Experimental Synchronization of Independent Entangled Photon Sources

We report the generation of independent entangled photon pairs from two synchronized but mutually incoherent laser sources. The quality of synchronization is confirmed by observing a violation of Bell’s inequality with 3.2 standard deviations in an entanglement swapping experiment. The techniques developed in our experiment are not only important for realistic linear optical quantum-information processing, but also enable new tests of local realism.

Figure 1

Experimental setup of synchronized femtosecond pulse lasers. F1 and F2 are lens to focus the pumping 532 nm laser from two Verdi laser systems; Ti1 and Ti2 are Ti:sapphire crystals; M5 and M10 are high reflection flat mirrors; M1-M4 and M6-M9 are concave mirrors of 10 cm radius of curvature; P1-P4 are prisms; T1 and T2 are 20% output couplers. On the top of the figure, Ti1, M1-M5, P1, P2, and T1 constitute the first mode-lock femtosecond laser cavity. An analogous mode-lock femtosecond laser cavity shown at the bottom of the drawing is constituted of Ti2, M6-M10, P3, P4, and T2. KM is a Ti:sapphire crystal for synchronization. One end mirror M5 is driven by a translation stage (TS) to match the two laser cavity lengths. Both 788 nm infrared laser pulses are detected by fast photodiodes (PD1 and PD2) behind beam samplers (SM1 and SM2). Hence we can monitor the synchronization between two laser pulses on an oscilloscope.

Figure 2

The schematic drawing of experimental setup of quantum entanglement swapping. The two synchronized lasers produce two pairs of polarization entangled photons, respectively, by up-conversion and type-II SPDC. In order to compensate the birefringence of the BBO crystals, we place a half wave plate (HWP) and a compensating 1 mm BBO crystal on each path of the four photons. Interference filters (IF) with $\Delta {\lambda }_{\mathrm{FWHM}}=2.8\mathrm{nm}$ are place before each single photon detector ($D_1–D_4$). The beam splitter (BS) performs as a Bell-state measurement here. To meet the condition of temporal overlap, we used a step motor which minimum step is $0.1\mu \mathrm{m}$ to search for the position where the two photons arrived in the BS at the same time.

Figure 3

Entanglement verification. Fourfold coincidences, resulting from twofold $D_1^{+}{D}_4$ and $D_1^{-}{D}_4$ coincidences conditioned on the twofold coincidences of the Bell-state measurement. When varying the polarizer angle ${\theta }_4$, the two complimentary sine curve with a visibility of 82% demonstrate that photons 1 and 4 are polarization entangled.