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arXiv:2604.13375

Photoemission and absorption under coherent and entangled-photon-pair illumination

Malvin Carl Teich, Mark C. Booth, Francesco Lissandrin, Bahaa E. A. Saleh

quant-phphysics.opticsSubmitted Apr 15, 2026• Announced Apr 16, 2026• 0 citations
v1 · Apr 15, 2026v2 · May 4, 2026
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Abstract

The phenomena of subthreshold photoemission and absorption under coherent and entangled-photon-pair illumination are reviewed, and the generation and properties of entangled-photon pairs are surveyed. Three prominent forms of subthreshold photoemission are examined: one-photon Fermi-tail photoemission (FTP), two-photon photoemission (TPP), and entangled-two-photon photoemission (ETPP). Experimental methods for measuring subthreshold photocurrents and photoelectron count rates are discussed, along with strategies for enhancing selected contributions. Experimental observations of FTP from a CsK$_2$Sb photocathode in a photomultiplier tube (PMT), under both coherent and entangled-photon-pair illumination, are reviewed, and the role of FTP as a noise source in two-photon measurements is elucidated. TPP from Na and CsK$_2$Sb photocathodes in a PMT under classical-light illumination is considered, as are TPP and ETPP from a CsK$_2$Sb photocathode in a channel photomultiplier (CPM) under coherent and entangled-photon-pair illumination. The observation of ETPP is facilitated by the use of a CPM, which suppresses FTP, and by low-intensity illumination, which minimizes TPP. Quantum models of TPP and ETPP accord well with experiment. Entangled-two-photon absorption (ETPA) is analyzed, as are its applications in entangled-two-photon fluorescence microscopy (ETPFM) and entangled-two-photon spectroscopy (ETPS). The three principal forms of subthreshold absorption parallel those of subthreshold photoemission: singleton-induced Boltzmann-tail absorption; cousin-induced/singleton-pair-induced two-photon absorption; and twin-induced ETPA. Heuristic particle and fully quantum models of these processes are compared, and experimental studies of ETPA and ETPFM, together with methods for enhancing their observability, are summarized.

Comments
113 pages, 25 figures, 404 references, review paper

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The phenomena of subthreshold photoemission and absorption under coherent and entangled-photon-pair illumination are reviewed, and the generation and properties of entangled-photon pairs are surveyed. Three prominent forms of subthreshold photoemission are examined: one-photon Fermi-tail photoemission (FTP), two-photon photoemission (TPP), and entangled-two-photon photoemission (ETPP). Experimental methods for measuring subthreshold photocurrents and photoelectron count rates are discussed, along with strategies for enhancing selected contributions. Experimental observations of FTP from a CsK2_22​Sb photocathode in a photomultiplier tube (PMT), under both coherent and entangled-photon-pair illumination, are reviewed, and the role of FTP as a noise source in two-photon measurements is elucidated. TPP from Na and CsK2_22​Sb photocathodes in a PMT under classical-light illumination is considered, as are TPP and ETPP from a CsK2_22​Sb photocathode in a channel photomultiplier (CPM) under coherent and entangled-photon-pair illumination. The observation of ETPP is facilitated by the use of a CPM, which suppresses FTP, and by low-intensity illumination, which minimizes TPP. Quantum models of TPP and ETPP accord well with experiment. Entangled-two-photon absorption (ETPA) is analyzed, as are its applications in entangled-two-photon fluorescence microscopy (ETPFM) and entangled-two-photon spectroscopy (ETPS). The three principal forms of subthreshold absorption parallel those of subthreshold photoemission: singleton-induced Boltzmann-tail absorption; cousin-induced/singleton-pair-induced two-photon absorption; and twin-induced ETPA. Heuristic particle and fully quantum models of these processes are compared, and experimental studies of ETPA and ETPFM, together with methods for enhancing their observability, are summarized.
A quantum theory of two-photon volume photoemission from metals and semiconductors is developed when the incident source of light comprises collinear down-converted entangled-photon pairs with entanglement time Te.{T}_{e}.Te​. Despite the fact that the process involves the absorption of pairs of photons, the entangled-photon photocurrent varies linearly with the incident photon flux density. This is a consequence of the fact that the presence of one photon of an entangled-photon pair signals the presence of the other; it is in sharp contrast with the quadratic dependence of the classical two-photon photocurrent on incident photon flux density. Calculations are carried out for sodium metal (Na) and for K2CsSb,{\mathrm{K}}_{2}\mathrm{CsSb},K2​CsSb, a bialkali-antimonide semiconductor material often used as a cathode in photomultiplier tubes. The photocurrent is found to vary inversely with entanglement time although nonmonotonic behavior emerges over certain ranges of Te.{T}_{e}.Te​. Entangled-photon photoemission may well be useful for enhancing the range of two-photon photoemission spectroscopy and might find particular use in the investigation of surface and image states of various materials.
We present an ongoing experiment to investigate the potential enhancement of the two-photon absorption (TPA) rate of an atom using time–frequency-entangled photon pairs, a process known as entangled two-photon absorption (ETPA). There has been significant debate in the literature regarding the existence and magnitude of such enhancement. Previous studies have primarily focused on ETPA in molecular fluorophores, which are relevant for biological imaging applications but typically exhibit relatively low two-photon cross sections, often not exceeding 10⁻⁵⁰ cm⁴s/photon. In this work, we sacrifice broad applicability for experimental practicality, and instead probe ETPA in rubidium vapor, which has a comparatively large two-photon cross section (∼10⁻¹⁸ cm⁴s/photon) relative to most molecular fluorophores used in prior studies. We have developed and characterized an entangled photon-pair source, and have observed two-photon absorption in rubidium vapor. We have also reproduced previous theoretical models of ETPA. Preliminary results are presented, along with a discussion of the future direction of the experiment.
Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.
We closely replicated and extended a recent experiment ("Spatial properties of entangled two-photon absorption," Phys. Rev. Lett. 129, 183601, 2022) that reportedly observed enhancement of two-photon absorption rates in molecular samples by using time-frequency-entangled photon pairs, and we found that in the low-flux regime, where such enhancement is theoretically predicted in-principle, the two-photon fluorescence signal is below detection threshold using current state-of-the-art methods. The results are important in the context of efforts to enable quantum-enhanced molecular spectroscopy and imaging at ultra-low optical flux. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observed two-photon-induced fluorescence in the high-gain regime but in the low-gain regime any fluorescence was below detection threshold. We supplemented the molecular fluorescence experiments with a study of nonlinear-optical sum-frequency generation, for which we are able to observe the low-to-high-gain crossover, thereby verifying our theoretical models and experimental techniques. The observed rates (or lack thereof) in both experiments are consistent with theoretical predictions and with our previous experiments, and indicate that time-frequency photon entanglement does not provide a practical means to enhance in-solution molecular two-photon fluorescence spectroscopy or imaging with current techniques.
The Two-photon absorption processes (TPA) is of great interest due to its variety of technical applications. Two-photon transitions can be excited using light with different statistical properties, such as laser light (coherent light), pseudo-thermal light and entangled photons-pairs (ETPA). For a TPA process driven with entangled photons (ETPA), a linear dependence of the signal with the photon flux has been theoretically predicted and experimentally demonstrated by various groups. The main motivation for studying ETPA relies on the capability of inducing TPA with a low photon flux. This is advantageous to analyze, among other things, biological samples with a reduced probability of destroying them in the process. However, ETPA experiments developed to date raise many questions regarding the difficulty of isolating a signal in organic molecules. One of the main problems in these experiments is the difficulty to study independently all the systematic effects due to the complex structure of organic molecules. An alternative to clarify the origin of the signal is to study the ETPA process in atomic systems. In this case, the possibility to excite narrow band transitions with tunable single frequency lasers can provide a clear signature of the ETPA process. To achieve this objective, the first step consists of understanding TPA with coherent light, or classical TPA, and measuring the corresponding cross-section in atomic systems. Once the classical regime is characterized, the ETPA process can be studied considering decoherence effects and coherence between intermediate states. For this, the ETPA cross section must be calculated and measured. In this work we present a theoretical calculation of the ETPA transition probability and cross-section with the density operator formalism for the transition 62S1/2 →82S1/2. The theoretical results will be complemented with an experiment with atomic Cesium based in a coincidence detection scheme.
Nonlinear optics has had extensive application into a vast array of scientific fields. One such nonlinear process, two-photon absorption (TPA), has had a wildly successful \nadoption into the field of biological imaging and microscopy. As far and as fast as this field is progressing, limitations stemming from the use of ultrafast lasers are starting to appear. \nIn this work, an alternative nonclassical light source will be motivated for the application \nof low photon-flux two-photon microscopy. The origin and properties of the chosen \nnonclassical source, spontaneous parametric downconversion (SPDC), will be discussed along with the spatial and spectral properties modelled. Nonlinear processes such as TPA and sum frequency generation (SFG) will be viewed as "ultrafast coincidence measurements" of two photons arriving at a molecule within the time window of excitation. These ultrafast coincidence measurements will be viewed in an alternative manner: in terms of the second-order coherence from a light source. This degree of second-order coherence can be subdivided into two categories arising from different combinations of correlations within and between entangled photon pairs. Of interest, the energy-time correlations within the photon pair allow for enhancements in ultrafast coincidence rates over coherent light sources. The makings of an experimental setup to demonstrate enhanced rates from ultrafast two-photon coincidences taking place in SFG in a nonlinear crystal will be discussed.