The newly observed effect makes the atoms transparent to certain frequencies of light

A newly discovered phenomenon called collectively induced transparency (CIT) causes groups of atoms to suddenly stop reflecting light at certain frequencies.

CIT was discovered by confining ytterbium atoms inside a photocavity—essentially a small box of light—and blasting them with a laser. Although the laser light will bounce off the atoms to a point, as the light’s frequency is tuned, a transparent window appears in which the light simply passes through the cavity unobstructed.

says Andrei Faraon (BS ’04) of the California Institute of Technology (BS ’04), William L. Valentine Professor of Applied Physics and Electrical Engineering and co-author of a paper about the discovery published April 26 in the journal nature. “Our research became primarily a journey to find out why.”

Analysis of window transparency indicates that it is the result of interactions in the cavity between groups of atoms and light. This phenomenon is similar to destructive interference, whereby waves from two or more sources can cancel each other out. Clusters of atoms constantly absorb and re-emit light, which generally results in reflection of laser light. However, at the CIT frequency, there is an equilibrium caused by the light being re-emitted from each atom in an ensemble, which leads to a decrease in reflectance.

“A group of atoms that are strongly coupled to the same optical field can lead to unexpected results,” says co-lead author Mei Li, a graduate student at Caltech.

The optical resonator, which is only 20 μm in length and includes features smaller than 1 μm, was fabricated at the Kavli Institute for Nanoscience at Caltech.

“Through traditional quantum optics measurement techniques, we have found that our system has reached an unexplored regime, revealing new physics,” says graduate student Rikuto Fukumori, co-lead author of the paper.

Besides the phenomenon of transparency, the researchers also note that a group of atoms can absorb and emit light from a laser either much faster or much slower compared to a single atom depending on the intensity of the laser. These processes, called superradiation and subduction, and their underlying physics are still poorly understood due to the large number of interacting quantum particles.

“We’ve been able to observe and control the quantum mechanical interactions between light and matter on a nanoscale,” says co-author Joonhee Choi, a former postdoctoral researcher at Caltech and now an assistant professor at Stanford.

Although the research is primarily fundamental and expands our understanding of the mysterious world of quantum effects, this discovery has the potential to one day help pave the way for more efficient quantum memories where information is stored in an array of highly-coupled atoms. Farron also worked to create quantum storage by manipulating the interactions of multiple vanadium atoms.

“Besides memories, these experimental systems provide important insights into the development of future communications between quantum computers,” says Manuel Endres, Professor of Physics and Rosenberg Scholar, a co-author of the study.