When is a laser a real laser?

The study highlighted how FERMI Free Electron Laser is a unique source for quantum optics experiments.

A study published in the latest issue of Nature Communications has provided a major contribution in the definition of one of the main properties of free electron lasers, such as FERMIin Trieste. A research team from the German Electron Synchrotron, DESY, and from the National Research Nuclear University of Moscow, led by Ivan Vartaniants, performed an experiment on the free electron laser source in Trieste, highlighting its virtually unique properties vs. existing facilities in the U.S., Japan, Germany, Switzerland and China.

Lasersare intense and coherent light sources and the most complex systems are probably the so-called Free Electron Lasers (FELs), such as FERMI, that can generate intense light at much shorter wavelengths than visible light, from VUV to X-rays. “A major property of lasers is coherence, i.e. the ability to predict the light properties in a point in space or in time, when known” – explains Luca Giannessi, Coordinator of Fermi Machine Physics Team. First order coherence is an effect that is produced, for instance, in diffraction, resulting from the correlation between the amplitudes of a wave at different points in space (transverse coherence) or time (longitudinal coherence.) However, a high degree of first order coherence is not enough to define a laser, according to the Nobel laureate Roy Glauber, who stated that a laser can be defined as a source that is coherent in all orders. The higher order correlations are between intensity at different points in time and space. How are these correlations measured? The statistics of the photons is applied to measure correlations.

Glauber’s work was inspired by the famous Hanbury Brown and Twiss experiment, in which coincidences of photons (i.e. correlations) were measured with photons coming from distant stars. By varying the distance between two detectors, Hanbury Brown and Twisswere able to determine the degree of coherence of a star, and extract other information. This experiment was key to measure the second order coherence of a light source: the intensity of light at different points is measured in coincidence, and statistical analysis is performed. This experiment, according to many, has opened the way to the field of quantum optics.

Measurements at other FELs, based on the Self Amplified Stimulated Emission process, have shown that although they are first order coherent, they are not coherent at higher orders. However, FERMI* measurements have shown that the nature of the light from this laser is actually different: it’s not only first order coherent, but it’s also second order coherent. And this is in line with Glauber definition. Some quantum optics experiments require high order coherence, i.e. they require lights with order and predictability properties that are typical of quantum lasers. Therefore, FERMI so far is the reference facility at global level.

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* FERMI  is based on an external seeding scheme. This seeding scheme works like the note of a music instrument that is transmitted into an amplifier reproducing it accurately. The seeding scheme is produced by a solid-state laser, which therefore has the higher order coherence of quantum lasers. The experiment, based on the Handbury Brown and Tiss approach, was performed by a team lead by Ivan Vartaniants and it has shown that the FERMI scheme, based on amplification and frequency conversion of the seeding, preserve its properties, including higher order coherence properties. The article on the experiment, with the title “Seeded X-ray free-electron laser generating radiation with laser statistical properties”, was published in the issue of  Nature Communications of29 October 2018.

Press release of AREA Science Park

PRESS REVIEW

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