07 April 2015 A new quantum material in Nature Photonics

alligator photonic crystal waveguide coupling two atoms

Strong interactions between atomic spins, phonons, and photons in photonic crystals Researchers at ICFO, in collaboration with scientists at Caltech and the Max Planck Institute for Quantum Optics, have proposed a novel way to realize strong long-range interactions between atomic spins, motion, and photons. This rich interplay between many degrees of freedom is made possible by coupling cold atoms to photonic crystals, and taking advantage of the ability to significantly alter the way that light propagates in these structures. One paper, authored by Dr. James Douglas and Hessam Habibian, led by ICFO Prof. Darrick Chang, proposes the existence of this unique quantum material in already-existing one-dimensional waveguide systems. A second paper investigates the possibility of realizing this physics in two-dimensional photonic crystal membranes. Both of these works appear in Nature Photonics.

Isolated from the environment and routinely manipulated, cold atoms are ideal systems to investigate quantum behavior. However, as neutral point particles, atoms are not naturally able to interact over long distances, which limits the kinds of many-body phenomena that can be accessed. In these articles, the researchers have proposed a novel solution, which takes advantage of recent experimental successes to trap atoms in the vicinity of photonic crystal structures and couple atoms to their guided modes.

A photonic crystal is a periodic dielectric structure that controls the propagation of light. An interesting feature of photonic crystals is that for certain frequency ranges of light known as \"band gaps,\" the periodicity of the structure causes such strong constructive interference that light is completely forbidden from propagating. Thus, ideally, an excited atom whose transition frequency sits within the band gap would not be able to spontaneously emit a photon as in free space, but instead a localized and stable photonic \"cloud\" forms around the atom. The photonic cloud is able to facilitate the exchange of excitations between this atom and a second one in the vicinity, to realize effective long-range \"spin\" interactions. It can also exert strong dispersive forces on nearby atoms, to couple atomic motion and spin together.

The ability to achieve simultaneous strong and long-range coupling between spins, phonons, and photons is unprecedented. The researchers believe that this system should lead to many exciting and unexpected phenomena in the next few years, and should also provide novel routes toward the exploration of many-body physics and quantum information processing with atomic systems.