Quantum Ghosts: Atoms become transparent to certain frequencies of light

Collectively induced transparency

Collectively induced transparency

Artist’s visualization of a laser hitting atoms in an optical resonator. Scientists discovered a new phenomenon called collectively induced transparency (CIT), in which groups of atoms stop reflecting light at certain frequencies. The team found this effect by confining ytterbium atoms in an optical cavity and exposing them to laser light. At certain frequencies, a window of transparency was created, allowing light to flow unhindered past the cavity. Credit: Ella Maru Studio

Newly observed effect makes atoms transparent to certain light frequencies

Researchers at Caltech have discovered a new phenomenon, ‘collectively induced transparency’ (CIT), in which light at certain frequencies passes unhindered through groups of atoms. This finding could potentially improve quantum memory systems.

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

CIT was discovered by trapping ytterbium atoms in an optical cavity — essentially a tiny box for light — and blasting them with a laser. Although the light from the laser bounces off the atoms up to a point, adjusting the light frequency creates a transparent window where the light simply passes unhindered through the cavity.

“We never knew this transparency window existed,” says Caltech’s Andrei Faraon (BS ’04), William L. Valentine Professor of Applied Physics and Electrical Engineering and co-author of an article about the discovery published April 26 in the journal Nature. “Our research has become primarily a journey to find out why.”

Analysis of the transparency window indicates that it is the result of interactions between atomic groups and light in the cavity. This phenomenon is similar to destructive interference, where waves from two or more sources can cancel each other out. The groups of atoms constantly absorb and re-emit light, which generally results in reflection of the laser light. However, at the CIT frequency, there is an equilibrium created by the re-emitted light from each of the atoms in a group, resulting in a drop in reflectance.

“An ensemble of atoms strongly coupled to the same optical field can lead to unexpected results,” says co-lead author Mi Lei, a graduate student at Caltech.

The optical resonator, which is just 20 microns long with features less than 1 micron, was fabricated at Caltech’s Kavli Nanoscience Institute.

“Through conventional quantum optical measurement techniques, we found that our system has reached an uncharted territory that reveals new physics,” says PhD student Rikuto Fukumori, co-lead author of the work.

In addition to the transparency phenomenon, the researchers also observed that the cluster of atoms can absorb and emit light from the laser either much faster or much more slowly than a single one[{” attribute=””>atom depending on the intensity of the laser. These processes, called superradiance and subradiance, and their underlying physics are still poorly understood because of the large number of interacting quantum particles.

“We were able to monitor and control quantum mechanical light–matter interactions at nanoscale,” says co-corresponding author Joonhee Choi, a former postdoctoral scholar at Caltech who is now an assistant professor at Stanford University.

Though 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 to more efficient quantum memories in which information is stored in an ensemble of strongly coupled atoms. Faraon has also worked on creating quantum storage by manipulating the interactions of multiple vanadium atoms.

“Besides memories, these experimental systems provide important insight about developing future connections between quantum computers,” says Manuel Endres, professor of physics and Rosenberg Scholar, who is a co-author of the study.

Reference: “Many-body cavity quantum electrodynamics with driven inhomogeneous emitters” by Mi Lei, Rikuto Fukumori, Jake Rochman, Bihui Zhu, Manuel Endres, Joonhee Choi and Andrei Faraon, 26 April 2023, Nature.
DOI: 10.1038/s41586-023-05884-1

Coauthors include Bihui Zhu of the University of Oklahoma and Jake Rochman (MS ’19, PhD ’22). This research was funded by the Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, and the Office of Naval Research.

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