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Photonics Research

Tunable photon bunching and antibunching in braided giant-atom arrays via Fano interference

Wenju Gu, Liangbin He, Haixia Pang, Zhen Yi, Lihui Sun, Yan Yan, Su-jing Liu, Gaoxiang Li

Published Sep 8, 2026• 0 citations
Publisher

Abstract

Deterministic generation and manipulation of nonclassical light are the central goals of quantum photonics. Waveguide quantum electrodynamics with atomic arrays enable strong photon–photon interactions via collective effects. Recently, giant atoms with multiple coupling points have opened new avenues for engineering photon correlations. In particular, the braided configuration offers unique features; however, most studies on quantum nonlinearity are limited to two atoms. Here, we extend the investigation to arrays of more braided giant atoms and uncover two distinct regimes controlled by phase parameters. One regime corresponds to Lorentzian resonances in the single-excitation subspace, where the system behaves as an effective two-level model at resonance, producing bunched transmitted light and antibunched reflected light. The two-excitation nonlinearity is weaker and tends to bunch photons, and at triple resonance the system loses tunability. The second regime corresponds to the Fano resonance arising from the spectral coalescence of the collective modes. In this regime, we achieve near-unity transmission with strong photon antibunching in transmission, and significant photon bunching in reflection. Our results establish braided giant-atom arrays as a scalable, cavity-free platform for engineering nonclassical photon correlations.

DOI
10.1364/prj.609840

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Deterministic generation and manipulation of nonclassical light are the central goals of quantum photonics. Waveguide quantum electrodynamics with atomic arrays enable strong photon–photon interactions via collective effects. Recently, giant atoms with multiple coupling points have opened new avenues for engineering photon correlations. In particular, the braided configuration offers unique features; however, most studies on quantum nonlinearity are limited to two atoms. Here, we extend the investigation to arrays of more braided giant atoms and uncover two distinct regimes controlled by phase parameters. One regime corresponds to Lorentzian resonances in the single-excitation subspace, where the system behaves as an effective two-level model at resonance, producing bunched transmitted light and antibunched reflected light. The two-excitation nonlinearity is weaker and tends to bunch photons, and at triple resonance the system loses tunability. The second regime corresponds to the Fano resonance arising from the spectral coalescence of the collective modes. In this regime, we achieve near-unity transmission with strong photon antibunching in transmission, and significant photon bunching in reflection. Our results establish braided giant-atom arrays as a scalable, cavity-free platform for engineering nonclassical photon correlations.
We investigate the scattering processes of two photons in a one-dimensional waveguide coupled to two giant atoms. By adjusting the accumulated phase shifts between the coupling points, we are able to effectively manipulate the characteristics of these scattering photons. Utilizing the Lippmann-Schwinger formalism, we derive analytical expressions for the wave functions describing two-photon interaction in separate, braided, and nested configurations. Based on these wave functions, we also obtain analytical expressions for the incoherent power spectra and second-order correlation functions. In contrast to small atoms, the incoherent spectrum, which is defined by the correlation of the bound state, can exhibit more tunability due to the phase shifts. Additionally, the second-order correlation functions in the transmission and reflection fields could be tuned to exhibit either bunching or antibunching upon resonant driving. These unique features offered by the giant atoms in waveguide QED could benefit the generation of nonclassical itinerant photons in quantum networks.
The generation of high-performance nonclassical light remains a cornerstone of quantum technologies, yet faces a fundamental trade-off between emission purity and brightness. Here, we demonstrate that cavity-mediated many-body spin-exchange interactions provide a route to overcome this constraint by collectively amplifying spectral anharmonicity. In a cavity-coupled atomic array with a programmable relative phase φφφ, the resulting interference-interaction mechanism reshapes the dressed-state manifold and enables deterministic switching between distinct quantum emission regimes. For φ=0φ=0φ=0, constructive interference yields high-purity single-photon emission with antibunching improved by four orders of magnitude while preserving strong photon flux. Conversely, for φ=πφ=πφ=π, destructive interference creates a dark single-photon manifold, resonantly activating two-photon processes to produce bright and pure photon-pair bundles. Our work establishes interference-engineered many-body interactions as a scalable mechanism for on-demand quantum light generation and open a new avenue for harnessing collective many-body physics in quantum photonics.
The giant-atom topological-waveguide-QED systems have recently emerged as a promising platform for manipulating light-matter interactions. The combination of the multiple-point couplings and topological phase effect could lead to rich physical phenomena and effects. Here, we study single-photon scattering in a Su-Schrieffer-Heeger (SSH) waveguide coupled to either one or two two-level giant atoms. We assume that each giant atom is coupled to the waveguide via two coupling points and hence there exist four and sixteen coupling configurations for the single-giant-atom case and two-giant-atom separate coupling case, respectively. By solving the single-photon scattering problem in the real space, we obtain the exact expressions of the single-photon scattering amplitudes. It is found that a single photon in the SSH waveguide can be completely reflected or transmitted by choosing proper coupling configurations, coupling-point distances, atomic resonance frequency, and dimerization parameter. In addition, under proper parameter conditions, the scattering spectra are periodically modulated by the coupling-point distances. We also find that the collective behavior of the two giant atoms can be adjusted by quantum interference effect and topological effect and that the single-photon scattering spectra can exhibit the Lorentzian, super-Gaussian, electromagnetically induced transparencylike, and asymmetric Fano line shapes for some coupling configurations. This work will inspire the development of controllable single-photon devices based on the giant-atom topological-waveguide-QED systems.
The Fano effect arising from the interference between two dissipation channels of the radiation continuum enables tuning of the photon statistics. Understanding the role of the Fano effect and exploiting it to achieve strong photon correlations are of both fundamental and applied significance. We present an analytical description of Fano-enhanced photon correlations based on cavity quantum electrodynamics to show that the Fano effect in atom-cavity systems can improve the degree of antibunching by over four orders of magnitude. The enhancement factors and the optimal conditions are explicitly given, and found to relate to the Fano parameter q. Remarkably, the Fano enhancement manifests robustness against the decoherence processes and can survive in the weak coupling regime. We expect our work to provide insight to tuning the photon statistics through the Fano effect, which offers a new, to the best of our knowledge, route to enhance the photon correlations, as well as the possibility of generating nonclassical light in a wider diversity of systems without the need of a strong light-matter interaction.
We study the coherent single-photon scattering in a one-dimensional waveguide coupled to a giant artificial molecule consisting of two coupled giant atoms. Since each giant atom couples to the waveguide via two coupling points, the couplings of the molecule with the waveguide have three different coupling configurations: the separated-, braided-, and nested-coupling cases. We obtain the exact expressions of the single-photon transmission and reflection amplitudes with the real-space approach. It is found that the behavior of the scattering spectra depends on the phase shift between two neighboring coupling points, the coupling configuration, and the coupling between the two giant atoms. Concretely, we study the photon scattering in both the Markovian and non-Markovian regimes, in which the photon propagating time between two neighboring coupling points is neglected and considered, respectively. Under the Markovian limit, the asymmetric Fano line shapes in different coupling configurations of the giant-molecule waveguide-QED system can be obtained by choosing proper phase shift, and the transmission window can be adjusted by the coupling strength between the two giant atoms in these three coupling configurations. In particular, multiple reflection peaks and dips in these configurations are revived in the non-Markovian regime. This paper will pave the way for the study of controllable single-photon devices based on the giant-molecule waveguide-QED systems.
A strong optical nonlinearity arises when coherent light is scattered by a semiconductor quantum dot coupled to a nanophotonic waveguide. We exploit the Fano effect in such a waveguide to control the phase of the quantum interference underpinning the nonlinearity, experimentally demonstrating a tunable quantum optical filter which converts a coherent input state into either a bunched or an antibunched nonclassical output state. We show theoretically that the generation of nonclassical light is predicated on the formation of a two-photon bound state due to the interaction of the input coherent state with the quantum dot. Our model demonstrates that the tunable photon statistics arise from the dependence of the sign of two-photon interference (either constructive or destructive) on the detuning of the input relative to the Fano resonance.