Scientists from the University of California, Santa Barbara, led by David Weld, have for the first time directly tested a theoretical prediction about the behavior of waves in an optical resonator - a pair of mirrors that confine light. The work was published in the journal Physical Review Letters.
Optical resonators underlie lasers, atomic clocks and many precision sensors. If the distance between the mirrors changes quickly enough, some theories predict that the waves inside will begin to gather into short, intense pulses. Until now, however, this had not been directly tested: for an experiment with real light, a mirror would have to be accelerated to a significant fraction of the speed of light in billionths of a second, which is not yet technically possible.
To circumvent this limitation, the researchers replaced light with atoms. At ultralow temperatures, atoms behave like waves. The team cooled a gas of lithium atoms almost to absolute zero and placed it in a lattice of laser light. This lattice caused the "matter waves" to move at an almost constant speed - less than one meter per second.
The atoms were confined by two sheets of laser light that played the role of weightless mirrors. One of the sheets was moved back and forth about 100 times per second. The scientists then recorded the distribution of the atoms over time and compared the results with theoretical predictions.
After only a few cycles, the moving sheet of light turned the initially diffuse cloud into a compact packet of atoms that followed a repeating path. Packets starting from different positions left the unstable trajectory and converged on a single stable one - the very unidirectional motion that, according to theory, resembles the event horizon of a black hole or a hypothetical "white hole."
When the frequency of the drive was doubled, the researchers obtained two clumps, each on its own trajectory. By sharply shifting the timing of the sheet's motion, they could swap the stable and unstable paths, reversing the motion of the packets. Subtle effects were also observed that go beyond the ideal picture, related to the fact that the speed of the atoms is not as constant as that of light.
The authors hope the effect will find application for compressing and decompressing optical signals, possibly in solid materials with electrically controlled optical properties - without moving mirrors. In addition, the setup may help physicists test other fundamental predictions, for example about the birth of pairs of light particles from the vacuum between rapidly moving mirrors.