Coherent interaction between free electrons and a photonic cavity

Abstract

Since its inception, research of cavity quantum electrodynamics (CQED) 1,2 has greatly extended our understanding of light-matter interactions and our ability to utilize them. Thus far, all the work in this field has been focused on light interacting with bound electron systems -such as atoms, molecules, quantum dots, and quantum circuits. In contrast, markedly different physical phenomena and applications could be found in free-electron systems, the energy distribution of which is continuous and not discrete, implying tunable transitions and selection rules. In addition to their uses for electron microscopy 3,4 , the interaction of free electrons with light gives rise to important phenomena such as Cherenkov radiation, Compton scattering, and free-electron lasing 5,6 . Advances in the research of ultrafast electron-light interactions have also enabled the development of powerful tools for exploring femtosecond dynamics at the nanoscale 7,8 . However, thus far, no experiment has shown the integration of free electrons into the framework of CQED, because the fundamental electron-light interaction is limited in strength and lifetime. This limit explains why many phenomena have remained out of reach for experiments with free electrons. In this work, we developed the platform for studying CQED at the nanoscale with free electrons and demonstrated it by observing their coherent interaction with cavity photons for the first time. To demonstrate this concept, we directly measure the cavity photon lifetime via a free electron probe and show more than an order of magnitude enhancement in the electron-photon interaction strength. These capabilities may open new paths toward using free electrons as carriers of quantum information, even more so after strong coupling between free electrons and cavity photons will have been demonstrated.Efficient electron-cavity photon coupling could also allow new nonlinear phenomena of cavity opto-electro-mechanics and the ultrafast exploration of soft matter or other beam-sensitive materials using low electron current and low laser exposure.

The coherence of light is fundamentally tied to the quantum coherence of the emitting particle

“…This property can be measured from the radiation autocorrelations and spectrum. With the advent of laser-driven electron sources, for example, in ultrafast electron microscopes ( 25 – 27 , 63 , 66 , 67 ), the particle coherent energy uncertainty is believed to be dictated by the laser linewidth ( 26 , 68 ), e.g., spanning tens of millielectron volts for excitations with femtosecond lasers. With the ability to coherently control the spatial electron wave function ( 23 , 24 ), the transverse momentum uncertainty can be further lowered.…”

Attosecond metrology in a continuous-beam transmission electron microscope

“…If the electron beam is coherent in space, then it should also be possible to advance electron holography or picodiffraction ( 44 , 45 ) to highly dynamical setting at frequencies approaching those of light, and atomic subcycle diffraction may reveal the complex real-space motions of valence electron densities that are the atomistic origin of the macroscopic optical properties of materials ( 16 , 17 ). Quantum optics with free-electron wave functions ( 23 , 46 – 48 ) and general three-body interactions of light, electrons, and matter ( 49 ) may profit from the extremely narrow spectroscopic bandwidth that is offered by a continuous laser wave ( 30 ). In cryogenic electron microscopy, the orientation of biomolecules might be identified by their optical dipole response, and a modulated electron beam in particle colliders might reveal the physics of the collisions as a function of time.…”

Spontaneous and stimulated electron–photon interactions in nanoscale plasmonic near fields

“…As a consequence, the initial electron energy spectrum is expanded with sidebands, evenly spaced by the photon energy hω L . The population of these sidebands varies with the near-field integral along the electron trajectory and the statistics of the incident light 25 , enabling spatially resolved near-field measurements [26][27][28][29][30][31][32] with fs-and astemporal 31,33 and meV-spectral resolutions 34 . The key to the PINEM mechanism is the fact that the evanescent near field provides spatial Fourier components with sufficiently large momenta to bridge the phase mismatch between the electron field and the optical pump field in free space.…”