Using shaped laser pulses in the extremely high energy range, researchers have succeeded in manipulating helium atoms. Oldenburg physicist Matthias Wollenhaupt was involved in the study. 

An international team of scientists led by Dr Lukas Bruder from the University of Freiburg has demonstrated a new method for studying and controlling processes at the atomic level using ultrashort intense, high-energy laser pulses. Prof. Dr Matthias Wollenhaupt, a physicist at the University of Oldenburg, also contributed to the study, which was published in the journal Nature. The scientists report that among other applications their method could form the basis for unprecedented precision in the investigation and control of chemical reactions.

The team used the FERMI Free Electron Laser for their experiments. Named after the Italian nuclear physicist Enrico Fermi, this device is located in Trieste, Italy, and can generate ultrashort and intense light pulses in the extreme ultraviolet (EUV) range of the spectrum. The researchers were able to precisely "shape" light pulses in this short-wavelength domain for the first time and use them to manipulate helium atoms. "Pulse shaping has long been an established technique in the visible light range. The fact that this has now also been achieved in the extreme UV range provides us with a new, powerful tool for quantum control," says Wollenhaupt, who heads the Ultrafast Coherent Dynamics research group at the University of Oldenburg. The term "quantum control" refers to the control of atomic-level processes that obey the peculiar laws of quantum physics.

Using „chirped” laser pulses to induce transient quantum states

In their experiment, the researchers used special EUV laser pulses the frequency of which varies over the duration of the pulse. Experts refer to them as "chirping" or "chirped" pulses because comparable acoustic signals resemble the chirping of birds. With these pulses, they induce transient quantum states in helium atoms in which a "coupling" between the light field and the helium atoms takes place. Since the atoms of the noble gas helium are particularly stable, laser pulses of extremely high-energy light were required to induce these quantum states. "These states are referred to as 'dressed states'. They only last as long as the laser pulse lasts," explains Wollenhaupt. In these states, the energy levels of the electrons in the atom shift compared to the unperturbed system and split up into what is known as a "doublet". The researchers explain in their paper how they were able to manipulate and selectively control the shape of this doublet using differently chirped laser pulses for the first time.

The current experiment builds on the work of a team led by Wollenhaupt, the results of which were published in 2006, when the physicist was still based at the University of Kassel. At that time the researchers used laser pulses in the visible and infrared range of light to control the corresponding doublet in potassium atoms with chirped pulses. The Oldenburg physicist is an expert in quantum control using tailored laser pulses. This expertise was also crucial in the current experiment in which chirped laser pulses in the EUV range were used for quantum control. "The technique we developed opens up a new field of research," said the lead author Bruder. „This includes new possibilities for making experiments with free electron lasers much more efficient and selective, or for gaining new insights into fundamental quantum systems, which are not accessible with visible light.”

In addition to Wollenhaupt and the Freiburg team, researchers from the Max Planck Institute for the Physics of Complex Systems in Dresden, the Institute of Photonics and Nanotechnology in Milan (Italy), the University of Innsbruck (Austria), the University of Gothenburg (Sweden), the Institute of Materials in Trieste, the National Institute for Nuclear Physics in Rome (Italy), the Deutsches Elektronen-Synchrotron DESY in Hamburg, Aarhus University (Denmark) and the University of Hamburg were involved in the publication.