Motion-Induced Directionality of Collective Emission in a Nonchiral Waveguide.

We report the experimental observation of motion-induced directionality in collective atomic emission within a hollow-core waveguide, establishing a general principle: directional interactions can emerge from collective phase engineering alone. Remarkably, neither single-emitter asymmetry nor any asymmetry in the geometric arrangement of the system is required-both the atom-field coupling and the spontaneous emission are fully isotropic in our system. Instead, Raman-induced effective two-level emitters with spatially oscillating transition dipole phases and atomic motion give rise to controllable directionality, reaching values up to 0.89(1). We study the correlations of the superfluorescent bursts close to and well above the threshold to collective emission; we find thermal statistics below and a buildup of coherence above it. Numerical simulations based on the truncated Wigner approximation for spins yield good agreement. Additionally we present a simple model based on position uncertainty capable of reproducing the observed directionality. Our results open a new route to directional interactions in nonchiral systems, with direct implications for the design of directional metamaterials and photonic structures built from isotropic constituents.