1. Department of Physics and Astronomy, University of Nebraska—Lincoln, 116 Brace Laboratory, PO Box 880111, Lincoln, Nebraska 68588-0111, USA
2. Present address: Department of Physics, Fort Hayes State University, 600 Park Street, Hays, Kansas 67601-4099, USA.

Correspondence to: Herman Batelaan¹ Correspondence and requests for materials should be addressed to H.B. (e-mail: Email: hbatelaan2@unl.edu).

Abstract

In their famous 1927 experiment, Davisson and Germer observed¹ the diffraction of electrons by a periodic material structure, so showing that electrons can behave like waves. Shortly afterwards, Kapitza² and Dirac³ predicted that electrons should also be diffracted by a standing light wave⁴. This Kapitza–Dirac effect is analogous to the diffraction of light by a grating, but with the roles of the wave and matter reversed. The electron and the light grating interact extremely weakly, via the 'ponderomotive potential'⁵, so attempts to measure the Kapitza–Dirac effect had to wait for the development of the laser. The idea⁶ that the underlying interaction with light is resonantly enhanced for electrons in an atom led to the observation⁷ that atoms could be diffracted by a standing wave of light. Deflection of electrons by high-intensity laser light, which is also a consequence of the Kapitza–Dirac effect, has also been demonstrated⁸. But the coherent interference that characterizes wave diffraction has not hitherto been observed^{9, 10}. Here we report the diffraction of free electrons from a standing light wave—a realization of the Kapitza–Dirac effect as originally proposed.