The integration of nanophotonics and atomic physics has been a long-sought goal that would open new frontiers for optical physics, including novel quantum transport and many-body phenomena with photon-mediated atomic interactions. Reaching this goal requires surmounting diverse challenges in nanofabrication and atomic manipulation. Here we report the development of a novel integrated optical circuit with a photonic crystal capable of both localizing and interfacing atoms with guided photons. Optical bands of a photonic crystal waveguide are aligned with selected atomic transitions. From reflection spectra measured with average atom number N ¼ 1:1 AE 0:4, we infer that atoms are localized within the waveguide by optical dipole forces. The fraction of single-atom radiative decay into the waveguide is G 1D /G 0 C(0.32 ± 0.08), where G 1D is the rate of emission into the guided mode and G 0 is the decay rate into all other channels. G 1D /G 0 is unprecedented in all current atom-photon interfaces.

We report observations of superradiance for atoms trapped in the near field of a photonic crystal waveguide (PCW). By fabricating the PCW with a band edge near the D 1 transition of atomic cesium, strong interaction is achieved between trapped atoms and guided-mode photons. Following short-pulse excitation, we record the decay of guided-mode emission and find a superradiant emission rate scaling as Γ SR ∝NΓ 1D for average atom number 0.19 ≲N ≲ 2.6 atoms, where Γ 1D =Γ 0 ¼ 1.0 AE 0.1 is the peak singleatom radiative decay rate into the PCW guided mode, and Γ 0 is the radiative decay rate into all the other channels. These advances provide new tools for investigations of photon-mediated atom-atom interactions in the many-body regime. DOI: 10.1103/PhysRevLett.115.063601 PACS numbers: 42.50.Ct, 37.10.Gh, 42.70.Qs Interfacing light with atoms localized near nanophotonic structures has attracted increasing attention in recent years. Exemplary experimental platforms include nanofibers [1][2][3], photonic crystal cavities [4], and waveguides [5,6]. Owing to their small optical loss and tight field confinement, these nanoscale dielectric devices are capable of mediating long-range atom-atom interactions using photons propagating in their guided modes. This new paradigm for strong interaction of atoms and optical photons offers new tools for scalable quantum networks [7], quantum phases of light and matter [8,9], and quantum metrology [10].In particular, powerful capabilities for dispersion and modal engineering in photonic crystal waveguides (PCWs) provide opportunities beyond conventional settings in atomic, molecular and optical physics within the new field of waveguide QED [2,3,6,[11][12][13]. For example, the edge of a photonic band gap aligned near an atomic transition strongly enhances single-atom emission into the one-dimensional (1D) PCW due to a "slow-light" effect [14][15][16]. Because the electric field of a guided mode near the band edge approaches a standing wave, optical excitations can be induced in an array of trapped atoms with little propagation phase error, resulting in phase-matched superradiant emission [17,18] into both forward and backward waveguide modes of the PCW. Superradiance has important applications for realizing quantum memories [19][20][21][22][23], single-photon sources [24,25], laser cooling by way of cooperative emission [26,27], and narrow linewidth lasers [28]. Related cooperative effects are predicted in nanophotonic waveguides absent an external cavity [29], including atomic Bragg mirrors [30] and selforganizing crystals of atoms and light [31][32][33].Complimentary to superradiant emission is the collective Lamb shift induced by proximal atoms virtually exchanging off-resonant photons [34][35][36][37]. With the atomic transition frequency placed in a photonic band gap of a PCW, real photon emission is largely suppressed. Coherent atomatom interactions then emerge as a dominant effect for QED with atoms in band-gap materials [38][39][40][41][42][43]. Both the strength and lengt...

Tailoring the interactions between quantum emitters and single photons constitutes one of the cornerstones of quantum optics. Coupling a quantum emitter to the band edge of a photonic crystal waveguide (PCW) provides a unique platform for tuning these interactions. In particular, the cross-over from propagating fields E(x) ∝ e ±ikx x outside the bandgap to localized fields E(x) ∝ e −κx jxj within the bandgap should be accompanied by a transition from largely dissipative atom-atom interactions to a regime where dispersive atom-atom interactions are dominant. Here, we experimentally observe this transition by shifting the band edge frequency of the PCW relative to the D 1 line of atomic cesium for N = 3.0 ± 0.5 atoms trapped along the PCW. Our results are the initial demonstration of this paradigm for coherent atomatom interactions with low dissipation into the guided mode.quantum optics | nanophotonics | atomic physics R ecent years have witnessed a spark of interest in combining atoms and other quantum emitters with photonic nanostructures (1). Many efforts have focused on enhancing emission into preferred electromagnetic modes relative to vacuum emission, thereby establishing efficient quantum matter-light interfaces and enabling diverse protocols in quantum information processing (2). Photonic structures developed for this purpose include high-quality cavities (3-7), dielectric fibers (8-13), metallic waveguides (14-16), and superconducting circuits (17-19). Photonic crystal waveguides (PCWs) are of particular interest, because the periodicity of the dielectric structure drastically modifies the field propagation, yielding a set of Bloch bands for the guided modes (GMs) (20). For example, recent experiments have shown superradiant atomic emission because of a reduction in group velocity for an atomic frequency near a band edge of a PCW (21).A quite different paradigm for atom-light interactions in photonic crystals was proposed in the works in refs. 22-25 but has yet to be experimentally explored. In particular, when an atomic transition frequency is situated within a bandgap of a PCW, an atom can no longer emit propagating waves into GMs of the structure. However, an evanescent wave surrounding the atoms can still form, resulting in the formation of atom-photon-bound states (26,27). This phenomenon has attracted new interest recently as a means to realize dispersive interactions between atoms without dissipative decay into GMs. The spatial range of atomatom interactions is tunable for 1D and 2D PCWs and set by the size of the photonic component of the bound state (28, 29). Manybody physics with large spin exchange energies and low dissipation can thereby be realized in a generalization of cavity quantum electrodynamics (CQED) arrays (30,31). Fueled by such perspectives, there have been recent experimental observations with atoms (21, 32, 33) and quantum dots (34, 35) interacting through the GMs of PCWs, albeit in frequency regions outside the bandgap, where GMs are propagating fields.In this manuscript, we re...