New quantum technology combines free electrons and photons

Faster computers, tap-proof communication, sensors beyond standard quantum limit – quantum technologies have the potential to revolutionize our lives just as once the invention of computers or the internet. Experts worldwide are trying to implement findings from basic research into quantum technologies.

To this end, they sometimes require individual particles, such as photons – the elementary particles of light - with special properties. However, obtaining individual particles is complicated and requires complex methods. Various applications already use free electrons to generate light, such as the case in X-ray tubes.

In a new study, recently published in the journal Science, scientists from EPFL’s Laboratory of Photonics and Quantum Measurement, Göttingen Max Planck Institute for Multidisciplinary Sciences (MPI-NAT) and the University of Göttingen demonstrate a novel method for generating cavity-photons using free electrons, in a form of pair states. To do so, they used chip-based photonic integrated circuits in an electron microscope.

Fundamental Particle Physics in Electron Microscopes

In the experiment, the beam of an electron microscope passes on a built-in integrated photonic chip, consisting of a micro-ring resonator and optical fiber output ports. This new approach, using photonic structures fabricated at EPFL for transmission electron microscope (TEM) experiments performed at MPI-NAT, was established in a recent study.

Whenever an electron interacts with the vacuum evanescent field of the ring resonator, a photon can be generated. In this process, obeying the laws of energy and momentum conservation, the electron loses the energy quantum of a single photon. Through this interaction, the system evolves into a pair state. Thanks to a newly developed measurement method, the scientists could precisely detect simultaneously both electron energy and generated photons, revealing the underlying electron-photon pair states.

Future quantum technology with free electrons

Besides observing this process for the first time at the single particle level, these findings implement a novel concept for generating single-photon or electron. Specifically, the measurement of the pair state enables heralded particle sources, where the detection of one particle signals the generation of the other. This is necessary for many applications in quantum technology and adds to its growing toolset.

“The method opens up fascinating new possibilities in electron microscopy. In the field of quantum optics, entangled photon pairs already improve imaging. With our work, such concepts can now be explored with electrons,” explains Claus Ropers, MPI-NAT Director.

In the first proof-of-principle experiment, the researchers make use of the generated correlated electron-photon pairs for photonic mode imaging, achieving a three-orders of magnitude contrast enhancement. Dr. Yujia Yang, a postdoc at EPFL and a co-lead author of the study, adds: “We believe our work has a substantial impact on the future development in electron microscopy by harnessing the power of quantum technology.”

A particular challenge for future quantum technology is how to interface different physical systems. “For the first time, we bring free electrons into the toolbox of quantum information science. More broadly, coupling free electrons and light using integrated photonics could open the way to a new class of hybrid quantum technologies,” says Tobias Kippenberg, professor at EPFL and head of the Laboratory of Photonics and Quantum Measurement.

The work from the collaboration between the two teams contributes to the currently emerging field of free-electron quantum optics, and demonstrates a powerful experimental platform for event-based and photon-gated electron spectroscopy and imaging. “Our work represents a critical step to utilize quantum optics concepts in electron microscopy. We plan to further explore future directions like electron-heralded exotic photonic states, and noise reduction in electron microscopy,” says Guanhao Huang, PhD student at EPFL and co-lead author of the study.