Motion-induced directionality of collective emission in a non-chiral waveguide

We discuss the first observation of motion-induced directionality in the collective emission of light from atoms confined within a hollow-core waveguide [1]. The atoms form effective two-level systems created by a far off-resonant Raman process an as a consequence their effective dipole matrix element has a phase factor which oscillates in space. This phase factor has no consequence for atoms at rest. However, in a thermal cloud of atoms the combination of atomic motion and the spatially oscillating phase of the dipoles leads to a preferred collective emission in the forward direction. To understand the underlying effect we use the Truncated Wigner Approximation for spins [2] to simulate the collective emission of a reduced number of moving atoms coupled to a waveguide. We see good agreement between experimental results and the theoretical description and can explain the origin of the asymmetry between forward and backward superradiant emission. Furthermore, we study the second-order correlation function of the emitted light close to and well above the threshold to collective emission, showing a buildup of coherence during the superfluorescent bursts while the emitted light below the threshold exhibits thermal statistics.

[1] Y. Spahn, J.Hartmann, B. Saalfrank, M.Fleischhauer, T. Halfmann, T. Peters (in preparation)

[2] J. Hartmann, T. Schlegel, C. Mink, M. Fleischhauer, Truncated Wigner Approximation for unitary and open many-body spin systems (in preparation)