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Physical Review Letters

Coherent Control of an Embedded Bound State without a Spectral Gap

Anonymous

Published Sep 1, 2026• 0 citations
Publisher

Abstract

Bound states in the continuum (BICs) can confine photonic excitations in open systems without conventional cavities or band gaps, making them promising candidates for long-lived quantum storage and single-photon control. Their use as quantum resources is limited, however, by two basic difficulties: they are dark to incident photons, and they are not protected by a spectral gap from the surrounding continuum. We address these difficulties in a giant atom coupled to a one-dimensional waveguide by separating radiative access from BIC-preserving manipulation. Atomic-frequency modulation dynamically breaks and restores the destructive-interference condition, allowing a mode-matched single photon to be captured into, and released from, the BIC. Symmetric coupling modulation, in contrast, preserves the BIC condition throughout the evolution and continuously changes the atomic and photonic weights of the stored state. This BIC-preserving deformation realizes a gapless adiabatic control problem: for a regular embedded continuum, the intrinsic leakage probability is proportional to the ramp rate rather than quadratically suppressed as in gapped systems. The linear scaling originates from near-resonant scattering states at the BIC energy and is independent of microscopic dispersion except through the prefactor. These results establish a coherent capture-control-release protocol for a populated BIC and identify the fundamental error law for manipulating quantum states that remain embedded in a gapless environment.

DOI
10.1103/8mrx-82gp

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Bound states in the continuum (BICs) can confine photonic excitations in open systems without conventional cavities or band gaps, making them promising candidates for long-lived quantum storage and single-photon control. Their use as quantum resources is limited, however, by two basic difficulties: they are dark to incident photons, and they are not protected by a spectral gap from the surrounding continuum. We address these difficulties in a giant atom coupled to a one-dimensional waveguide by separating radiative access from BIC-preserving manipulation. Atomic-frequency modulation dynamically breaks and restores the destructive-interference condition, allowing a mode-matched single photon to be captured into, and released from, the BIC. Symmetric coupling modulation, in contrast, preserves the BIC condition throughout the evolution and continuously changes the atomic and photonic weights of the stored state. This BIC-preserving deformation realizes a gapless adiabatic control problem: for a regular embedded continuum, the intrinsic leakage probability is proportional to the ramp rate rather than quadratically suppressed as in gapped systems. The linear scaling originates from near-resonant scattering states at the BIC energy and is independent of microscopic dispersion except through the prefactor. These results establish a coherent capture-control-release protocol for a populated BIC and identify the fundamental error law for manipulating quantum states that remain embedded in a gapless environment.
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.
Bound states in the continuum (BICs) provide a mechanism for preserving entanglement in waveguide quantum electrodynamics. Here, using quantum control techniques in a configuration of two braided giant atoms, we propose a robust approach to create high-fidelity entangled BICs. The protocol combines quasi-Floquet modulation of the effective atom--waveguide couplings with an independent phase-controlled atomic exchange, which provides restricted counterdiabatic assistance for preparing the complete dressed BIC. In the lossless effective model, we obtain a full-state BIC fidelity of 0.999920.999920.99992 and an unconditional atomic Bell fidelity of 0.995880.995880.99588 using a fast control protocol. Microscopic finite-mode simulations validate the central-sideband description during the passage, while static calibration-error scans quantify its control tolerances. A separate microscopic study with relaxation and dephasing, using experimentally motivated component scales, identifies a finite useful entanglement window and a benefit from shaping the coupling envelopes. These results support controlled preparation, retention, and retrieval of dressed entanglement within the stated control and noise assumptions.
Abstract We investigate phase-controlled bound states in a one-dimensional photonic waveguide coupled to an artificial giant atom at two distant sites. Specifically, we identify the bound state out of the continuum (BOC) and the bound state in the continuum (BIC) and derive the exact existence condition for the BOC. Furthermore, we analytically determine the BIC’s frequency and photonic distribution profile. Remarkably, our analysis reveals quantum beats in both atomic and photonic dynamics, arising from coherent oscillations between the BIC and BOC. These results establish a novel approach for manipulating waveguide quantum electrodynamics via engineered bound states, with potential applications in quantum information processing.
The giant atom paradigm, where a single quantum emitter couples to a continuum at multiple discrete points, has enabled unprecedented control over light-matter interactions, including decoherence-free subspaces and chiral emission. However, realizing these non-local effects beyond the microwave regime remains a significant challenge due to the diffraction limit. Here, we theoretically propose a photonic analog of giant atoms operating at optical frequencies, utilizing a quantum emitter resonantly coupled to a pair of spatially separated single-mode cavities interacting with a common 1D photonic continuum. By rigorously deriving the effective non-Hermitian Hamiltonian and integrating out the bath degrees of freedom, we demonstrate that the interference between cavity-mediated emission pathways leads to the formation of robust Bound States in the Continuum (BICs). These interference-induced dark states allow for the infinite trapping of excitation within the emitter-cavity subsystem, effectively shielding it from radiative decay. Our results extend the giant atom toolbox to the optical domain, offering a scalable architecture for integrated quantum photonics and quantum interconnects.
It is known that the superposition of two bound states in the continuum (BICs) leads to the phenomenon of an oscillating bound state, where excitations mediated by the continuum modes oscillate persistently. We perform exact calculations for the oscillating BICs in a one-dimensional photonic lattice coupled to a ``giant atom'' at multiple points. Our work is significantly distinct from previous proposals of oscillating BICs in continuous waveguide systems due to the presence of a finite energy band contributing band-edge effects. In particular, we show that the bound states outside the energy band are detrimental to the oscillating BIC phenomenon, and can be suppressed by increasing either the number of coupling points or the separation between each coupling point. Crucially, non-Markovianity is necessary for the existence of oscillating BICs, and the oscillation amplitude increases with the characteristic delay time of the giant atom interactions. We also propose an initialization scheme in the BIC subspace. Our work be experimentally implemented on current photonic waveguide array platforms and opens up prospects in utilizing reservoir engineering for the storage of quantum information in photonic lattices.
Abstract We investigate the dynamics of two two-level giant atoms coupled to a one-dimensional waveguide in separate and braided configurations. Using the resolvent method, the quantum dynamics are determined by the poles of the Green’s function. We identify conditions under which three real poles with distinct eigenfrequencies emerge, corresponding to three bound states in the continuum (BICs). Their superposition gives rise to oscillating BICs. In this regime, the atomic excitation oscillates between the two atoms, while the photon field remains confined between the coupling points and exhibits persistent oscillations. The maxima of the photon field distribution coincide with those of the atomic excitations, reflecting the atom-photon bound-state nature. The revival of the photon distribution reveals the coherent interaction between the two giant atoms mediated by the BICs. This work broadens the fields of quantum information storage and manipulation by leveraging larger Hilbert spaces enabled by multiple nonradiative BICs.