverse-electric transversemagnetic splitting to exploit states where these two degrees of freedom are combined. To this end we develop a model based on a spin-orbit Poincaré hypersphere. Then we consider the example of semiconductor polariton systems and demonstrate full ultrafast Stark control of spin-orbit states. Moreover, by controlling states on three different spin-orbit spheres and switching from one sphere to another we demonstrate the control of different logic bits within one single physical system. PACS numbers: 71.36.+c, 42.50.Tx, 71.70.Ej, 78.55.Cr The polarisation of photons and the spin of photonemitters, such as atoms, quantum dots and vacancydefect centres, are among the most exploited physical properties for the implementation of classical as well as quantum information processing [1][2][3][4][5]. In recent years, considerable efforts have been devoted to the use of structured light beams with orbital angular momentum (OAM) to maximise information processing capabilities. Significant quantum effects such as entanglement of multi-photon states with high values of OAM and OAM Hong-Ou-Mandel interference have been demonstrated [5][6][7][8][9][10]. The next natural step is to use higher-dimensional Hilbert spaces, like for example Spin-Orbit (SO) coupled states [11][12][13][14][15], which might allow simplify quantum logic [16].The strong coupling of photons with photon-emitters leads to the formation of polaritons, new half-light halfmatter dressed states. A particular advantage offered by these hybrid quasiparticles is that they allow not only ultrafast manipulation through their light component [17] but also through their matter component, opening the way to a more extended and flexible control. This can be achieved taking advantage of the AC Stark effect that allows controlling the excitation energy of photon emitters [18,19] without modifying the population. This effect, recently been demonstrated for semiconductor microcavities [20,21], but can in principle also be applied to other systems such as: semiconductor or colloidal quantum dots and defect centres.In this Letter we develop a theoretical model based on a SO hypersphere [22] and use red-detuned laser pulses to manipulate angular momentum and polarisation of polariton states. This model has the unique advantage of combining multiple logical bits in one single physical system, and of allowing them to be independently manipulated. For the sake of clarity we will limit our analysis to the case of OAM l = ±1, but the theory can be generalised to higher values of l .Our underlying general theory is valid for emitters in strong coupling with light in the presence of circular-symmetric confinement and transverse-electric transverse-magnetic (TE-TM) splitting. Parameters for stat...

Non-classical correlations in quantum optics as resources for quantum computation are important in the quest for highly-specialized quantum devices. The standard way to investigate such effects relies on either the characterization of the inherent features of sources and circuits or the study of the output radiation of a given optical setup. The latter approach demands an extensive description of the output fields, but often overlooks the dynamics of the sources. Conversely, the former discards most of the information about the single trajectories, which are observed in experimental measurements. In this work we provide a natural link between the two frameworks by exploiting the thermodynamics of quantum trajectories. This procedure enables investigation of the quantum properties of the photon fields from a generic source via the analysis of the fluctuations and correlations of time-integrated quantities associated with the photon counting of the emitted light.Introduction. During the last decades, several platforms have been proposed for implementing efficiently quantum computing [1-3]: all of them suffer from the effect of decoherence, given by the coupling to the environment [4], which ultimately deteriorate the non-classical properties of the systems considered. In fact, for a quantum computational scheme to outperform a classical one, one requires that at least one of its component exhibits genuinely quantum features [5]. When the environment is the electromagnetic vacuum causing photon emission, such as in dissipative optical networks [6], the statistical analysis of the output light contains the information about the dynamical features of the open quantum systems [7]. In particular, the emitted photons can be used as a resource for quantum information processing [8]. Hence, the detection and optimization of non-classical correlations in the photons emitted by a general optical setup is of primary relevance for a variety of technological applications.