Experimental Diagnostics of Entanglement Swapping by a Collective Entanglement Test

This paper reports on the experimental diagnostics of an entanglement-swapping protocol by means of collective entanglement witness. Our approach is suitable to detect disturbances occurring in the preparation of quantum states, quantum-communication channels, and imperfect Bell-state projections. More specifically, we demonstrate that our method can distinguish disturbances such as depolarization, phase damping, amplitude damping, and imperfect Bell-state measurement by observing four probabilities and estimating collective entanglement witness. Since entanglement swapping is a key procedure for quantum repeaters, quantum relays, device-independent quantum communications, and entanglement-assisted error correction, this can aid in faster and practical resolutions of quality of transmission related problems as our approach requires fewer measurements than other means of diagnostics.