Quantum Advancement Combines Free Electrons and Photons

A collaboration between Swiss and German researchers demonstrated the generation of electron-photon pair states for the first time in a controlled way, using integrated photonic circuits on a chip. Using a new technique, they precisely detected the involved particles. The experiment could enable quantum-enhanced electron microscopy and adds free electrons to the toolbox of quantum technologies.

The study demonstrates a novel technique for generating free-electron/cavity-photon pairs using chip-based photonics integrated circuits in an electron microscope. The work was conducted by a collaboration between the groups of Claus Ropers at the Göttingen Max Planck Institute for Multidisciplinary Sciences and the University of Göttingen, and Tobias Kippenberg from EPFL, the Swiss Federal Institute of Technology in Lausanne.

In work that demonstrated the generation of electron-photon pair states for the first time in a controlled way, researchers performed an experiment in which the beam of an electron microscope passed a built-in integrated photonic chip, consisting of a micro-ring resonator and optical fiber output ports. The optical chip is shown with a ring-shaped light storage, called a micro-ring resonator, and a fiber-optic coupling. The chip is only 3 mm wide, and the ring resonator at its tip has a radius of 0.114 mm. Courtesy of Armin Feist/Max Planck Institute for Multidisciplinary Sciences.

In the experiment, the beam of an electron microscope passes a built-in integrated photonic chip, consisting of a micro-ring resonator and optical fiber output ports. The approach used photonic structures fabricated at EPFL for TEM experiments performed at MPI-NAT (Göttingen). Whenever an electron interacted with the vacuum evanescent field of the ring resonator, the system evolved into an electron-photon pair state, a superposition between the cases of one photon generated and no photon generated.

Obeying the laws of energy- and momentum conservation, in the case when one photon is generated, the electron lost the energy quantum of a single photon. With a newly developed measurement method, both electron energy and generated photons were detected simultaneously, revealing the underlying electron-photon pair states.

Aside from observing this process for the first time at the single-particle level, these findings implement a novel concept for single-photon or electron generation. 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,” said Max Planck Director Claus Ropers.

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

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,” said Tobias Kippenberg, professor at EPFL.

The work contributes to the emerging field of free-electron quantum optics, and it 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,” said Guanhao Huang, a Ph.D. student at EPFL and co-lead author of the study.

The research was published in Science (www.doi.org/10.1126/science.abo5037).