A new method of shining light through gas trapped in a silicon chip may point the way to simpler, more portable timekeepers, chemical sensors and test beds for communications networks that run on quantum weirdness. Researchers pumped light into one end of a hollow rectangular chamber embedded in silicon and filled with gaseous rubidium atoms. The specially coated cavity funneled the light to the other end, where the group could detect which frequencies of light the gas had absorbed along the way.

Shining light across atoms is a fundamental tool for chemists, who can identify substances by the telltale frequencies they soak up, as well as for researchers studying how light and matter interact. But so-called spectroscopy systems had never fit with integrated circuits before, says experimental physicist Holger Schmidt and his co-workers at the University of California, Santa Cruz.

Schmidt and co-workers solved this problem by coating a hollow centimeter-long chamber with layers of silicon oxide and silicon nitride. Like an optical fiber in reverse, the mirrorlike layers bounced light back into the rubidium-filled chamber instead of letting it escape. A different arrangement of layers on either end of the cavity allowed laser light to enter and emerge from the ends through optical fibers, according to a recent report published online by Nature Photonics.

Schmidt says one possible application is to sample air by pumping it into such a chip and scan for low concentrations of chemicals in factories or in areas at risk for chemical weapons attacks. Closer at hand, he says, would be a method to make semiconductor laser frequencies more precise by tuning them to the vibrations of rubidium or other atoms—a useful trick for chemists.

Similarly, it could turn those vibrations into precise timekeepers, which might benefit computer networks, says optics researcher Michael Raymer of the University of Oregon. Farther out, Schmidt says, researchers might use the system to perform experiments in which atoms drastically slow light or to store information encoded in light's fuzzy quantum states.

Raymer says the most exciting applications are yet to come, because the device bridges the previously separate fields of atomic physics and integrated optics. "When you bring two fields together, that's often where totally unexpected ideas come from."