Abstract: Starting from the physical intuition that light (massless fields) is delocalized while massive matter is localized, we develop a three-dimensional quantum field theory model to study photon absorption by a material detector. The detector is modeled as a two-level system coupled to a massless scalar field via a linear interaction. We consider three progressively realistic scenarios: (i) a point-like detector absorbing an s-wave spherical photon, (ii) a point-like detector absorbing a plane-wave photon, and (iii) a finite-size detector with a spatial distribution. Using Fermi's golden rule, we compute the transition rates and absorption cross sections. We show that for a point detector, the absorption cross section for s-wave photons is finite and frequency-independent, and for plane-wave photons it is independent of the incident direction. For a finite-size detector, the absorption efficiency is modulated by a form factor that suppresses absorption when the photon wavelength is shorter than the detector size. This provides a quantitative explanation for why microscopic objects (atoms) absorb light efficiently, while macroscopic objects are transparent or reflective. Our model offers an operational understanding of how a localized detector registers an event from a delocalized quantum field, linking quantum measurement theory with concrete detector models.
Abstract: Starting from the physical intuition that light (massless fields) is delocalized while massive matter is localized, we develop a three-dimensional quantum field theory model to study photon absorption by a material detector. The detector is modeled as a two-level system coupled to a massless scalar field via a linear interaction. We consider three progressively realistic scenarios: (i) a point-like detector absorbing an s-wave spherical photon, (ii) a point-like detector absorbing a plane-wave photon, and (iii) a finite-size detector with a spatial distribution. Using Fermi's golden rule, we compute the transition rates and absorption cross sections. We show that for a point detector, the absorption cross section for s-wave photons is finite and frequency-independent, and for plane-wave photons it is independent of the incident direction. For a finite-size detector, the absorption efficiency is modulated by a form factor that suppresses absorption when the photon wavelength is shorter than the detector size. This provides a quantitative explanation for why microscopic objects (atoms) absorb light efficiently, while macroscopic objects are transparent or reflective. Our model offers an operational understanding of how a localized detector registers an event from a delocalized quantum field, linking quantum measurement theory with concrete detector models.
Light absorption Whereas receiving radio waves is a macroscopic process and hence belongs to the area of classical electrodynamics – in a macroscopic antenna an electric voltage is induced whereby a large number of electrons follow the electric field strength of the incident wave, in a kind of collective motion – the detection of light, so far as the elementary process is concerned, takes place in microscopic type objects such as atoms and molecules. As a consequence, the response of an optical detector is determined by the microstructure of matter. In particular, it is impossible – due to the enormously high frequency of light (in the region of 10 15 Hz) – to measure the electric field strength. What is in fact detectable is the energy transfer from the radiation field to the atomic receiver, and this allows us to draw conclusions about the (instantaneous) intensity of light. We might ask what we can say about the above-mentioned absorption process from an experimentalist's point of view. Among the basic experiences that provide an insight into the structure of the micro-cosmos is the resonance character of the interaction between light and an atomic system. The atomic system, when hit by light, behaves like a resonator with certain resonance frequencies; i.e. it becomes excited (takes up energy) only when the light frequency coincides with a value that is characteristic for the particular atom. Hence, an incident light wave with an initial broadband frequency spectrum that has passed through a gas exhibits in its spectrum dark zones, the so-called absorption lines.
Quantum detection processes in quantum field theory (QFT) must play a key role in the description of quantum-field correlations, such as the appearance of entanglement, and of causal effects. We consider the detection in the case of a simple QFT model with a suitable interaction to exact treatment, consisting of a quantum scalar field coupled linearly to a classical scalar source. We then evaluate the response function to the field quanta of two-level pointlike quantum model detectors, and analyze the effects of the approximation adopted in standard detection theory. We show that the use of the RWA, which characterizes the Glauber detection model, leads in the detector response to nonlocal terms corresponding to an instantaneously spreading of source effects over the whole space. Other detector models, obtained with nonstandard or no application of RWA, give instead local responses to field quanta, apart from source-independent vacuum contribution linked to preexisting correlations of zero-point field.
This work develops a structural framework in which light is understood not as a propagating particle or wave, but as a process of electromagnetic field reorganization constrained by geometry and material conditions.Emission is treated as a local disruption of field coherence rather than the creation of a traveling entity. Between emission and absorption, the field enters an open, pre-realizational phase in which no localized physical event occurs. Geometry, material boundaries, and environmental structure act as physical selectors that restrict admissible field configurations. A photon is realized only at absorption, as the localized closure of this open field organization.The framework integrates established empirical phenomena — including spontaneous emission, interference with single quanta, cavity quantum electrodynamics, detector discreteness, and mirror behavior — into a single causal architecture, without introducing new field equations or modifying the formalism of Maxwellian electrodynamics or quantum electrodynamics.The speed of light is interpreted as a structural limit governing the maximal rate at which coherent field organization can be maintained across spatial separation, rather than as the velocity of a transported object. The work further identifies a continuity between quantum and gravitational regimes, in which both emerge from a common phase structure but correspond to different domains of realizability.The proposal is structural rather than dynamical in scope. It does not introduce new laws or entities, but specifies where and when physical events are taken to occur within existing electromagnetic theory, thereby providing a unified physical account of emission, interference, reflection, and photon detection.
A quantum-mechanical theory of photon detection is presented which takes fully into account the attenuation of the field due to the detection process. The time evolution of the joint quantum state of the detector and the field is found, to all orders in perturbation theory. Formulas are derived for the probabilities of absorbing specified numbers of photons within a given time interval, and for the correlations in the positions and times at which absorptions take place. The results are free from the inconsistencies which arise in the conventional theory of photon detection, and remain valid even when the field becomes appreciably attenuated during the experiment. It is found that an initially coherent field state remains coherent during its interaction with a given detector, and that its amplitude becomes attenuated by an amount which is completely independent of the number of counts which the detector records. This independence is shown to be a simple consequence of the Poisson quantum-number distribution of coherent fields. The counting statistics for arbitrary fields are expressed in a way which shows explicitly the relationship between the attenuation of the field and the absorption of quanta by the detector. The analysis is performed by first treating the case in which the field is confined within a homogeneous detecting medium throughout the experiment, and then generalizing to the case in which the field spends a limited amount of time in a spatially localized detecting region. The detector is assumed to consist of harmonic oscillators, which are shown to represent a suitable formal model for the absorption of radiation by large numbers of conventional detecting atoms.
We present a universal coupled-mode-theory treatment of free-space scattering of waves from resonant objects. The range of applicability of the presented approach is fairly broad: it can be used for almost any linear wave system, as long as the resonant scatterer has either three-dimensional (3D) spherical or 2D cylindrical symmetry, or else is sufficiently smaller than the resonant wavelength of the incident wave. The presented framework, while being intuitive and analytically simple, can nevertheless provide quantitatively very accurate modeling of scattering cross sections, absorption cross sections, and many other quantities of interest. We illustrate this approach by showing how it applies to the particular examples of scattering of light from spherically symmetric resonant objects and atoms, and scattering of neutrons off nuclei.
Whilst quantum electrodynamics (QED) underwent an impressive development and reached its maturity in the middle of the last century, one of its basic concepts—the photon wave function in free space—was deprived of such fortune. Although the photon wave function in coordinate representation was introduced already in 1930 by Landau and Peierls, the concept was found to suffer from inherent difficulties that were not overcome during the century —see review (Bialynicki-Birula, 1996). The common explanation presented in textbooks, e.g., (Akhiezer & Berestetskii, 1965; Mandel & Wolf, 1995), may be summed up as follows: (i) no position operator exists for the photon, (ii) while the position wave function may be localized near a space-time point, the measurable quantities like the electromagnetic field vectors, energy, and the photodetection probability remain spread out due to their non-local relation with the position wave function.