The easiest way to control light propagation is by using mirrors. Mirrors are normally relatively big and are made up of a huge number of atoms. Prof. Julien Laurat at Pierre and Marie Curie University in Paris (Laboratoire Kastler Brossel-LKB) and his team recently reported in an article in the Physical Review Letters journal that they have managed to create an efficient mirror using only 2000 atoms.

William Henry Bragg and his son William Lawrence Bragg earned the Nobel Prize in 1915 for developing the so-called Bragg reflection in crystalline solids. Laurat’s research fulfills the necessary conditions for this well-known physical effect by each trapped atom contributing with a small reflectance. The constructive interference of multiple reflections is then achieved by the engineered position of each atom.

Neil Corzo, a Marie-Curie postdoctoral fellow and the lead author of this work, explained that while tens of millions of atoms were needed to achieve a certain level of reflectance in previous experiments, only 2000 atoms trapped near a fiber were required to reach the same level of reflectance. He attributes this to the atom position control and strong atom photon coupling they have managed to achieve in their system.

A nanoscale fiber with a diameter of only 400 nm is a key ingredient in the system. A large portion of the light travels outside the fiber in a short-lived field where it is strongly focused over the 1 cm nanofiber length. Cold cesium atoms are trapped near the fiber in well-defined chains using this strong transversal confinement by implementing an all-fibered dipole trap. By using pairs of beams, the team generate two chains of trapping potentials around the fiber. Each site is occupied by a single atom. The distance between atoms in the chains are engineered to be close to half the resonant wavelength of the cesium atoms by carefully selecting the colors of the trap beams. This fulfills the necessary conditions for Bragg reflection.

Waveguide quantum electrodynamics is an emerging field with applications in quantum simulation, quantum networks and quantum nonlinear optics, and this technique signifies an important advance in the field. It will allow for innovative quantum network capabilities and many-body effects evolving from long-range interactions between multiple spins, something that is virtually impossible to achieve in free space.

This research is a continuation of other works done by Laurat’s group in recent years, including the creation of an all-fibered optical memory.

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