Synchrotron radiation has long been a valuable tool for investigating the structure of matter. However, synchrotron sources produce incoherent radiation limited to relatively low peak brilliance. As a next step in the development of radiation sources, the free-electron laser (FEL) has been shown to be capable of producing radiation pulses with a peak brilliance several orders of magnitude larger than that of synchrotrons.
There are currently several large international projects dedicated to developing FELs for the production of short-wavelength radiation by Self Amplified Spontaneous Emission (SASE): LCLS in the USA, FLASH in Hamburg, Germany and SCSS in Japan. While the brightness of these (classical) SASE-FELs far exceeds that of conventional synchrotron sources, the radiation produced by SASE-FELs is not ideal for many applications requiring a high degree of temporal coherence as the radiations produced consists of many random superradiant spikes with a broad noisy spectrum.
A possible alternative to classical SASE-FEL emission for coherent short-wavelength generation arises from the fact that in quantum theory the radiation emission process is fundamentally discrete. When an electron emits a photon, the momentum recoil is ħk, where k is the photon wavenumber. Hence, the electron momentum recoil is naturally quantized and can change in only discrete amounts. Including the discreteness of the electron recoil in a quantum FEL (QFEL) theory, the quantum regime of FEL operation appears promising for a quasi-monochromatic X-ray source capable of lower powers than in a classical SASE-FEL, but of greatly improved temporal coherence relative to classical SASE-FELs.
