One-dimensional ultracold medium of extreme optical depth

We report on the preparation of a one-dimensional ultracold medium in a hollow-core photonic crystal fiber, reaching an effective optical depth of 1000(150). We achieved this extreme optical depth by transferring atoms from a magneto-optical trap into a far-detuned optical dipole trap inside the hollow-core fiber, yielding up to 2.5(3)×10(5) atoms inside the core with a loading efficiency of 2.5(6)%. The preparation of an ultracold medium of such huge optical depth paves the way toward new applications in quantum optics and nonlinear optics.