Science at the European X-ray Free Electron Laser

X-ray Free Electron Lasers (XFELs) have greatly enhanced our ability to observe transient nuclear and electronic motions in real time at atomic resolution, thereby deepening our fundamental understanding of matter across different disciplines. Moreover, XFELs offer several significant advantages in High Energy Density (HED) science, which deals with matter under extreme conditions of temperature and pressure. For instance, XFELs can probe structure and ionization dynamics in warm dense matter, a regime between solid and plasma states, investigate material response to ultra-high pressures, help refine models of radiation transport at extreme conditions, and recreate and study conditions inside gas giants or white dwarfs.
Most XFELs today generate pulses that consist of amplified noise, leading to significant shot-to-shot fluctuations. While these pulses exhibit high transverse coherence, their longitudinal coherence remains very low. In this talk, I will present two methods for controlling longitudinal coherence and demonstrate their application in X-ray spectroscopy. Such experiments are made possible only by the resulting exceptional spectral brilliance of XFEL sources. Specifically, I will discuss a nuclear clock transition in a specific scandium isotope. 
Because of the Heisenberg uncertainty principle, the structure of a molecule fluctuates about its mean geometry, even in the ground state. I will show the observation of this fundamental quantum effect experimentally, particularly, revealing the collective nature of the structural quantum fluctuations, by Coulomb Explosion Imaging for complex molecules. In detail, an 11-atom molecule was investigated by inducing its Coulomb explosion with an x-ray free-electron laser. The structural fluctuations manifest themselves in correlated variations of ion momenta obtained through coincident detection of the atomic fragments from individual molecules.
Lastly, I will discuss several applications of XFELs in the area of high energy density science. For example, I will present the first experimental evidence of liquid carbon, formed by shock-compressing graphite with a high-energy laser and probing it transiently using ultrashort XFEL pulses. Additionally, I will show the first experimental observation of plasma compression driven by relativistic currents in a cylindrical geometry; this effect was predicted over two decades ago but never confirmed until now. These experiments underscore the transformative impact of XFELs on advancing inertial fusion energy research.
 
The European X-Ray Free Electron Laser (EuXFEL), located near Hamburg and operated as a non-profit collaboration of 12 member states, is one of the world’s most advanced scientific research facilities. It provides scientists with extremely bright, coherent, and ultrafast X-ray flashes that allow exploration of matter at atomic length and ultrafast time scales. Since its completion in 2018, the facility, with 580 staff members from over 60 countries and more than 1’200 users per year, has produced more than 1’500 scientific publications and >30 PB of data annually.
The EuXFEL accelerates up to 27’000 electron pulses per second to energies up to 17.5 GeV. These electron pulses generate soft to hard X-rays in three different undulator systems, which feed seven operational instruments: FXE, SPB/SFX, SCS, SQS, SXP, HED, and MID, with an eighth under construction. These instruments support studies ranging from time-resolved crystallography to high-energy-density physics. The EuXFEL’s first operation mode as a free-electron laser (FEL) uses self-amplified spontaneous emission (SASE), but through self-seeding, spectral brightness and coherence can be significantly enhanced. The most recent further evolution toward coherent laser-like X-ray radiation, with appropriate feedback, marks a key milestone: the realization of the first hard X-ray laser oscillator. Several scientific areas will significantly benefit, for instance nuclear spectroscopy. An especially notable application is the resonant excitation of the potential nuclear clock isomer ⁴⁵Sc, whose ultranarrow 12.4 keV nuclear transition with linewidth of approximately 1.4 feV promises unprecedented timekeeping precision. 
In the Quantum World section, the presentation highlighted how EuXFEL enables direct imaging of small quantum systems and molecular dynamics at atomic resolution. Using X-ray-induced Coulomb explosion imaging (CEI), researchers can reconstruct molecular structures by measuring the three-dimensional momenta of ions produced when intense X-ray pulses strip electrons of the constituting atoms. CEI offers equal sensitivity to all atomic species, including hydrogen unlike diffraction-based methods. Recent work revealed how ground-state molecules exhibit collective quantum fluctuations, small correlated atomic motions detectable through momentum correlations.
European XFEL also enables unique experiments at conditions that simulate the interior conditions of planets and stars, exploring temperature-pressure regimes from ambient up to multi-terapascal and million-kelvin ranges. Using X-ray diagnostics and laser-based compression, scientists can reproduce environments akin to Earth’s core or the center of the Sun. Research topics include: The phase diagrams of iron and water, essential for understanding planetary interiors and exoplanet composition. Discovery of superionic ice, a phase where hydrogen ions move freely within a solid oxygen lattice, and the prediction of metallic ice phases at pressures above 1.5 TPa (Nature Communications 2024). Moreover, experimental insights into diamond formation from hydrocarbons (Nature Astronomy 2024) and the melting behavior of carbon under extreme compression (Nature 2025). These results deepen our grasp of planetary structures and high-pressure chemistry. 
The EuXFEL also contributes to the pursuit of nuclear fusion. Its ultrafast, high-brightness X-rays allow researchers to observe fusion dynamics in real time, offering data that can guide, for instance, the design of more efficient inertial-confinement fusion targets. Current collaborations aim to quantify energy transfer, ablator behavior, and plasma evolution to move toward viable, sustainable fusion energy. 

A conceptual electricity budget illustrates the challenge: achieving a gain of 160 requires balancing input from lasers (10 % efficiency) and power conversion (45 %), demanding several hundred megawatts of grid and recirculated energy.
European XFEL research spans multiple scientific frontiers:

* **Astrophysics**: simulating matter at stellar cores and planetary interiors.
* **Chemistry & Biology**: revealing protein and enzyme structures at atomic resolution, enabling studies of photosynthesis and artificial energy conversion.
* **Material Science & Technology**: creating and characterizing novel materials with tailored electronic and magnetic properties.
* **Quantum Physics & Digitalization**: advancing ultrafast control of quantum systems and data processing methods.

The European XFEL stands as a **flagship for European and international science**, merging cutting-edge accelerator physics, quantum optics, and high-energy-density research. It opens entirely new ways to visualize and manipulate matter—from probing the structure of planets and molecules to driving innovations in timekeeping and sustainable energy. As the facility continues expanding its capabilities and collaborations, it is poised to play a defining role in understanding the universe at its most fundamental scales.