Interfacing cold atoms with nanoscopic dielectric devices offers exciting opportunities for quantum technologies. We focus on enhancing light-matter coupling via slow-mode nanophotonic crystals while addressing challenges in design, nanofabrication, and precise atom delivery near surfaces.

Novel platforms interfacing trapped cold atoms and guided light in nanoscale waveguides are a promising route to achieve a regime of strong coupling between light and atoms in single pass, with applications to quantum non-linear optics and quantum simulation. A strong challenge for the experimental development of this emerging waveguide-QED field of research is to combine facilitated optical access for atom transport, atom trapping via guided modes and robustness to inherent nanofabrication imperfections. In this endeavor, here we propose to interface Rubidium atoms with a photonic-crystal waveguide based on a large-index GaInP slab. With a specifically tailored half-W1 design, we show that a large chiral coupling to the waveguide can be obtained and guided modes can be used to form two-color dipole traps for atoms at 116~nm from the edge of the structure. This optimized device should greatly improve the level of experimental control and facilitate the atom integration.

Topological lasers based on optical cavity arrays, such as the Su-Schrieffer-Heeger array, in active media were recently realized on various platforms. In all these studies the coupling coefficient between any adjacent sites of the lattice was positive. Consequently, the lasing mode featured an out-of-phase oscillation which is typically impractical for any application because it weakens the far-field intensity. This problem can be mitigated by altering the coupling signs along the array. However, implementing negative evanescent coupling in microrings, microdisks and micropillars is not possible using standard designs and fabrication processes. In this work, we design and experimentally implement a photonic crystal cavity array that consists of three sites where the sign of one coupling parameter is inverted, which enables the observation of an in-phase symmetry-protected zero mode. The photonic crystal array presented here utilizes the recently proposed "image-barrier" technique, in which the photonic barriers are copied at opposite sides of the array, which mitigates chain-termination effects and at the same time enables sign flip from one barrier to the adjacent one, resulting in "twisted coupling." Consequently, the overall symmetry of the zero mode is inverted and becomes even, which is experimentally demonstrated. Our work opens the door for implementing a new generation of phase-locked topological laser arrays that oscillate in the same phase with enhanced far-field optical intensities.

We describe a project underway since 2015 at the Université de Franche-Comté in France where we have been preserving the history of optics and photonics, with the particular aim of ensuring our students are made aware of this rich scientific heritage. We have successfully located and preserved a wide range of instrumentation and archival material dating from the mid-19th century to the 1960s, including some of the first European studies of lasers, holograms, and their applications. We are currently placing an emphasis on recording oral histories of current and former researchers and educators to ensure that our history during the latter part of the 20th century is fully recorded whilst memories are still fresh, and whilst supporting equipment and laboratory material can be found and archived.

Nanolasers are most appealing candidates for integrated nanosources on photonic microchips because of their ability to operate in the so-called thresholdless regime, compatible with ultra-low energy consumption operation. Remarkably, nanolaser technology has experimented a huge progress in the recent years, and today, a myriad of cavity designs and materials come to maturity [1]. Such technological advances enable the realization of coupled nanolaser arrays [Fig. 1(a), (d)] with unprecedented control over the geometrical (coupling), nonlinear (carrier-induced refractive index change) and non-Hermitian (gain/loss) parameters, and give a handle to generate and tailor laser beams with symmetry and/or topological protection.

Photonic modes resistant to imperfections or perturbations are of paramount importance in many photonic applications. In this context, zero-modes have several advantages. Unfortunately, they are often difficult to observe. In this work, focusing on coupled photonic crystal cavities, we propose and demonstrate a technique that allows control of inter-cavity coupling without introducing concomitant frequency mismatch, thus allowing observation of zero-modes in a non-Hermitian system and testing of their robustness against asymmetries of coupling. This is done through an original "image barrier" engineering approach.

Symmetry-protected zero modes in arrays of coupled optical elements have attracted considerable attention because they are expected to be robust against coupling disorders. In the Hermitian limit, zero modes are dark ones, i.e. the intensity in one sublattice vanishes; yet, in a non-Hermitian counterpart, zero modes can be bright and feature π/2 phase difference between sublattices. In this work, we report on the direct observation of a lasing zero mode in a non-Hermitian three coupled nanocavity array. We show efficient excitation for nearly equal pump power in the two extreme cavities. Furthermore, its efficiency can be dynamically controlled by pumping the center cavity. The realization of zero mode lasing in large arrays of coupled nanolasers has potential applications in laser-mode engineering and it opens up promising avenues in optical computing.

Interfacing cold neutral atoms and photons guided in nanoscale waveguides has raised a large interest over the recent years, with a wealth of emerging opportunities. Arrays of atoms can be trapped in the evanescent field of guided modes and the strong transverse confinement enables to increase the individual atom-photon coupling in single pass. Remarkable experimental advances have been obtained with optical nanofibers [1], but photonic crystals waveguides (PCW) are very promising as they allow for precise dispersion engineering. Despite these promises, trapping atoms in the vicinity of such PCWs is still at its infancy.

Fêter le quarantième anniversaire de la SFO ne peut se concevoir sans célébrer ses Clubs et Commissions qui sont les forces vives de notre société savante. Cet article présente une description synthétique des Clubs aux profils variés, tout en montrant leurs complémentarités. Il introduit aussi nos Commissions, véritables fer de lances des valeurs d’inclusion, de diffusion des savoirs et de mutualisation qui fondent notre SFO.

We report on the first experimental observation of spontaneous mirror symmetry breaking (SSB) in coherently driven-dissipative coupled optical cavities. SSB is observed as the breaking of the spatial or mirror Z_{2} symmetry between two symmetrically pumped and evanescently coupled photonic crystal nanocavities, and manifests itself as random intensity localization in one of the two cavities. We show that, in a system featuring repulsive boson interactions (U>0), the observation of a pure pitchfork bifurcation requires negative photon hopping energies (J<0), which we have realized in our photonic crystal molecule. SSB is observed over a wide range of the two-dimensional parameter space of driving intensity and detuning, where we also find a region that exhibits bistable symmetric behavior. Our results pave the way for the experimental study of limit cycles and deterministic chaos arising from SSB, as well as the study of nonclassical photon correlations close to SSB transitions.

Zero modes are symmetry protected ones whose energy eigenvalues have zero real parts.In Hermitian arrays,they arise as a consequence of the sublattice symmetry,implying that they are dark modes.In non-Hermitian systems that naturally emerge in gain/loss optical cavities,particle-hole symmetry prevails instead;the resulting zero modes are no longer dark but feature π/2 phase jumps between adjacent cavities.Here,we report on the direct observation of zero modes in a non-Hermitian three coupled photonic crystal nanocavities array containing quan-tum wells.Unlike the Hermitian counterparts,the observation of non-Hermitian zero modes upon single pump spot illumination requires vanishing sublattice detuning,and they can be identified through far-field imaging and spectral filtering of the photoluminescence at selected pump locations.We explain the zero-mode coalescence as a parity-time phase transition for small coupling.These zero modes are robust against coupling disorder and can be used for laser mode engineering and photonic computing.

Coupling quantum emitters and nanostructures, in particular cold atoms and waveguides, has recently raised a large interest due to unprecedented possibilities of engineering light-matter interactions. However, the implementation of these promising concepts has been hampered by various theoretical and experimental issues. In this work, we propose a new type of periodic dielectric waveguide that provides strong interactions between atoms and guided photons with an unusual dispersion. We design an asymmetric comb waveguide that supports a slow mode with a quartic (instead of quadratic) dispersion and an electric field that extends far into the air cladding for an optimal interaction with atoms. We compute the optical trapping potential formed with two guided modes at frequencies detuned from the atomic transition. We show that cold Rubidium atoms can be trapped as close as 100 nm from the structure in a 1.3-mK-deep potential well. For atoms trapped at this position, the emission into guided photons is largely favored, with a beta factor as high as 0.88 and a radiative decay rate into the slow mode 10 times larger than the free-space decay rate.

Triple-photon generation (TPG) is based on a third-order nonlinear optical interaction, which is the most direct way to produce pure quantum three-photon states. These states can exhibit three-body quantum correlations, and their statistics cannot be reproduced by any Gaussian statistics of coherent sources or optical parametric twin-photon generator, making them potentially useful for quantum information processing tasks such as quantum state distillation, quantum error-correction and universal quantum computing. Furthermore, the generation of entangled photon pairs heralded by the detection of a third photon can be used in advanced quantum communication protocols. We made the first experimental demonstration of TPG in 2004 using a bi-stimulation scheme in a bulk KTP crystal, followed by the quantum theory. The new challenges are now to achieve a spontaneous TPG and the corresponding quantum experiments and protocols using oriented ridge KTP waveguides, which ensures both birefringence phase-matching and light confinement. The waveguides are cut by a precision dicing saw. We recently performed their characterization using third-harmonic generation measurements, which showed their good quality. A rate of about 5 triplets per second is expected when pumping a 5-cm-long waveguide with a 5-W 532 nm beam in the CW regime. Such a spontaneous TPG exhibits low rate of triple photons, which makes the certification of quantum features hard. In this article, we review our theoretical and experimental work on TPG and the associated quantum modeling. We also develop theoretical tools for the certification of quantum features of spontaneous triple-photon states. Graphic abstract

Reaching a regime of strong coupling between light and atoms is a long-sought goal in quantum optics, as it will enable non-linear optics at the photon level. Interfacing atoms with guided light in nanophotonic waveguides is a promising route to achieve such regimes. Experimental platforms include nanofibers [1] and photonic crystal waveguides (PCWs), owing to the large transverse confinement of their guided modes. With PCWs, tuning the dispersion relation gives rise to the additional benefit of a very low group velocity of the guided light [2] , allowing for strong coupling even in single pass. Moreover these systems can help explore waveguide-QED and probe exciting novel bandgap physics [3] . While encouraging values of couplings have been observed with first corrugated devices [4] , a photonic platform with trapped atoms via guided modes in the vicinity of a waveguide and lying deep in the strong coupling regime has yet to be demonstrated.

We present a proposal for trapping Rubidium cold atoms near a novel design of a GaInP photonic crystal waveguide with characteristics optimized through systematic and inverse design. Purcell factors higher than unity are predicted.

The nonlinear directional coupler (NDC) is a core device in integrated optics with application for instance in all-optical switching [1] . Recently, the χ (2) NDC has found a flourishing field of application: quantum optics. Its key strengths in quantum information processing as a source of entangled photons and entangled field quadratures are actively explored. Particularly, strong continuous-variable entanglement in a second harmonic generation (SHG) configuration has been predicted when supermodes are excited at the input of the NDC [2] . Supermode-based SHG has remained elusive up to now. Here we experimentally demonstrate classical supermode-based SHG through a specifically-designed integrated nonlinear interferometer ( Figure 1a ) made of linear (LDC) and nonlinear (NDC) directional couplers (DC) with a fully-fibered pump.

Spontaneous symmetry breaking (SSB) is a fundamentally important process that underlies many outstanding physical phenomena, such as phase transitions in metamaterials, and that can be exploited for a wide variety of applications. In particular, SSB has been observed in single coherently driven micro or macro ring resonators, where it respectively occurs between the forward and the counter propagative waves [1] , and between the two elliptically polarized components of the resonator [2] , and it has notably been proposed for new computing schemes. Nanodevices constitute a highly promising platform that could be harnessed for targeting a reduction of the photon number or improved integration capacity. SSB has recently been observed in evanescently coupled nanolasers, incoherently driven above the laser threshold, as the breaking of the mirror (or left/right) symmetry [3] . However, incoherent pumping suffers from thermal noise and a lack of control since the only control parameter is the pumping level in the nanolaser system, whereas the detuning can also be changed in the coherently driven system. In addition, most theoretical developments on few photon SSB in nonlinear coupled cavities have been carried out in a coherently driven-dissipative framework and have notably predicted important quantum features such as entanglement [4] . These driven-dissipative nonlinear coupled-cavity models constitute a paradigm of open quantum systems. Importantly, SSB in a coherently driven-dissipative system has not been demonstrated to date.