Chill a bacterium-size mirror to a cold enough temperature and it will begin to quiver not from heat but from its quantum state--a so-called superposition in which the object is both vibrating and still. Aside from the experimental thrill of conjuring up quantum states in big, messy things, such ultracold objects could form the foundation of exquisitely fine sensors for weighing individual atoms. Three new studies show that researchers are making headway in their efforts to create nearly quiverless mirrors.
Quantum superpositions occur naturally in the tiny world of atoms, but as objects get bigger, heat and other external influences more easily disrupt these delicate states. Researchers have devised strategies for cooling nanometer-size slivers nearly to the point at which quantum effects take over. Micrometer-size mirrors are an even bigger challenge given their larger number of atoms, each one a potential source of disruption. In principle, though, light alone could chill such a device. A mirror can act as a sort of plunger that caps a cavity full of photons. If the mirror's vibrations happen to drive it outward, some photons escape the cavity, and the plunger feels a resulting suction force as the pressure inside the cavity diminishes. If the mirror moves into the cavity, pressure builds and pushes it back out. Either process would damp down the mirror's motions and keep it still.
The trick is keeping the photons inside the cavity long enough so that physicists can adjust their frequency and enable them to push and tug the mirror in the right way, like pushing a swing at the proper moment. In the November 2 Nature researchers demonstrated two ways to do so: In the first, a French and an Austrian group chose to construct highly reflective cavities, each capped on one end by a flexible beam that acted as a high quality mirror. Both teams cooled their devices from room temperature down to 10 kelvins or less. In the second method, a group from the University of California, Santa Barbara, replaced the beam with a diving-board-like cantilever holding a mirror. By bouncing laser light into the cavity the physicists monitored the position of the cantilever. They then fed this information to a second laser that actively damped the cantilever's motion by applying pressure near its base. The result: a chilly 0.135 kelvin.
"Although spotting intriguing quantum mechanical effects is probably still some distance away, the research marks a promising step in that direction," writes physicist Khaled Karrai of Ludwig Maximilians University in Munich in an accompanying editorial. Observing quantum superpositions will require cooling to a few thousandths of a kelvin or less, Karrai points out. Right now heat is probably seeping in through the mirror's flexible attachments and by absorption of light, he notes.