Entanglement Persistency of Multiphoton Entangled States

We experimentally demonstrate the entanglement persistency when losing photons in three- and four-photon polarization-entangled states. The entanglement properties of the mixed states obtained from multiphoton spontaneous parametric down-conversion are studied via witness and positive partial transpose approaches. Together with a quantification of the bipartite entanglement such analysis enables intuitive understanding of novel multiparty quantum communication protocols.

Figure 1

(top) Schemes of the experimental setups and (bottom) graphs illustrating the bipartite entanglement of (left) $|\mathrm{W}⟩$ and (right) $|{\Psi }^{(4)}⟩$ states. In the graphs the vertices represent qubits, and the solid (dashed) lines represent bipartite entanglement (separability).

Figure 2

Coincidence detection probabilities for the three- and four-qubit states (a) $|W⟩$ and (b) $|{\Psi }^{(4)}⟩$. Real parts of the experimentally determined two photon density matrices of the states (c) ${\rho }_{ab}^W$ and (d) ${\rho }_{ac}^{{\Psi }^{(4)}}$ in the $\{H,V\}$ polarization basis.