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arxiv: 2605.17878 · v1 · pith:NQWVLU2Nnew · submitted 2026-05-18 · 🪐 quant-ph

Bound state in the continuum and dynamics via phase modulation in giant-atom waveguide setups

Ji Qi , Xiaojun Zhang , Honngwei Yu , Zhihai Wang This is my paper

Pith reviewed 2026-05-20 11:00 UTC · model grok-4.3

classification 🪐 quant-ph
keywords bound states in the continuumgiant atomswaveguide QEDcoupling phasequantum dynamicsinterferencenonlocal light-matter coupling
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The pith

Coupling phase between two giant atoms controls the number and profiles of bound states in the continuum, enabling tailored quantum dynamics.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines a system of two giant atoms that each couple to a waveguide at multiple separated points, plus a direct link between the atoms that carries a controllable phase. Adjusting that phase changes how many bound states in the continuum form and what their spatial shapes look like for both the photons and the atoms themselves. The resulting bound states then produce several distinct patterns of time evolution for the overall quantum state. The work therefore identifies phase choice as a direct handle for shaping quantum behavior in these nonlocal light-matter systems. A reader would care because it supplies a concrete, tunable knob for interference-based control in emerging giant-atom quantum devices.

Core claim

In the two-giant-atom waveguide-QED model, the direct coupling phase determines both the number of bound states in the continuum and their profiles for photons and atoms. The presence of these BICs then produces a variety of dynamical behaviors that serve as an effective mechanism for tailoring quantum-state evolution.

What carries the argument

The direct coupling phase between the two giant atoms, which sets the interference conditions that create bound states in the continuum at the multi-point waveguide connections.

If this is right

  • Varying the coupling phase changes the number of BICs present in the system.
  • The spatial profiles of BICs for both photonic and atomic components can be adjusted by the phase.
  • Diverse dynamical behaviors emerge from the BICs and affect how quantum states evolve.
  • This supplies an effective mechanism for tailoring quantum-state evolution in giant-atom waveguide-QED systems.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same phase-tuning approach could scale to chains or networks of more than two giant atoms to create controllable multi-particle interference effects.
  • Phase engineering might be combined with frequency or distance adjustments to achieve more robust state preparation or error suppression.
  • Circuit-QED implementations with tunable couplers offer a direct route to test the predicted dependence of BIC count on phase.

Load-bearing premise

The two-giant-atom waveguide-QED model with its direct coupling phase accurately represents the physical system, including the validity of multi-point coupling beyond the dipole approximation and the absence of other decoherence channels.

What would settle it

Measure the photonic transmission spectrum or the time-dependent atomic excitation probabilities in a physical realization while varying the coupling phase; the predicted discrete changes in the number of bound states and the resulting evolution patterns should appear or disappear accordingly.

Figures

Figures reproduced from arXiv: 2605.17878 by Honngwei Yu, Ji Qi, Xiaojun Zhang, Zhihai Wang.

Figure 1
Figure 1. Figure 1: Schematic diagram of two braided giant atoms cou￾pled to a coupled-resonator waveguide (CRW) in Eq. (4). In the following sections, we consider that the giant atoms are coupled to the CRW at sites n1 and n2 (m1 and m2). tal quantum resource, such as entanglement, for a wide range of quantum-information-processing tasks. It is therefore natural to ask whether BICs can still emerge, and how they can be contr… view at source ↗
Figure 2
Figure 2. Figure 2: Dependence of the imaginary parts of the eigenval￾ues of the effective matrix M on the phase ϕ.(a)N = 12, ∆ = 4, (b)N = 14, ∆ = 2 and N = 17, ∆ = 5. The other parame￾ters are set as g = 0.1ξ, λ = 1.6g 2 /ξ. setup N = 17, ∆ = 5, but I1 = I2 < 0, implying the complete disappearance of the BICs. The dynamics of the atomic amplitudes α1(t) and α2(t) is determined by the eigenvalues of the effective non￾Hermiti… view at source ↗
Figure 3
Figure 3. Figure 3 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Profiles of the BICs and the corresponding atomic￾state tomography. (a,b) Spatial profiles of the BICs for N = 12 and ∆ = 4 at coupling phases ϕ = 0 and ϕ = π, respec￾tively. (c,d) Density matrices of the corresponding atomic states associated with the BICs shown in (a) and (b), respec￾tively. The other parameters are set as g = 0.1ξ, λ = 1.6g 2 /ξ. distribution |βi | 2 reaches its maxima when i − n1 is od… view at source ↗
read the original abstract

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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript studies a two-giant-atom waveguide-QED system that includes both multi-point waveguide couplings and a direct atom-atom interaction with tunable phase φ. It claims that varying φ controls the number and spatial profiles of bound states in the continuum (BICs) for both photons and atoms, and that the resulting BICs produce a variety of dynamical behaviors that can be used to tailor quantum-state evolution.

Significance. If the central claims are valid, the work identifies coupling-phase engineering as a practical control knob for interference, BIC formation, and dynamics in nonlocal light-matter systems. This could be relevant for giant-atom quantum networks, though its impact depends on how cleanly the phase modulation separates from existing waveguide-mediated effects.

major comments (1)
  1. [Model Hamiltonian] The effective Hamiltonian (presumably introduced in the model section) superposes a direct coupling term J e^{iφ} σ₁†σ₂ onto the multi-point waveguide couplings that already generate distance-dependent, complex-mediated interactions. No explicit subtraction of the overlapping virtual-photon contribution is described, raising the possibility that φ cannot be varied independently without altering the mediated part. This directly affects the claimed ability to tune BIC number and profiles via φ alone.
minor comments (1)
  1. [Abstract] The abstract contains a minor grammatical issue ('for both of photon and atoms').

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for identifying an important point regarding the construction of the effective Hamiltonian. We address the concern about the independence of the direct coupling phase φ below and will revise the manuscript accordingly to improve clarity.

read point-by-point responses

  1. Referee: [Model Hamiltonian] The effective Hamiltonian (presumably introduced in the model section) superposes a direct coupling term J e^{iφ} σ₁†σ₂ onto the multi-point waveguide couplings that already generate distance-dependent, complex-mediated interactions. No explicit subtraction of the overlapping virtual-photon contribution is described, raising the possibility that φ cannot be varied independently without altering the mediated part. This directly affects the claimed ability to tune BIC number and profiles via φ alone.

    Authors: We agree that the manuscript would benefit from a clearer derivation and justification of the effective Hamiltonian. In the model, the term J e^{iφ} σ₁†σ₂ represents a direct atom-atom interaction that is assumed to be engineered independently of the waveguide (for example, via an auxiliary tunable coupler in a circuit-QED realization). The waveguide-mediated interactions arising from the multi-point couplings are retained in full, and the direct term is added on top without double-counting because the virtual-photon exchange is already fully accounted for by the non-Markovian waveguide propagator. Nevertheless, we acknowledge that the current text does not explicitly state the separation or show the subtraction step. We will revise the model section to include a brief derivation sketch that isolates the direct coupling and confirms that φ can be varied independently while keeping the mediated part fixed. This clarification will support the subsequent claims about BIC control. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained within the effective model

full rationale

The paper constructs an effective Hamiltonian for two giant atoms coupled to a waveguide at multiple points, augmented by a direct atom-atom interaction term with tunable phase φ. Bound states in the continuum are obtained by solving the resulting eigenvalue problem in the single-excitation sector, and dynamics follow from time evolution under the same Hamiltonian. These steps rely on standard waveguide-QED interference and do not reduce any claimed prediction to a fitted input, self-definition, or load-bearing self-citation chain. The reported control of BIC number and profiles is a direct algebraic consequence of the phase-dependent matrix elements rather than a renaming or tautological re-expression of the inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claims rest on a standard waveguide-QED Hamiltonian for giant atoms plus an added direct-coupling term whose phase is treated as a free control parameter; no new particles or forces are introduced.

free parameters (1)
  • direct coupling phase
    Tunable phase offset between the two giant atoms that is varied to control BIC number and profiles.
axioms (2)
  • domain assumption Giant-atom multi-point coupling to the waveguide is valid beyond the dipole approximation
    Invoked in the model setup to justify spatially separated connection points.
  • standard math Waveguide supports propagating modes that interfere at the connection points
    Standard assumption in waveguide-QED treatments.

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Reference graph

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