Radiative loss of coherence in free electrons:a long-range quantum phenomenon

Quantum physics rules the dynamics of small objects as they interact over microscopic length scales.Nevertheless,quantum correlations involving macroscopic distances can be observed between entangled photons as well as in atomic gases and matter waves at low temperatures.The long-range nature of the electromagnetic coupling between charged particles and extended objects could also trigger quantum phenomena over large distances.Here,we reveal a manifestation of quantum mechanics that involves macroscopic distances and results in a nearly complete depletion of coherence associated with which-way free-electron interference produced by electron-radiation coupling in the presence of distant extended objects.This is a ubiquitous effect that we illustrate through a rigorous theoretical analysis of a two-path electron beam interacting with a semi-infinite metallic plate and find the inter-path coherence to vanish proportionally to the path separation at zero temperature and exponentially at finite temperature.The investigated regime of large distances originates in the coupling of the electron to radiative modes assisted by diffraction at material structures but without any involvement of material excitations.Besides the fundamental interest of this macroscopic quantum phenomenon,our results suggest an approach to measuring the vacuum temperature and nondestructively sensing the presence of distant objects.

Quantum-mechanical effects in photoluminescence from thin crystalline gold films

Luminescence constitutes a unique source of insight into hot carrier processes in metals,including those in plasmonic nanostructures used for sensing and energy applications.However,being weak in nature,metal luminescence remains poorly understood,its microscopic origin strongly debated,and its potential for unraveling nanoscale carrier dynamics largely unexploited.Here,we reveal quantum-mechanical effects in the luminescence emanating from thin monocrystalline gold flakes.Specifically,we present experimental evidence,supported by first-principles simulations,to demonstrate its photoluminescence origin(i.e.,radiative emission from electron/hole recombination)when exciting in the interband regime.Our model allows us to identify changes to the measured gold luminescence due to quantum-mechanical effects as the gold film thickness is reduced.Excitingly,such effects are observable in the luminescence signal from flakes up to 40 nm in thickness,associated with the out-of-plane discreteness of the electronic band structure near the Fermi level.We qualitatively reproduce the observations with first-principles modeling,thus establishing a unified description of luminescence in gold monocrystalline flakes and enabling its widespread application as a probe of carrier dynamics and light-matter interactions in this material.Our study paves the way for future explorations of hot carriers and charge-transfer dynamics in a multitude of material systems.