We dispersively interface an ensemble of one thousand atoms trapped in the evanescent field surrounding a tapered optical nanofiber. This method relies on the azimuthally-asymmetric coupling of the ensemble with the evanescent field of an off-resonant probe beam, transmitted through the nanofiber. The resulting birefringence and dispersion are significant; we observe a phase shift per atom of ∼\,1\,mrad at a detuning of six times the natural linewidth, corresponding to an effective resonant optical density per atom of 0.027. Moreover, we utilize this strong dispersion to non-destructively determine the number of atoms.
@article{arxiv.1108.2469,
title = {Dispersive Optical Interface Based on Nanofiber-Trapped Atoms},
author = {S. T. Dawkins and R. Mitsch and D. Reitz and E. Vetsch and A. Rauschenbeutel},
journal= {arXiv preprint arXiv:1108.2469},
year = {2015}
}