Coherence and decoherence in photon spin-qubit entanglement

Abstract

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.