  We study the dynamics of spontaneous generation of coherence and photon
spin-qubit entanglement or "flying qubits" in a $\Lambda$ system with
non-degenerate lower levels. The cases of entanglement in frequency only and
frequency and polarization are compared and the reduced density matrix and
entanglement entropy are analyzed. We explore in detail how which-path
information manifest when the energy difference between the qubit states is
larger than the linewidth of the excited state suppresses coherence. A
framework is provided to describe the complete dynamics of spontaneous
generation of coherence and (ideal) photodetection obtaining the post
measurement qubit density matrix. A simple model of photodetection with a
quantum eraser to suppress which-path information in the detection measurement
is implemented. It is found that such quantum eraser purifies the post
measurement qubit density matrix establishing contact with recent experiments.
