  We study a system of two distant quantum emitters coupled via a
one-dimensional waveguide where the electromagnetic field has a
direction-dependent velocity. As a consequence, the onset of collective
emission is non-simultaneous and, for appropriate parameters, while one of the
emitters exhibits superradiance the other can be subradiant. Interference
effects enable the system to radiate in a preferential direction depending on
the atomic state and the field propagation phases. We characterize such
directional emission as a function of various parameters, delineating the
conditions for optimal directionality.

Quantum Physics