  By performing a full analysis of the projected local density of states (LDOS)
in a photonic crystal waveguide, we show that phase plays a crucial role in the
symmetry of the light-matter interaction. By considering a quantum dot (QD)
spin coupled to a photonic crystal waveguide (PCW) mode, we demonstrate that
the light-matter interaction can be asymmetric, leading to unidirectional
emission and a deterministic entangled photon source. Further we show that
understanding the phase associated with both the LDOS and the QD spin is
essential for a range of devices that that can be realised with a QD in a PCW.
We also show how quantum entanglement can completely reverse photon propagation
direction, and highlight a fundamental breakdown of the semiclassical dipole
approximation for describing light-matter interactions in these spin dependent
systems.
