Innovative imaging technique uses the quantum properties of X-ray light

An international team of researchers including scientists from FAU has, for the first time, used X-rays for an imaging technique that exploits a particular quantum characteristic of light. In their article, which has now been published in the journal Physical Review Letters, the researchers detail how this process could be used for imaging non-crystallized macromolecules.

The research team used the extremely short and very intensive X-ray pulses at the X-ray laser European EXFEL in Hamburg in order to generate fluorescence photons that arrived almost simultaneously at the detector—in a time window of less than one femtosecond (one quadrillionth of a second). By calculating the photon-photon correlations in the fluorescence of the illuminated copper atoms, it was possible to create an image of the light source.

On the atomic scale, the structures of materials and macromolecules are usually determined using X-ray crystallography. While this technique relies on coherent X-ray diffraction, the scattering of X-ray light can cause incoherent processes such as fluorescence emissions, which can dominate, even though they do not make a useful contribution to the diffraction measurement. Instead, they add a functionless haze or background to the measurement data.

As long ago as the 1950s, two British astronomers proved that it is possible to gain structural information from such self-luminous light sources, in their case it was the light from stars. Robert Hanbury Brown and Richard Twiss, whose method is known as intensity interferometry, opened a new door to the understanding of light and founded the field of quantum optics.

Recently, scientists from FAU, the Max Planck Institute for the Structure and Dynamics of Matter and the Deutsches Elektronen-Synchrotron (DESY) suggested that intensity interferometry could be adapted for atomic-resolution imaging using X-ray fluorescence. The challenge in extending this idea to X-rays is that the coherence time of the photons, which dictates the time interval available to perform photon–photon correlations, is extremely short. It is determined by the radiative decay time of the excited atom, which is about 0.6 femtoseconds for copper atoms.

Together with scientists from Uppsala University and the European XFEL, the group has now overcome that challenge by using femtosecond-duration XFEL pulses from that facility to initiate X-ray fluorescence photons within the coherence time. The team generated a source consisting of two fluorescing spots in a foil of copper and measured the fluorescence on a million-pixel detector placed eight meters away.

Only about 5,000 photons were detected on each illumination pulse, and the cumulative sum over 58 million pulses produced just a featureless uniform distribution. However, when the researchers instead summed photon-photon correlations across all images from the detector, a striped pattern emerged, which was analyzed like a coherent wave field to reconstruct an image of the fluorescent source, consisting of two well-separated spots of light.

The scientists now hope to combine this new method with conventional X-ray diffraction to image single molecules. Element-specific fluorescent light could expose substructures that are specific to certain atoms and even to certain chemical states. This could contribute to a better understanding of the functions of important enzymes such as those involved in photosynthesis.