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arXiv:2609.32125

Counterdiabatic quasi-Floquet control for the generation of entangled BICs using giant atoms

Alexis R. Legón, Pedro Orellana, Ariel Norambuena

quant-phSubmitted Sep 26, 2026• Announced Sep 29, 2026• 0 citations
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Abstract

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 $0.99992$ and an unconditional atomic Bell fidelity of $0.99588$ 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.

Comments
25 pages, 9 figures

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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.
Giant atoms, which couple to a waveguide through multiple spatially separated connection points beyond the dipole approximation, provide a versatile route for quantum information processing based on interference-induced bound states in the continuum (BICs). While multi-giant-atom architectures are being developed toward giant-atom quantum networks, the role of direct coupling between the giant atoms, in particular the associated coupling phase, in atomic dynamics remains insufficiently understood. Here we take a first step toward addressing this issue by studying a two-giant-atom waveguide-QED model. We show that the coupling phase can be used to control both the number of BICs and their profiles for both of photon and atoms. More interestingly, the presence of BICs gives rise to a variety of dynamical behaviors, providing an effective mechanism for tailoring quantum-state evolution in giant-atom waveguide-QED systems. Our results highlight coupling-phase engineering as a useful tool for controlling interference, bound states, and quantum dynamics in nonlocal light--matter interfaces.
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) convert dissipative open systems into effectively closed quantum subspaces through destructive interference. We show that two identical giant atoms coupled to a one-dimensional waveguide support BICs that coincide with maximally entangled atomic states. Most importantly, entanglement is predominantly determined by the geometric design; the ratio of intra-atomic connection lengths fixes the concurrence, while the propagation phase between atoms selects a family of Bell-like states. We further analyze the dynamical stability of these maximally entangled BICs under exact time evolution, revealing a clear hierarchy of robustness against parameter perturbations. Our results establish an analytical bridge between symmetry, geometry, entanglement, and BICs in giant-atom waveguide platforms.
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Bound states in the continuum (BICs) have attracted intense interest, yet their many-particle counterparts remain largely unexplored in waveguide quantum electrodynamics. We identify and characterize a bound state embedded in the doublon continuum (BIDC) that emerges when four atoms couple to a coupled-resonator waveguide with strong on-site interaction. Exploiting this interaction-enabled BIDC, we show that (i) a distant, four-atom entangled state can be prepared with high fidelity, and (ii) quantum entangled states can be coherently transferred between spatially separated nodes. Our results establish a scalable mechanism for multi-particle state generation and routing in waveguide platforms, opening a route to interaction-protected quantum communication with many-particle BICs.
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.