Scientists working at the FERMI free-electron laser in Italy directed extreme UV radiation at helium atoms to manipulate electrons' energy levels. [Image: Alessia Candeo - Politecnico di Milano]
Scientists in Europe have shown how to manipulate the quantum states of atoms using intense extreme ultraviolet (XUV) radiation from a free-electron laser (Nature, doi: 10.1038/s41586-024-08209-y). They say that their work, which relied on seeding to modify the phase of the laser pulses, might be exploited in chemistry and quantum computing.
Achieving strong coupling
Intense laser light can be used to substantially alter the energy levels of atomic systems, potentially offering a way to control chemical reactions, quantum computations and other quantum-based phenomena. But until now, such “strong coupling” has only been possible at relatively long wavelengths (via the control of valence electrons). Researchers have begun to exploit light sources at XUV and soft X-ray wavelengths but usually have to combine this radiation with infrared fields to enter the strong coupling regime.
A collaboration headed by Lukas Bruder at the University of Freiburg in Germany, and including scientists in Italy, Austria and Denmark as well as Germany, has shown how to overcome these limitations by precisely shaping XUV pulses at the FERMI free-electron laser in Trieste, Italy. Free-electron lasers generate high-frequency electromagnetic waves by accelerating electrons to very high energies and tapping the radiation they give off as they are wiggled from side to side in devices known as undulators.
To carry out their work, Bruder and colleagues exposed FERMI's relativistic electron bunches to precisely timed and shaped seed pulses from an intense femtosecond laser. The laser's phase modulation transferred to the electron bunches, and then the bunches passed through a dispersive magnet and split up into micro-bunches. Those micro-bunches subsequently traveled through a series of undulators to yield a coherent XUV pulse, which shares the phase modulation of the original laser pulses.
A unique regime
The researchers say that by using XUV radiation, they were able to manipulate both helium's bound states and its more energetic unbound states—something impossible to do when using near infrared radiation of a similar intensity.
The researchers directed the XUV pulses at a sample of helium atoms in order to induce Rabi oscillations—rapid fluctuations in the population of electrons that lead to a splitting of the atoms' energy levels. They were able to observe these oscillations by placing a spectrometer after the undulators and measuring the distribution in kinetic energy of ionized electrons.
Bruder and his team found that the electron populations split in line with theoretical expectations. When generating laser pulses with no chirp (no change in frequency over time), they observed, as expected, that the two states in question had roughly equal numbers of electrons. But when introducing chirp, they instead saw a population imbalance—a rise in frequency along the pulse leading to more electrons in the lower state, while a frequency drop conversely led to greater numbers in the higher state.
The researchers say that by using XUV radiation, they were able to manipulate both helium's bound states and its more energetic unbound states—something impossible to do when using near infrared radiation of a similar intensity. As such, they argue, XUV “provides access to a unique regime.”
Looking to the future, Bruder and colleagues anticipate that recent progress at seeded free-electron lasers will extend their pulse-shaping scheme to soft X-ray wavelengths and may bring “pulse-shaping applications on the attosecond time scale within reach.” Such developments, they envisage, will allow quantum dynamics to be controlled with a temporal resolution and atomic selectivity not possible using longer-wavelength laser light.