Figure 1

The relevant level structure of the alkali-metal atoms in the ensembles. We consider a double-$\Lambda$-type five-level configuration of atomic states of ${}^{87}\mathrm{Rb}$. $\mid g⟩$, $\mid h⟩$, and $\mid v⟩$ with energy differences ${\omega }_{gh}$ and ${\omega }_{gv}$ $(\hslash =1)$ can be pairs of Zeeman sublevels of electronic ground state $5S_{1∕2}$, corresponding to $\mid F=1,M_F=0⟩$, $\mid F=2,M_F=2⟩$, and $\mid F=2,M_F=-2⟩$, respectively. $\mid e_h⟩$ and $\mid e_v⟩$ with energy splittings ${\omega }_{g{e}_h}$ and ${\omega }_{g{e}_v}$ to the ground state $\mid g⟩$ are the Zeeman sublevels of the electronic excited state $5P_{1∕2}$, corresponding to $\mid F=2,M_F=1⟩$ and $\mid F=2,M_F=-1⟩$, respectively.Reuse & Permissions

Figure 2

The purification scheme for three-particle entanglement. Alice, Bob, and Charlie superimpose their photons on polarizing beam splitters. They keep only those cases where there is exactly one photon in each output mode. They perform a polarization measurement in the 45° basis in modes a4, b4, and c4. Alice performs a specific operation on the photon in mode a3 according to the results. After that the state of the remaining photons in modes a3, b3, and c3 will have a higher fidelity than the two original states.Reuse & Permissions