Figure 6. Experimental setup in a Sagnac structure.

A 60 mW continuous-wave (CW) laser beam with a central wavelength of 404 nm is focused onto a type-II BBO crystal to create entangled photon pairs. The PBS part of the cubes convert the polarization-entangled state to spatial-entangled state. A1, B1, A2 and B2 are four single-qubit gates. By post-selecting the case where the two photons exit at ports 1 and 2, one would effectively realize a two-qubit quantum gate (A1A2+B1B2). The displaced-Sagnac structure makes the phase between modes r1 and b1 (r2 and b2) inherently stable.