Rabi dynamics in a 2-level system at XUV wavelength

The discovery of photoelectric effect and its subsequent explanation by Albert Einstein led to the development of quantum mechanics in the beginning of 20th century. Since then, light-matter interaction has occupied the center stage in modern physics. The experimental realization of laser sources revolutionized the field in the 1950s, which led to the opening of numerous research fields in physics, chemistry and biology. Of particular interest in fundamental physics is the coherent interaction between a photon and an atom. Following the pioneering works of Stanley Autler and Charles Townes in 1955, scientists finally realized how to manipulate a two-level quantum system interacting with an electromagnetic field.

In the next decades, our perception about light-matter interaction would change completely thanks to the path-breaking approach of Claude Cohen-Tannoudji, who introduced the notion of an atom dressed with a light field and how to use it to understand the underlying physics. The subsequent development of quantum optics produced spectacular advances in precision metrology, entanglement and quantum information technology, just to name a few.

Typically, the laser sources used around the world to do such experiments can be fitted in a room. These sources provided photons with longer wavelength regime of the electromagnetic spectra, ranging from infrared to visible. Over the last decade, a new type of laser sources has started to being built around the world, known as free-electron lasers or FELs. These FELs are user facilities measuring few hundreds of meters in size. They provide photon pulses, but in the extreme ultraviolet or, the X-ray domain, where, finally, we can have lasers intense and coherent enough that match the capabilities of the tabletop sources! Using one such laser source, we studied the coherent interaction between helium atoms and XUV pulses.

We used the coherent pulses from the FERMI FEL facility in Trieste, Italy, to study the photoionization of helium atoms in the XUV domain. The photoelectron spectroscopy measurements were carried out at the LDM (Low Density Matter) Beamline of FERMI. The project, which was led by CNRS researcher Saikat Nandi from Institut Lumière Matière, in Lyon, is an international collaboration between 29 scientists from France, Italy, Sweden and Germany. In particular, the group of Jan Marcus Dahlström from Lund University in Sweden provided state-of-the-art theoretical calculations, crucial for interpreting the experimental results.

Because of the coherent nature of the light pulses, which is one of the distinctive capabilities of FERMI, we could drive the population between two bound states in the helium atom periodically following absorption of an XUV photon, thereby effectively constructing a two-level system, see Figure 1. The oscillations of populations between the two-levels driven by an electromagnetic field are known as Rabi oscillations, named after Issac Rabi who first discovered it in 1930s. As we varied the photon energy of the FEL pulse across the resonant transition in helium, an Autler-Townes doublet was formed within 50 femtoseconds. We could interpret the ultrafast emergence of the Autler-Townes doublet in terms of the dressed-atom picture with coupled atom – field energies. Indeed, close to the transition energy between the quantum levels, the coupled energies form a level anti-crossing: the evidence of the formation of a dressed ‘atom + photon’ state in the XUV domain.

The FEL pulse is usually very intense, reaching peak intensity up to 1013 – 1014 W/cm2 at the interaction region. This allowed us to observe quantum interference between two ionization pathways that manifested as a blue shift of the said avoided crossing.

Our experimental findings show that one can drive coherent light-matter interaction even at short wavelengths, such as XUV. This opens up new avenues, where quantum matter can be manipulated using XUV or X-ray radiation. Most importantly, it will allow the scientists to take advantage of the element specificity of short-wavelength radiation for tweaking the outcome of light-matter interactions using novel coherent control techniques.

image illustrating Rabi dynamics in a 2-level system at XUV wavelength

Figure 1: As the relativistic electron bunch travels through the undulator array of FERMI, it produces coherent radiation in the XUV domain. We used the XUV pulses to drive Rabi oscillations in helium atoms, whose signature is imprinted on the detected photoelectrons as an avoided crossing.

This research was conducted by the following research team:

Saikat Nandi1, Edvin Olofsson2, Mattias Bertolino2, Stefanos Carlström2, Felipe Zapata2, David Busto2, Carlo Callegari3, Michele Di Fraia3, Per Eng-Johnsson2, Raimund Feifel4, Guillaume Gallician5, Mathieu Gisselbrecht2, Sylvain Maclot2,4, Lana Neoričić2, Jasper Peschel2, Oksana Plekan3, Kevin C Prince3, Richard J Squibb4, Shiyang Zhong2, Philipp V Demekhin6, Michael Meyer7, Catalin Miron5,8, Laura Badano3, Miltcho B Danailov3, Luca Giannessi3,9, Michele Manfredda3, Filippo Sottocorona3,10, Marco Zangrando3,11, Jan Marcus Dahlström2

1Université de Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, Villeurbanne, France.
2Department of Physics, Lund University, Lund, Sweden.
3Elettra-Sincrotrone Trieste, Trieste, Italy.
4Department of Physics, University of Gothenburg, Gothenburg, Sweden.
5Université Paris-Saclay, CEA, CNRS, LIDYL, Gif-sur-Yvette, France.
6Institute of Physics and CINSaT, University of Kassel, Kassel, Germany.
7European XFEL, Schenefeld, Germany
8ELI-NP, "Horia Hulubei" National Institute for Physics and Nuclear Engineering, Magurele, Romania.
9Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Italy.
10Università degli Studi di Trieste, Trieste, Italy.
11IOM-CNR, Istituto Officina dei Materiali, Trieste, Italy.

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S. Nandi, E. Olofsson, M. Bertolino, S. Carlström, F. Zapata, D. Busto, C. Callegari, M. Di Fraia, P. Eng-Johnsson, R. Feifel, G. Gallician, M. Gisselbrecht, S. Maclot, L. Neoričić, J. Peschel, O. Plekan, K. C. Prince, R. J. Squibb, S. Zhong, P. V. Demekhin, M. Meyer, C. Miron, L. Badano, M. B. Danailov, L. Giannessi, M. Manfredda, F. Sottocorona, M. Zangrando, J. M. Dahlström, “Observation of Rabi dynamics with a short-wavelength free-electron laser, Nature 608, 488 – 493 (2022), DOI: 10.1038/s41586-022-04948-y

Last Updated on Wednesday, 07 September 2022 16:55