An x-ray laser may sound like something you'd only find in a James Bond movie, but scientists have made the device a reality. Today, physicists at the SLAC National Accelerator Laboratory in Menlo Park, California, announced that they have coaxed test beams out of their Linac Coherent Light Source (LCLS), the first laser working at "hard" x-ray wavelengths. With further refinement, the LCLS might be able to determine the structure of a protein by blasting just a single molecule with its beam; it also might be able to squeeze matter to high pressures and temperatures to simulate conditions in the centers of planets.
X-rays are key to probing the atomic-scale structure of materials. In recent decades, physicists have built hugely intense x-ray sources that have been a boon to condensed matter physics, materials science, and structural biology. These sources rely on circular particle accelerators called synchrotrons; the particles circulating in them radiate x-ray photons as they whirl around. The LCLS could eventually be a billion times brighter than these sources. What's more, it will produce bona fide x-ray laser beams, meaning that all the photons in them will march in quantum-mechanical lockstep and give the beam especially useful properties.
The LCLS works differently than most lasers. In a standard laser, a light-emitting material, such as a certain type of crystal, sits between two mirrors, and the light bouncing back and forth stimulates the atoms in the material to crank out lots more light in the form of a laser beam. There are no mirrors for x-rays, however. So instead, the LCLS relies on part of SLAC's 3-kilometer-long linear accelerator to fire a beam of electrons at light speed through specialized magnets called undulators. The magnets make the beam wiggle and produce some x-rays. The x-rays then travel along with the electrons and separate them into bunches, and the bunches produce x-rays far more efficiently. Thanks to that feedback, an x-ray laser beam emerges--as it did last week, SLAC officials report today.
The ultrashort, ultraintense pulses of an x-ray laser could be used for a variety of wild, new experiments. For example, current x-ray sources based on synchrotrons have been used to determine the structures of thousands of proteins. But those sources require samples with many individual copies of a molecule frozen into an orderly crystal. In principle, the LCLS should be able to do the same thing by blasting just one molecule.
The development of the $420 million LCLS signals a shift at SLAC from a focus on particle physics to a much broader program emphasizing x-ray studies. Just 2 years ago, the flagship machines at the lab were its PEP-II particle smasher and the jumbo BaBar particle detector it fed. At the time, SLAC researchers were locked in a race with their counterparts at Japan's KEK laboratory in Tsukuba to collect as much data as possible on fleeting particles called B mesons. But PEP-II shut down for good last April, bringing to an end the lab's 46-year-run as a center for particle physics experiments.
SLAC officials seem to be managing the lab's change of course with aplomb, says Samuel Aronson, director of Brookhaven National Laboratory in Upton, New York. "It certainly looks like they're doing all the right things," Aronson says. But Alfonso Mondragón, a structural biologist at Northwestern University in Evanston, Illinois, says it remains to be seen if the x-ray laser will live up to its billing, especially for single-molecule studies. "First they need somebody to show that it works the way they said it will work," he says. "That won't happen next week."
SLAC plans to run its first real experiments with the laser this September. In the meantime, researchers in Germany and Japan are building similar x-ray sources.