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arXiv:2601.00622

Photonic Reservoir Engineering via 2D ΛΛΛ-Type Atomic Arrays in Waveguide QED

Thi Phuong Anh Nguyen, Le Phuong Hoang, Xuan Binh Cao

quant-phSubmitted Jan 2, 2026• Announced Jan 5, 2026• 0 citations
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Abstract

Electromagnetically induced transparency (EIT) in $Λ$-type atomic systems underpins quantum technologies such as high-fidelity memory and nonlinear optics, but conventional setups face intrinsic limitations. Standard geometries of one-dimensional atomic chains coupled to waveguides allow only a single bright superradiant channel, while subradiant modes remain weakly accessible, limiting control over collective radiative behavior and dark-state pathways. This leads to unwanted inelastic processes, degrading memory fidelity and reducing nonlinear photon generation efficiency. Here, we propose two two-dimensional (2D) atomic lattice geometries coupled to a photonic crystal waveguide, namely Zigzag and Orthogonal structures. In the Zigzag model, engineered collective super- and subradiant modes produce a flattened EIT window, broadening the transmission bandwidth and suppressing unwanted scattering to enhance quantum memory fidelity. In the Orthogonal model, four-wave mixing (FWM) intensity is amplified by up to six orders of magnitude relative to a conventional one-dimensional $Λ$-type EIT chain with identical $Γ_{1D}$, $Ω_c$, and probe intensity, with localized idler photons forming well-defined spectral modes. These results demonstrate a versatile route to engineer structured photonic reservoirs for on-demand photon generation, high-fidelity quantum storage, and enhanced nonlinear optical processes.

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Electromagnetically induced transparency (EIT) in ΛΛΛ-type atomic systems underpins quantum technologies such as high-fidelity memory and nonlinear optics, but conventional setups face intrinsic limitations. Standard geometries of one-dimensional atomic chains coupled to waveguides allow only a single bright superradiant channel, while subradiant modes remain weakly accessible, limiting control over collective radiative behavior and dark-state pathways. This leads to unwanted inelastic processes, degrading memory fidelity and reducing nonlinear photon generation efficiency. Here, we propose two two-dimensional (2D) atomic lattice geometries coupled to a photonic crystal waveguide, namely Zigzag and Orthogonal structures. In the Zigzag model, engineered collective super- and subradiant modes produce a flattened EIT window, broadening the transmission bandwidth and suppressing unwanted scattering to enhance quantum memory fidelity. In the Orthogonal model, four-wave mixing (FWM) intensity is amplified by up to six orders of magnitude relative to a conventional one-dimensional ΛΛΛ-type EIT chain with identical Γ1DΓ_{1D}Γ1D​, ΩcΩ_cΩc​, and probe intensity, with localized idler photons forming well-defined spectral modes. These results demonstrate a versatile route to engineer structured photonic reservoirs for on-demand photon generation, high-fidelity quantum storage, and enhanced nonlinear optical processes.
The absorption of photons in a three-level atom can be controlled and manipulated by applying a coherent drive at one of the atomic transitions. The situation where the absorption is fully canceled, and the atom thus has been turned completely transparent, has been coined electromagnetically induced transparency (EIT). The characteristics of EIT is a narrow transparency window associated with a fluorescence quench at its center frequency, indicating that inelastic scattering at this particular point is suppressed. The emergence of EIT-like transparency windows is common in waveguide quantum electrodynamics (QED) when multiple closely spaced quantum emitters are coupled to a waveguide. The transparency depends on the separation and energy detuning of the atoms. In this work, we study a number of different setups with two-level atoms in waveguide QED that all exhibit EIT-like transparency windows. Unlike the case of a genuine three-level atom, no drive fields are required in the systems we consider, and the coherent coupling of energy levels is mediated by the waveguide. We specifically distinguish between systems with genuine EIT-like dynamics and those that exhibit a transparency window but lack the fluorescence quench. The systems that we consider consist of both small and giant atoms, which can be experimentally realized with artificial atoms coupled to either photons or phonons. These systems can offer a simpler route to many EIT applications since the need for external driving is eliminated.
Recent experimental advances in preparing regular defect-free 2D arrays of atoms with optical traps have offered new opportunities to engineer strong collective couplings between light and atoms as quantum optical dipoles. Applications of such collective light-matter interactions include the realization of atom-thick mirrors, retrieval of quantum memories with high efficiency, or the implementation of topological quantum optics. I will present novel applications in the context of quantum networking. First, I will discuss the design of a chiral photonic quantum link, where distant single atoms interact by exchanging photons propagating in a unique direction in free space. This is achieved by coupling both single atoms, representing qubits implemented using a pair of Rydberg states, to bi-layer atomic arrays which act as quantum phased-array antennas. Exploiting the combination of a “Rydberg-dressing” laser and a control laser driving the atomic arrays, an effective Rydberg dipole-dipole interaction can be engineered between atoms and atomic arrays as quantum emitters, allowing to match the spatial modes of spontaneously emitted and absorbed photons to a Gaussian mode of interest. In this way, we realize a chiral quantum interface, i.e., where atoms couple to photonic modes with a unique direction of propagation. This setup provides a basic building block of a novel platform for quantum networks in free space, i.e., without requiring coupling atoms to modes of cavities or nanostructures, which I will illustrate with the deterministic coherent transfer of quantum states between single atoms with high fidelity. Second, I will discuss the collective radiation properties of two distant single-layer atomic arrays acting as quantum memories. This system can support a long-lived non-local superposition state of atomic excitations exhibiting strong subradiance. This “dark” mode consists of an atomic excitation which is delocalized between the two distant atomic arrays, in such a way that the overall radiation is strongly suppressed by quantum interference. I will describe the preparation of these states and their application as resource of non-local entanglement. In particular, by weakly driving the arrays, the state of the quantum memories, localized in each array, can be transferred to this “non-local” subradiant state, allowing again for a deterministic coherent quantum state transfer with high fidelity between the memories. Finally, I will discuss experimental realizations using cold atoms in optical trap arrays, and the effect of experimental imperfections in the arrays, such as finite defect probability, temperature, and depth of the optical traps.
Optical quantum memories are important components in the long-distance quantum communication based on quantum repeater protocol. To outperform the direct transmission of light with quantum repeaters, it is crucial to develop quantum memories with high fidelity, high efficiency and long storage time. Recently, we demonstrate that it is feasible to achieve a high storage efficiency of 92% for electromagnetically-induced-transparency (EIT)-based memory with weak coherent signal pulses in cold atomic ensembles [1]. To realize the highly-efficient memory with quantum light, we have built a bright and narrowband photon-pair source which can be locked to atomic transition based on the cavity-enhanced spontaneous parametric down conversion [2]. Here, we present our results on the storage of single photons generated by such a source in EIT-based memories. A storage efficiency of 36% is obtained in initial runs. Future improvements toward a high efficiency are discussed. Such a development paves the way for the applications of photon-pair-based quantum repeater and multi-photon synchronization.
The efficient and controllable generation of entangled photon pairs is a cornerstone for advancing quantum technologies, including quantum computing, communication, and sensing. Traditional methods, primarily relying on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) in bulk nonlinear crystals or waveguides, often suffer from low efficiency, bulky form factors, and limited control over the spectral and spatial properties of the generated photons. This paper explores a transformative approach leveraging resonant Bound States in the Continuum (BICs) within nanophotonic structures to overcome these limitations. BICs are extraordinary optical resonances characterized by theoretically infinite quality (Q) factors, enabling extreme light localization and immense field enhancement within subwavelength volumes. By integrating nonlinear optical materials into BIC-supporting structures, such as photonic crystal slabs or metasurfaces, the local electric field strength experienced by the nonlinear medium can be dramatically increased, boosting the efficiency of entangled photon generation. Furthermore, the sharp, tailor-able spectral response of BICs offers unprecedented control over the joint spectral amplitude of the photon pairs, promising high degrees of spectral purity and entanglement fidelity. This work outlines the theoretical underpinnings, potential device architectures, and anticipated advantages of BIC-enhanced entangled photon sources, paving the way for compact, high-brightness, and spectrally engineered quantum light sources critical for next-generation quantum technologies.
We describe an approach to achieve near-perfect unidirectional light-matter coupling to an effective quantum emitter that is formed by a subwavelength array of atoms in the Rydberg-blockade regime. The nonlinear reflection and transmission of such two-dimensional superatoms are exploited in different interferometric setups for the deterministic generation of tunable single photons and entangling two-photon operations with high fidelities, F≳0.999\mathcal{F}\gtrsim0.999F≳0.999. The described setup can function as a versatile nonlinear optical element in a free-space photonic quantum network with simple linear elements and without the need of additional mode confinement, optical resonators, or optical isolators.
Bound states in the continuum (BICs) can confine photonic excitations in open systems without conventional cavities or band gaps, making them natural candidates for long-lived quantum storage and single-photon control. Their use is limited, however, by two obstacles: they are dark to incident photons, and they lack spectral-gap protection from the surrounding continuum. We overcome both limitations in a giant atom coupled to a one-dimensional waveguide using two temporal control knobs. Atomic-frequency modulation breaks and restores the destructive-interference condition, enabling deterministic capture and release of mode-matched single photons. Coupling modulation instead preserves the BIC condition while tuning the atomic and photonic weights of the stored state. A key result is that this embedded state can nevertheless be controlled adiabatically despite the absence of a spectral gap, with an intrinsic leakage probability linear in the ramp rate. By separating radiative access from BIC-preserving deformation, the protocol turns a dark BIC into a single-photon memory whose fidelity is set by the intrinsic continuum-induced leakage law, providing a route to embedded-state control in open photonic platforms.