An international team of scientists, with participation of DESY, has succeeded in producing and directly controlling hybrid electron-photon quantum states in helium atoms. To this end, the team headed by Dr. Lukas Bruder, junior research group leader at the Institute of Physics, University of Freiburg generated specially prepared, highly intense extreme ultraviolet light pulses using the FERMI free electron laser in Trieste, Italy. The researchers achieved control of the hybrid quantum states using a new laser pulse-shaping technique. Their results have been published in the journal Nature.

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The experimental hall of FERMI. Image: Elettra Sincrotrone Trieste

As long as electrons are bound to an atom, their energy can only be of certain values. These energy values depend primarily on the atoms themselves. However, if an atom is in the beam of a very intense laser, the energy levels shift. Hybrid electron-photon states are created, known as ‘dressed states’. These occur at laser intensities in the range of ten to a hundred trillion watts per square centimetre. In order to be able to produce and control these special quantum states, laser pulses are necessary that achieve such intensities within a short time window of only a few trillionths of a second. 

Free electron laser for producing laser radiation in the extreme ultraviolet range

For their experiment, the scientists used the FERMI free electron laser which allows generation of laser light in the extreme ultraviolet spectral range at very high intensity. This extreme ultraviolet radiation has a wavelength of less than 100 nanometres, which is necessary to manipulate the electron states in helium atoms. 

In order to control the electron-photon states, the researchers used laser pulses that dispersed or contracted depending on the scenario. To this end, they adjusted the time lag of the different colour components of the laser radiation. The properties of the laser pulses were controlled using a ‘seed laser pulse’, which preconditioned the emission of the free electron laser. 

“Our research enabled us for the first time to directly control these transient quantum states in a helium atom,” says Bruder. “The technique we’ve developed opens up a new field of research: this includes new opportunities for making experiments with free electron lasers more efficient and selective or for gaining new insights into fundamental quantum systems, which are not accessible with visible light. In particular it may now be possible to develop methods to study or even control chemical reactions with atomic precision.”

“The excellent coherence properties of the ultra-short light pulses generated using the seeding technique at FERMI, in particular, are crucial for the experiments,” as DESY scientist Tim Laarmann points out, one of the co-authors of the study who is also involved in research at the “CUI: Advanced Imaging of Matter” cluster of excellence at the University of Hamburg. This special beam quality is also to be provided in future to researchers at DESY’s free-electron laser FLASH facility with a distinctly higher repetition rate as part of its present FLASH2020+ upgrade program.

The research project was funded, among other sources, by the Federal Ministry of Education and Research (BMBF) as part of the collaborative research projects “Longitudinal Coherence on Free-Electron Lasers (LoKoFEL)” and Advanced Seeding Techniques for XUV and Angstrom Radiation (STAR)” and involved scientists from the University of Freiburg, the Max Planck Institute for Physics of Complex Systems in Dresden, the University of Oldenburg, the IFN-CNR in Milan, the University of Innsbruck, the University of Gothenburg, the CNR-IOM Trieste, the Istituto Nazionale di Fisica Nucleare in Rome, the Deutsches Elektronen-Synchrotron DESY in Hamburg, the Hamburg Centre for Ultrafast Imaging, the University of Aarhus and the University of Hamburg.

Reference
Richter et al., Strong-field quantum control in the extreme ultraviolet using pulse shaping. Nature, 2024. DOI: 10.1038/s41586-024-08209-y