Entanglement swapping with quantum-memory-compatible photons

We report entanglement swapping with time-bin entangled photon pairs, each constituted of a 795-nm photon and a 1533-nm photon, that are created via spontaneous parametric down conversion in a nonlinear crystal. After projecting the two 1533-nm photons onto a Bell state, entanglement between the two 795-nm photons is verified by means of quantum state tomography. As an important feature, the wavelength and bandwidth of the 795-nm photons is compatible with ${\mathrm{Tm}:\mathrm{LiNbO}}_3$-based quantum memories, making our experiment an important step towards the realization of a quantum repeater.

Figure 1

Schematics of our setup. See text for details.

Figure 2

Coincidence detection probabilities (conditioned on a successful Bell-state measurement) of the 795-nm photons as a function of the phase difference in the measurement interferometers. The uncertainty bars indicate one standard deviation and are calculated from measured rates assuming Poissonian detection statistics. The visibility of the sinusoidal fit is ($56.2\pm 5.7)\%$. The blue curve features a visibility of 1/3, indicating the classical bound for a separable Werner state.

Figure 3

Density matrix characterizing the joint state of the two 795-nm photons after entanglement swapping. The fidelity with respect to the $|{\mathrm{\Psi }}^{+}\rangle$ Bell state—which we find to be the nearest maximally entangled state—is $(68\pm 3)\%$.

Figure 4

Joint spectral amplitude of photon pairs generated via an SPDC process with bandwidths of pump pulse and spectral filters indicated.