Opening the door to X-ray quantum optics

The possibility to generate coherent copies of X-ray pulses from FELs would facilitate a realm of X-ray techniques analogous to those currently available with optical laser light. Yet, this is a challenge due to the short wavelength of X-rays. Now, researchers have devised a new solution that ‘splits’ the electron bunch prior to generation of photons, producing two perfectly coherent copies of pulses.

X-ray free electron lasers (FELs) deliver ultra-bright, ultra-short and coherent X-ray pulses. Short pulses enable time-resolved studies of phenomena such as optical switching, whilst the short wavelength of X-rays, enables experiments with clear fingerprints of atomic species, which also resolve the distances between atoms in solids and liquids. In contrast, optical spectroscopies and microscopies primarily probe electrons shared across molecules, and provide neither images at the atomic scale nor allow unique identification of atomic constituents. Thus, X-ray FELs provide essential insights for fields ranging from pharmacology to electronic device engineering.

Yet, so far, part of the picture that is missing is the ability to do experiments with two – or more – coherent copies of beams. Splitting light is par for the course in optical spectroscopies. Phase-locked pulses – pulse pairs that have a fixed phase relation, split via mirrors, enable a host of interferometric and multidimensional spectroscopies. For X-ray light, the challenge comes with the advantage: the short wavelength. The wavelength – of the order of interatomic spacings – makes it extremely challenging to split the beam using mirrors, where even the smallest path length difference introduces phase jitter.

A perfect X-ray beam splitter

Now, researchers at the SwissFEL have come up with an ingenious solution. This takes a step back in the FEL and ‘splits’ the electron bunch in the accelerator, prior to the production of photons. Collaborators from both the electron and photon ends of SwissFEL have been a feature of this project, which uses a solution from the accelerator side to meet the photon requirements.

“What is really special is that from the beginning this has been a close interaction between quantum technologists and accelerator physicists. This is really facilitated by the interdisciplinarity of PSI, and its early recognition that the current, second quantum revolution will impact its core activity of building and operating large machines”, believes Simon Gerber, head of the Quantum Photon Science Group and corresponding author in the recent publication.

Read more on the PSI website

Image: Authors (L to R) Sven Reiche, Gabriel Aeppli and Simon Gerber standing outside the entrance to the SwissFEL

Credit: Paul Scherrer Institute / Mahir Dzambegovic