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arXiv:2606.01146

Most Subradiant Bound Photon Pairs from Chirality-Mediated Dispersion Softening

Kailin Tan, Xuanbing Jiang, Dong Wang, Saijun Wu

quant-phphysics.atom-phSubmitted May 31, 2026• Announced Jun 2, 2026• 0 citations
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Abstract

We study the subradiant bound states (BSs) in a two-level atom array chirally coupled to a one-dimensional waveguide. We demonstrate that the chiral interaction can drive BSs to become the most subradiant two-excitation states across a wide spacing range. This phenomenon is rooted in a mechanism of chirality-mediated dispersion softening, where the BS band distortion suppresses the band curvature $|α_2|$ at an extremum point. We rigorously prove that the BS decay rate follows the scaling $Γ\sim |α_2|/N^3$, revealing that the reduction of $|α_2|$ is key to suppressing emission and enhancing subradiance. We also show the existence of chiral BSs in a realistic nanofiber interface.

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We study the subradiant bound states (BSs) in a two-level atom array chirally coupled to a one-dimensional waveguide. We demonstrate that the chiral interaction can drive BSs to become the most subradiant two-excitation states across a wide spacing range. This phenomenon is rooted in a mechanism of chirality-mediated dispersion softening, where the BS band distortion suppresses the band curvature ∣α2∣|α_2|∣α2​∣ at an extremum point. We rigorously prove that the BS decay rate follows the scaling Γ∼∣α2∣/N3Γ\sim |α_2|/N^3Γ∼∣α2​∣/N3, revealing that the reduction of ∣α2∣|α_2|∣α2​∣ is key to suppressing emission and enhancing subradiance. We also show the existence of chiral BSs in a realistic nanofiber interface.
We study theoretically the radiative lifetime of bound two-particle excitations in a waveguide with an array of two-level atoms, realizing a one-dimensional Dicke-like model. Recently, Zhang et al. have numerically found an unexpected sharp maximum of the bound pair lifetime when the array period ddd is equal to 1/12th1/12\mathrm{th}1/12th of the light wavelength ${\ensuremath{\lambda}}_{0}$ [Phys. Rev. Research 2, 013173 (2020)]. We uncover a rigorous transformation from the non-Hermitian Hamiltonian with the long-range radiative coupling to the nearest-neighbor coupling model with the radiative losses only at the edges. This naturally explains the puzzle of long lifetime: the effective mass of the bound photon pair also diverges for $d={\ensuremath{\lambda}}_{0}/12$, hampering an escape of photons through the edges. We also link the oscillations of the lifetime with the number of atoms to the nonmonotonous quasi-flat-band dispersion of the bound pair.
We consider two two-level quantum emitters (QEs) with separations on the order of the wavelength which are chirally coupled to a one-dimensional (1D) waveguide, and the electromagnetic field of the 1D waveguide has a direction-dependent velocity, which produces two field propagation phases on the dynamics of QEs. Their spontaneous process is examined for QEs having unequal emission rates to the waveguide. It is found that radiation could be enhanced for both QEs, inhibited for both QEs, enhanced for one while inhibited for the other, completely suppressed for both QEs. In particular, the mechanism for radiation completely suppressed is the presence of a QE-photon bound state.
We present a theoretical study of the two-particle spectrum ω(K)ω(K)ω(K) for the chiral waveguide QED setup of an array of two-level atoms directionally interacting with photons propagating along the waveguide. We demonstrate that for each pair center-of-mass momentum KKK there exist distinct solutions with ℑω≤0\Imω\le 0ℑω≤0 in the two-particle spectrum, corresponding to bound, antibound and resonance states, in addition to the continuum of scattering states. Contrary to previous studies, which showed the bound and resonance-state spectra only over a limited range of KKK, the calculated spectrum is consistent across all KKK values. An interesting finding is that the real part of the spectrum ℜω(K)\Re ω(K)ℜω(K) in the chiral model is gapless. The calculated dispersion law ω(K)ω(K)ω(K) provides an effective model for the bound photon pairs also in a finite-size array, manifesting the topological non-Hermitian skin effect.
The properties of coupled emitters can differ dramatically from those of their individual constituents. Canonical examples include sub- and super-radiance, wherein the decay rate of a collective excitation is reduced or enhanced due to correlated interactions with the environment. Here, we systematically study the properties of collective excitations for regularly spaced arrays of quantum emitters coupled to a one-dimensional waveguide. We find that, for low excitation numbers, the modal properties are well-characterized by spin waves with a definite wavevector. Moreover, the decay rate of the most subradiant modes obeys a universal scaling with a cubic suppression in the number of emitters. Multi-excitation subradiant eigenstates can be built from fermionic combinations of single excitation eigenstates; such 'fermionization' results in multiple excitations that spatially repel one another. We put forward a method to efficiently create and measure such subradiant states, which can be realized with superconducting qubits. These measurement protocols probe both real-space correlations (using on-site dispersive readout) and temporal correlations in the emitted field (using photon correlation techniques).
We study the emergence of two types of two-photon bounds states in waveguides of any chirality. Specifically, we present a systematic way of analytically determining the eigenstates of a system consisting of a waveguide coupled to a partially chiral, infinite array of equidistant two-level emitters. Using an effective Hamiltonian approach, we determine the properties of the two-photon bound states by determining their dispersion relation and internal structure. The bound states come in two varieties, depending on the two-photon momentum and emitter spacing. One of these states is a long-lived true bound state, whereas the other, a scattering resonance, decays in time via coupling to free two-photon states, leading to resonances and corresponding phase shifts in the photon-photon scattering.