Interferometry is a method of investigation where waves, normally electromagnetic radiation, are superimposed to gather information about the path the waves took. If a coherent wave is spit into two perpendicular directions then reflected and brought back together they will constructively interfere if the paths they took were the same and destructively interfere if the paths they took were half a wavelength out. It was this process that allowed us to both disprove the aether argument of electrodynamics and show that gravitational waves existed (as in both cases the existence of these effects would lead to a distortion of a laser passing through space).
With any experimental technique there is a constant push to refine and improve it which often occurs in a series of incremental steps. Over the years interferometry and spectroscopy methods have been demonstrated in a wide range of wavelengths from radiowaves to nuclear magnetic resonance along with visible and ultraviolet lasers. In any case the ability to precisely control phases of the wave is essential to its application. Extreme ultraviolet pulses of light, perhaps only a few hundred attoseconds long, has been used in the last few years for probing energy, charge and information transport over minuscule distances in all manner of materials. For these techniques to be used at their full effectiveness a method for manipulating their phases has to be created, made much more difficult based on their short period of production.
Using a free electron laser (a laser where the excited medium is composed of high speed electrons that can move freely in a magnetic structure) it was demonstrated that the production of phase locked laser pulses could be produced that mirrored each other almost perfectly. Some of the suggested uses for the technique include the modelling of the electron system in glycine, the simplest of the amino acids; studying the correlated motion of holes and their paired electrons in materials like methyl iodide (CH3I); and finally for examining the 4d giant dipole resonance present in the xenon (Xe) atom.