arXiv subjects

Keiichirou Akiba

Publications and source records attributed to Keiichirou Akiba.

9 recordsLinked to original sources

arXiv2026

Electron-Photon Spatial Entanglement in Coherent Cathodoluminescence

Electron--photon quantum entanglement in an electron microscope paves the way for a new quantum platform, enabling the integration of quantum functionalities into electron microscopy and opening opportunities for quantum imaging and quantum sensing at the nanoscale. To realize such a platform, it is crucial to understand the degree and nature of electron--photon entanglement in cathodoluminescence (CL). However, its dependence on electron-beam properties, particularly transverse coherence, remains unclear. Here, we present a theoretical framework describing the quantum state of an electron--photon pair generated in coherent CL from a system with translational invariance in the plane perpendicular to the electron beam, as can be realized, for example, in transition radiation at a planar interface. By expressing the scattered state directly in terms of the luminescence spectrum, we evaluate the entanglement using both subsystem purity and an Einstein--Podolsky--Rosen-type criterion. These two measures enable a clear distinction among wave-like, particle-like, and classical regimes in terms of spatial and momentum entanglement in the electron--photon system. Our analysis identifies the roles of the electron's transverse and longitudinal coherence, as well as the photon's spectral width, and reveals the conditions under which strong spatial entanglement emerges. This unified perspective clarifies the nature of electron--photon quantum correlations in coherent CL and advances the development of quantum-enabled functionalities in electron microscopy.

quant-ph↗

arXiv2025

Nano light-source generation by electron beam irradiation of CsPbBr3/Cs4PbBr6 composites

Precise allocation of nano light-sources in photonic integrated circuits is essential for the development of next-generation optical technologies such as optical nano-circuits, quantum information processing, and quantum communication. However, controlling the position of such light sources is a challenging task. We here show that halide perovskite nanoparticle light sources can be generated at the desired positions by electron beam. We applied cathodoluminescence spectroscopy to CsPbBr3/Cs4PbBr6 composite produced by thermal evaporation and discovered that the intensity of green light emission from the CsPbBr3 nanoparticles increased with electron beam irradiation. Changes in the cathodoluminescence spectrum associated with the electron beam irradiation suggest CsPbBr3 nanoparticle formations. Furthermore, by taking advantage of the high spatial resolution and controllability of the electron beam, we demonstrate nano-light source patterning on the film.

physics.optics↗

arXiv2025

Roadmap for Quantum Nanophotonics with Free Electrons

Over the past century, continuous advancements in electron microscopy have enabled the synthesis, control, and characterization of high-quality free-electron beams. These probes carry an evanescent electromagnetic field that can drive localized excitations and provide high-resolution information on material structures and their optical responses, currently reaching the sub-ångström and few-meV regime. Moreover, combining free electrons with pulsed light sources in ultrafast electron microscopy adds temporal resolution in the sub-femtosecond range while offering enhanced control of the electron wave function. Beyond their exceptional capabilities for time-resolved spectromicroscopy, free electrons are emerging as powerful tools in quantum nanophotonics, on par with photons in their ability to carry and transfer quantum information, create entanglement within and with a specimen, and reveal previously inaccessible details on nanoscale quantum phenomena. This Roadmap outlines the current state of this rapidly evolving field, highlights key challenges and opportunities, and discusses future directions through a collection of topical sections prepared by leading experts.

cond-mat.mes-hall↗

arXiv2025

Unveiling the nature of cathodoluminescence from photon statistics

Cathodoluminescence (CL), the emission of light induced by accelerated free electrons, has been extensively utilized in various applications, such as displays, streak cameras, and high-spatial-resolution analysis of optical material, surpassing the diffraction limit of light. Despite its long history, the photon statistics of CL have only recently been examined, revealing unexpectedly large bunching of photons. Here we find that this peculiar photon bunching contains information of intervening excitation processes before the photon emission, which can be extracted from the photon statistics within each excitation event by a single free electron. Using this approach, we experimentally unveiled the statistical differences of coherent CL involving a single electromagnetic interaction process and incoherent CL involving multiple excitation processes. The developed formulation is universally applicable for particle generation processes in general to investigate the nature of cascade reactions.

physics.optics↗

arXiv2024

Competitive coexistence of ferromagnetism and metal--insulator transition of VO$_2$ nanoparticles

We investigated the magnetic and electric properties of nanometer-sized vanadium dioxide (VO$_2$) particles. VO$_2$ nanoparticles were formed by milling VO$_2$ powder. We measured the magnetic field dependence of the magnetization of the VO$_2$ powder and nanoparticles. The VO$_2$ powder did not exhibit ferromagnetism, whereas the VO$_2$ nanoparticles exhibited ferromagnetism. In addition, we fabricated samples by bridging between electrodes with the VO$_2$ nanoparticles, and the temperature dependence of their resistance was measured. Metal-insulator transitions (MITs) were observed, and the temperature range where the MIT occurred was wider than that in a typical bulk VO$_2$. The VO$_2$ nanoparticles exhibited these properties of ferromagnetism and MIT possibly because of the surface and size effects of the VO$_2$ nanoparticles. These results indicate the first observation of the competitive coexistence of ferromagnetism and MIT of VO$_2$ nanoparticles.

cond-mat.mtrl-sci↗

arXiv2023

Time-correlated electron and photon counting microscopy

Electron microscopy based on high-energy electrons allows nanoscopic analytical imaging taking advantage of secondarily generated particles. Especially for cathodoluminescence, the correlation between primary incident electrons and emitted photons includes information on the entire interaction process. However, electron-photon time correlation tracking the relaxation dynamics of luminescent materials has so far not been achieved. In this work, we propose time-correlated electron and photon counting microscopy, where coincidence events of primary electrons and generated photons are counted after interaction. The electron-photon time correlation enables extracting a unique lifetime of the emitter independent of the photon state, accounting for coherent and incoherent photon generation processes. We also introduce a correlation factor and discuss the correlation between electrons and generated coherent photons. Through momentum selection, we observe correlation changes indicating the presence of pair correlation originated from the electron-photon entanglement. The present work lays the foundation for developing next-generation electron microscopy based on quantum correlation.

physics.ins-det↗

arXiv2023

Superbunching in cathodoluminescence: a master equation approach

We propose a theoretical model of a master equation for cathodoluminescence (CL). The master equation describes simultaneous excitation of multiple emitters by an incoming electron and radiative decay of individual emitters. We investigate the normalized second-order correlation function, $g^{(2)}(τ)$, of this model. We derive the exact formula for the zero-time delay correlation, $g^{(2)}(0)$, and show that the model successfully describes giant bunching (superbunching) in the CL. We also derive an approximate form of $g^{(2)}(τ)$, which is valid for small excitation rate. Furthermore, we discuss the state of the radiation field of the CL. We reveal that the superbunching results from a mixture of an excited photon state and the vacuum state and that this type of state is realized in the CL.

cond-mat.mes-hall↗

arXiv2021

Purcell effect of nitrogen-vacancy centers in nanodiamond coupled to propagating and localized surface plasmons revealed by photon-correlation cathodoluminescence

We measured the second-order correlation function of the cathodoluminescence intensity and investigated the Purcell effect by comparing the lifetimes of quantum emitters with and without metal structure. The increase in the electromagnetic local density of state due to the coupling of a quantum emitter with a plasmonic structure causes a shortening of the emitter lifetime, which is called the Purcell effect. Since the plasmon-enhanced electric field is confined well below the wavelength of light, the quantum emitter lifetime is changed in the nanoscale range. In this study, we combined cathodoluminescence in scanning (transmission) electron microscopy with Hanbury Brown-Twiss interferometry to measure the Purcell effect with nanometer and nanosecond resolutions. We used nitrogen-vacancy centers contained in nanodiamonds as quantum emitters and compared their lifetime in different environments: on a thin SiO2 membrane, on a thick flat silver film, and embedded in a silver film. The lifetime reductions of nitrogen-vacancy centers were clearly observed in the samples with silver. We evaluated the lifetime by analytical calculation and numerical simulations and revealed the Purcell effects of emitters coupled to propagating and localized surface plasmons. This is the first experimental result showing the Purcell effect due to the coupling between nitrogen-vacancy centers in nanodiamonds and surface plasmon polaritons with nanometer resolution.

physics.optics↗

arXiv2008

Storage and Retrieval of a Squeezed Vacuum

Storage and retrieval of a squeezed vacuum was successfully demonstrated using electromagnetically induced transparency. 930ns of the squeezed vacuum pulse was incident on the laser cooled 87Rb atoms with an intense control light in a coherent state. When the squeezed vacuum pulse was slowed and spatially compressed in the cold atoms, the control light was switched off. After 3us of storage, the control light was switched on again and the squeezed vacuum was retrieved, as was confirmed using the time-domain homodyne method.

quant-ph↗