Localized Particle Detector Model

• Localized Particle Detector Model is a framework that couples quantum fields to space-confined interactions using Robin boundary conditions to generate a discrete bound-state sector.
• It leverages a self-adjoint extension approach by excising a point from Minkowski spacetime, producing a quantized bound-state solution that defines the detector’s internal dynamics.
• The model generalizes Unruh-DeWitt detectors by offering a covariant, autonomous description of detector-field interactions applicable across diverse spacetime geometries.

A localized particle detector model is a framework for coupling quantum systems or fields to quantum fields in such a way that the interaction is confined to a well-defined region of spacetime, enabling the detection or measurement of particle-like excitations with precise spatial localization. Such models can be realized by promoting the detector to a quantum field that is itself spatially localized via geometric constructions or boundary conditions, rather than through ad hoc potentials or external confining mechanisms. The model described in (Ramos et al., 24 Sep 2025) establishes a fully relativistic, field-theoretical approach by excising a point from Minkowski spacetime and imposing Robin boundary conditions at the excised location, thereby generating a discrete sector of localized bound-state modes—corresponding to the detector’s internal degrees of freedom—embedded within a continuum of modified field modes.

1. Field Construction and Boundary Conditions

The construction is initiated by puncturing Minkowski spacetime at the spatial origin (r = 0), thereby modifying the topology and domain of the field theory. The detector is modeled as a massive real scalar quantum field Ψ(t, r, θ, φ), with spatial domain restricted to r > 0. The field is decomposed into separable solutions using spherical coordinates:

$$\Psi_{ω\ell m}(t, r, \theta, \phi) = e^{-iω t} R_{ω\ell}(r) Y_{\ell m}(\theta, \phi)$$

The radial part, $R_{ω\ell}(r)$, satisfies a Calogero-type Sturm-Liouville eigenproblem:

$$A u(r) \equiv \left[ -\frac{d^2}{dr^2} + \frac{\ell(\ell + 1)}{r^2} \right] u(r) = p^2 u(r)$$

with $u(r) = r R_{ω\ell}(r)$ and $p^2 = \omega^2 - m_0^2$.

Self-adjointness of A is nontrivial at r = 0 due to the singularity. For ℓ > 0, only Dirichlet boundary conditions are admissible, but for the s-wave (ℓ = 0), a one-parameter family of self-adjoint extensions is allowed, corresponding to Robin boundary conditions:

$$\lim_{r \to 0^+} \left[ r R^{(\beta)}(r) + \beta \frac{d}{dr} (r R^{(\beta)}(r)) \right] = 0$$

where β ∈ ℝ is the extension parameter.

2. Discrete Bound-State Sector and Field Quantization

The Robin boundary condition permits, for ℓ = 0, the emergence of a normalizable discrete bound-state solution with frequency

$$\omega_b = \sqrt{m_0^2 - \frac{1}{\beta^2}}, \quad \text{with} \quad \beta m_0 > 1$$

The bound radial mode takes the exponentially decaying form:

$R_\text{bound}^{(\beta)}(r) \propto e^{-r/\beta}$

Upon quantization, the field operator decomposes as:

$$\hat{\Psi}_\beta(x) = \Psi_\text{bound}(x) \, \hat{a}_\text{bound} + \Psi_\text{bound}^*(x) \, \hat{a}_\text{bound}^\dagger + \sum_{\ell,m} \int_{m_0}^\infty d\omega \left[ \Psi_{\omega\ell m}(x) \, \hat{b}_{\omega\ell m} + \Psi_{\omega\ell m}^*(x) \, \hat{b}_{\omega\ell m}^\dagger \right]$$

This construction identifies the localized detector degree of freedom with the quantized bound-state sector generated intrinsically by the self-adjoint extension via Robin boundary conditions.

3. Two-Point Function Structure and Observables

The decomposition imposed by the boundary conditions naturally splits the two-point (Green's) function into three sectors:

$$G(x, x') = G_\text{bound}(x, x') + G_\beta(x, x') + G_\text{Dirichlet}(x, x')$$

• $G_\text{bound}(x, x')$ arises from the discrete bound mode,
• $G_\beta(x, x')$ captures the boundary-modified continuous s-wave sector (ℓ = 0),
• $G_\text{Dirichlet}(x, x')$ collects contributions from higher partial waves with Dirichlet boundary conditions.

A key result is that, upon careful summation, the contributions from the discrete sector cancel in physical observables—specifically, in the renormalized stress-energy tensor—leaving only the effect of the boundary-induced continuum modification. Practically, this means the detector’s localized degree of freedom is essential for the construction but does not appear explicitly in the net stress-energy content; instead, the observable energy density and flux encode the physical effect of localization.

4. Stress-Energy Tensor Evaluation and Conservation

The expectation value of the renormalized stress-energy tensor is computed using the Hadamard point-splitting prescription:

$$\langle T_{\mu\nu} \rangle_\text{ren} = \lim_{x \to x'} \left[ (g_\nu^{\ \nu'} \nabla_\mu \nabla_{\nu'} - \frac{1}{2} g_{\mu\nu} g^{\rho\sigma'} \nabla_\rho \nabla_{\sigma'} - \frac{1}{2} g_{\mu\nu} m^2) G_\text{ren}(x, x') \right]$$

The full two-point function $G_\text{ren}(x, x')$ is regularized by subtracting the Hadamard singularity. Covariant conservation $$\nabla^\mu \langle T_{\mu\nu} \rangle_\text{ren} = 0$$ is explicitly verified, confirming the self-consistency of the approach.

Numerical evaluation demonstrates that the stress-energy is sharply peaked near the puncture (r → 0) due to boundary effects, decaying rapidly for larger r, and vanishing at infinity.

5. Generalization and Relation to the Unruh-DeWitt Paradigm

Traditional Unruh-DeWitt (UDW) detectors are nonrelativistic two-level systems coupled to a field via a smeared monopole interaction:

$$H_I = \lambda \Lambda(x) [ e^{-i\Omega t} a + e^{i\Omega t} a^\dagger ] \phi(x)$$

The present construction replaces the UDW two-level system with a fully relativistic, localized field whose bound-state excitations play the detector’s "internal" role. The detector–field coupling is realized via

$$\mathcal{L}_I = -\lambda \zeta(x) \phi_D(x) \phi(x)$$

with $\phi_D(x)$ the bound-mode solution and $\zeta(x)$ enforcing spatial and temporal localization. At leading order in perturbation theory, this field-based detector model reproduces the standard UDW transition probability structures, but with a manifestly covariant and self-consistent quantum field origin for localization.

This construction is fundamentally autonomous: the spectral gap, spatial support, and internal structure are dictated by the boundary condition parameter β, not by explicit external confining potentials or arbitrary spatial cutoffs.

6. Applicability to Nontrivial Geometries and Broader Context

The mechanism by which the discrete localized mode arises—from self-adjoint extension via Robin boundary conditions—extends beyond Minkowski spacetime. In static spacetimes with singularities (e.g., conical or global monopole geometries), the same mathematical strategy yields localized discrete sectors, with the excised region now interpreted as a genuine physical singularity.

This unified treatment provides a general relativistic field-theoretic route to detector localization, accommodating a wide range of spacetime backgrounds while ensuring both self-adjointness and covariant conservation in the detector sector. The flexibility of the approach suggests broader implications for precise quantum measurements, vacuum excitation, and the dynamics of detectors in curved and singular spacetimes.

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In summary, the localized particle detector model based on boundary-induced discrete modes (Ramos et al., 24 Sep 2025) defines the detector as a quantized, spatially localized sector within a scalar field theory, realized by excising a point and imposing Robin boundary conditions. This approach delivers an intrinsically localized, covariant, and fully relativistic model for detector-field interactions that generalizes and deepens the foundational paradigm of the Unruh-DeWitt detector.