New quantum technology combines free electrons and photons

The findings of the study expand the toolbox of quantum technology.


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Quantum information, communication, and sensing rely on generating and controlling quantum correlations in complementary degrees of freedom. Experts worldwide are trying to implement findings from basic research into quantum technologies.

Sometimes they require individual particles, including photons with special properties. However, getting individual particles is challenging and calls very sophisticated techniques. Free electrons are already used in many applications to produce light, such as X-ray tubes.

In a new study, 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 the form of pair states. They created electron-photon pairs using integrated photonic circuits on a chip in an electron microscope.

In an experiment, scientists pass the beam of an electron microscope on a built-in integrated photonic chip. The chip consists of a micro-ring resonator and optical fiber output ports. This new approach uses photonic structures fabricated at EPFL for transmission electron microscope (TEM) experiments performed at MPI-NAT.

optical chip
An optical chip with ring-shaped light storage, called a microring resonator, and a fiber-optic coupling. The chip is only three millimeters wide, and the ring resonator at its tip has a radius of 0.114 millimeters. © Armin Feist / Max Planck Institute for Multidisciplinary Sciences

A photon can be produced whenever an electron interacts with the ring resonator’s vacuum evanescent field. The electron loses the energy quantum of a single photon in this process while adhering to energy and momentum conservation principles. The system develops into a pair state as a result of this interaction. The scientists’ accurate simultaneous detection of electron energy and produced photons, made possible by a newly created measurement technique, revealed the underlying electron-photon pair states.

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

Claus Ropers, MPI-NAT Director, said, “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.”

In experiment, scientists used the generated correlated electron-photon pairs for photonic mode imaging. They were able to achieve 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.”

Tobias Kippenberg, professor at EPFL and head of the Laboratory of Photonics and Quantum Measurement, said, “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.”

The study could lead to the currently emerging field of free-electron quantum optics. It could also demonstrate a powerful experimental platform for event-based and photon-gated electron spectroscopy and imaging.

Guanhao Huang, a Ph.D. student at EPFL and co-lead author of the study, said“Our work represents a critical step to utilize quantum optics concepts in electron microscopy. We plan to explore further future directions like electron-heralded exotic photonic states and noise reduction in electron microscopy.”

Journal Reference:

  1. Armin Feist, Guanhao Huang, et al. Cavity-mediated electron-photon pairs. Science, 377(6607), 777-780. DOI: 10.1126/science.abo5037


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