Spectrally and spatially varying ensembles of emitters embedded into waveguides are ever-present in both well-established and emerging technologies. If control of collective excitations can be attained, a plethora of coherent quantum dynamics and applications may be realized on-chip in the scalable paradigm of waveguide quantum electrodynamics (WQED). Here, we investigate inhomogeneously broadened ensembles embedded with subwavelength spatial extent into waveguides employed as single effective and coherent emitters. We develop a method permitting the approximate analysis and simulation of such mesoscopic systems featuring many emitters, and show how collective resonances are observable within the waveguide transmission spectrum once their linewidth exceeds the inhomogeneous line. In particular, this allows for near-unity and tailorable non-Lorentzian extinction of waveguide photons overcoming large inhomogeneous broadening present in current state-of-the-art implementations. As a particular illustration possible in such existing experiments, we consider the classic emulation of the cavity QED (CQED) paradigm here using ensembles of rare-earth ions as coherent mirrors and qubits and demonstrate the possibility of strong coupling given existing restrictions on inhomogeneous broadening and ensemble spatial extent. This work introduces coherent ensemble dynamics in the solid state to WQED and extends the realm to spectrally tailorable emitters.
Spectrally and spatially varying ensembles of emitters embedded into waveguides are ever-present in both well-established and emerging technologies. If control of collective excitations can be attained, a plethora of coherent quantum dynamics and applications may be realized on-chip in the scalable paradigm of waveguide quantum electrodynamics (WQED). Here, we investigate inhomogeneously broadened ensembles embedded with subwavelength spatial extent into waveguides employed as single effective and coherent emitters. We develop a method permitting the approximate analysis and simulation of such mesoscopic systems featuring many emitters, and show how collective resonances are observable within the waveguide transmission spectrum once their linewidth exceeds the inhomogeneous line. In particular, this allows for near-unity and tailorable non-Lorentzian extinction of waveguide photons overcoming large inhomogeneous broadening present in current state-of-the-art implementations. As a particular illustration possible in such existing experiments, we consider the classic emulation of the cavity QED (CQED) paradigm here using ensembles of rare-earth ions as coherent mirrors and qubits and demonstrate the possibility of strong coupling given existing restrictions on inhomogeneous broadening and ensemble spatial extent. This work introduces coherent ensemble dynamics in the solid state to WQED and extends the realm to spectrally tailorable emitters.
Waveguide quantum electrodynamics (wQED) has become a central platform for studying collective light-matter interactions in low-dimensional photonic environments. While conventional wQED systems rely on uniform chirality or reciprocal emitter-waveguide coupling, we propose a structured wQED framework, where the coupling directionality of each emitter can be engineered locally to control excitation transport in an atom-nanophotonic interface. For different combinations of patterned coupling directionalities of the emitters, we identify four representative configurations that exhibit distinct dynamical behaviors: centering, wave-like, leap-frog, and dispersion excitations. Spectral analysis of the effective non-Hermitian Hamiltonian reveals that these dynamics originate from interferences among subradiant eigenmodes. Variance analysis further quantifies the spreading of excitation as functions of interatomic spacing and global chirality, showing tunable localization-delocalization transitions. Including nonguided losses, we find that the transport characteristics remain robust for realistic coupling efficiencies (beta >= 0.99). These results establish structured wQED as a practical route to manipulate excitation localization, coherence, and transport through programmable directionality patterns, paving the way for controllable subradiant transport and chiral quantum information routing.
Quantum metasurfaces comprised of subwavelength atomic arrays emerged as a promising platform for enhanced atom-photon interaction. However, realizing such a system with solid-state emitters has been considered impractical due to strong inhomogeneous broadening, which was expected to suppress the photon-mediated interactions that underpin collective emission. Here we report the observation of collective emission from subwavelength arrays of silicon-vacancy centres in diamond -- solid-state emitters whose inhomogeneous broadening exceeds the natural linewidth by two orders of magnitude -- demonstrating that collective effects such as resonance shifts, modified decay rates and directional coherent emission survive this disorder. A crucial enabling element is the implantation of a high density of silicon ions at each array site. This creates so-called superatoms, local ensembles that probabilistically achieve frequency matching across the array and enhance the collective response. We support our observations
with a theoretical analysis explaining the mechanisms that preserve the collective effects even in the presence of inhomogeneity. These observations have direct implications for the realization of subwavelength arrays in any solid-state system, paving the way for quantum-emitter metasurfaces that are naturally integrated into nanophotonic environments.
Summary form only given. A current challenge and trend in optical sciences is to extend the toolbox generated from experiments with single emitters and single photons towards interfacing photons with a well-defined number of quantum emitters. The resulting complex optical network would enable studies of many-body quantum optical phenomena and could lend itself to quantum simulations. However, common light-matter interfaces via strong focusing or cavity enhancement do not scale favourably. Therefore, achieving many-body coupling while preserving control of the individual quantum systems poses remarkable experimental challenges.Here, we present an inherently scalable platform, which relies on the evanescent coupling of single dye molecules to subwavelength TiO2waveguides (nanoguides) that are embedded in an organic matrix (Fig. 1a). At liquid helium temperatures, we observe lifetime-limited linewidths for these molecules distributed over an inhomogeneously broadened spectrum. However, by using integrated microelectrodes, we can Stark shift the resonances over a large bandwidth. The dopant concentration determines the optical density of our system which can reach up to a few hundred molecules per cubic wavelength. The highly confined mode of a nanoguide allows a substantial mode overlap with the emission pattern of a single emitter, resulting in coupling efficiencies of up to 32%. Therefore, we can observe coherent extinction signals from many different molecules in the transmission signal of a nanoguide; an example is shown by the blue curve in Fig. 1b [1]. The green and red curves in Fig. 1b demonstrate the nonlinear behaviour of a molecule when driven with a second narrow-band laser [1, 2]. We show that by modifying the spectral properties of the molecules either via external optical or static electric fields we control the optical response of a nanoguide. In addition to discussing subtleties of design and fabrication, we elaborate on future efforts towards the realization of on-chip polaritonic states [3] and the integration of further optical elements such as micoresonators [4].
Cavity quantum electrodynamics (cavity QED) is conventionally described by the Jaynes- or Tavis-Cummings model, where quantum emitters couple to a single-mode cavity. The opposite scenario, in which an ensemble of emitters couples to a single spatial mode of a propagating light field, is described by waveguide QED, where emitters interact with a continuum of frequency modes. Here we present an experiment where an ensemble of cold atoms strongly couples to a fiber-ring resonator with variable length containing an optical nanofiber. By changing the length of the resonator we can tailor the density of frequency modes and thus explore the transition from cavity QED to waveguide QED. We analyse the response of the ensemble–cavity system after the sudden switch-on of resonant laser light and find that for progressively longer resonators, the Rabi oscillations typical of cavity QED disappear and the single-pass dynamics of waveguide QED appear. Our measurements shed light on the interplay between the single-pass collective response of the atoms to the propagating cavity field and the ensemble–cavity dynamics.
This review describes the emerging field of waveguide quantum electrodynamics (WQED) concerned with the interaction of photons propagating in a waveguide with localized quantum emitters. The collective emitter-photon interactions can lead to both enhanced and suppressed coupling compared to the case of independent emitters. Here, we focus on guided photons and ordered arrays, leading to super- and sub-radiant states, bound photon states and quantum correlations with promising quantum information applications. We highlight recent groundbreaking experiments performed with different quantum platforms, including cold atoms, superconducting qubits, semiconductor quantum dots, quantum solid-state defects, and we provide a comprehensive introduction to theoretical techniques to study the interactions and dynamics of these emitters and the photons in the waveguide.
We theoretically investigate excitation dynamics in one-dimensional arrays of quantum emitters coupled to a waveguide, focusing on localization and long-time population trapping. By combining time-domain simulations with spectral analysis of an effective non-Hermitian Hamiltonian, we identify two distinct mechanisms that give rise to localization: geometry-induced subradiance and disorder-induced Anderson-like confinement. Spatially modulated emitter arrangements—such as single- and double-Gaussian transverse profiles—enable long-lived subradiant modes even in the absence of disorder, with decay rates that can be finely controlled via geometric parameters. In contrast, localization in uniform arrays emerges only when disorder breaks spatial symmetry and suppresses collective emission through interference. We track the crossover between geometric and disorder-induced regimes, finding that double-Gaussian profiles exhibit clear spatial signatures of this transition, while single-Gaussian configurations display more gradual changes. These results establish geometry and disorder as complementary tools for engineering long-lived quantum states in waveguide QED systems, with direct relevance for scalable implementations in photonic platforms.