Phys. Rev. Lett. 117, 133603 (2016)

Measuring the interaction of light with small ensembles of atoms is important for investigating quantum electrodynamic effects. However, millions of atoms are typically required to obtain strong atom–photon interactions. Now, Neil Corzo and co-workers report how just 2,000 caesium atoms, carefully arranged in two lines either side of a nanoscale waveguide, can reflect 75% of light guided in the waveguide. The atoms were optically trapped near the surface of the fibre so that they could interact with the fibre's evanescent field. An optical lattice formed by counter-propagating laser beams (with a frequency slightly detuned to the atomic resonance) was used to perform the trapping and create the 1D periodic arrays of atoms. The atom chains effectively act like a Bragg amplitude grating and reflect light travelling in the waveguide. Due to the periodic arrangement of the atoms the small reflections from each atom constructively interfere with reflections from every other atom, resulting in a strong overall effect. The transmission and reflection of a probe laser beam sent through the waveguide was detected using avalanche photodiodes.