Free electrons generate quantum light with counted particles

Quantum technology with light is an area of intense research and development. However, this technology requires special “quantum light”, which has fundamentally different characteristics to normal light. It can contain a precise number of light particles, called photons, for example. Researchers are working on generating these light states efficiently and reliably.

A team led by Claus Ropers at the Max Planck Institute (MPI) for Multidisciplinary Sciences and the University of Göttingen, in collaboration with a team led by Tobias Kippenberg at the EPFL Lausanne, has developed a fundamentally new method to generate quantum light. Rather than using lasers in the conventional way, the researchers employed high-energy free electrons from an electron microscope. The results have now been published in the scientific journal Nature Physics.

Light from electrons

In these experiments, an electron beam is positioned close to an optical waveguide. If the electrons and light have the same velocity, the electrons can very efficiently generate photons inside the waveguide. This light is then guided along defined routes to optical fibers through which it leaves the electron microscope and gets detected. “The generation of light with electrons is a well-known phenomenon, used in old TV screens, for example,” says Germaine Arend, postdoctoral researcher at the MPI and first author of the publication. “However, our method is special because of the close connection between the generated light and the individual electrons that generate it.” This means that each electron can be detected alongside “its” photons. Additionally, measuring the change in electron energy indicates the number of photons generated.

The researchers then selected electrons that had lost a defined amount of energy. This allowed them to determine the number of photons created. This access to the electrons and their properties enabled them to generate quantum light with a defined, counted number of photons. This quantum light differs significantly from normal light sources, such as LEDs and lasers, which emit an unknown, fluctuating number of photons.

For the future

The generation method used a typical electron beam, similar to those found in many electron microscopes. However, modifying the quantum mechanical state of the electrons prior to light generation could improve this process. “This could enable the tailored generation of quantum light with even more complex properties,” Max-Planck director Claus Ropers says. Thus, the newly developed method holds potential to foster new and exciting applications in quantum optics and quantum technology.