Figure 1

Operation principle of the device. (a), (b) Counterpropagating phase-matching scheme. (a) Interaction 1 with a pump angle $+{\theta }_{\mathrm{deg}}$ roduces an $H$-polarized signal and $V$-polarized idler; (b) Interaction 2 with a pump angle $-{\theta }_{\mathrm{deg}}$ produces a $V$-polarized signal and $H$-polarized idler. In both cases we call the signal (idler) the photon exiting from the right (left) facet. Phase matching is obtained automatically along the $z$ direction and through a periodic modulation of the waveguide core in the $x$ direction. The cladding consists of two Bragg reflectors to enhance the conversion efficiency. (c) Sketch of the experimental setup to generate Bell states: a laser pump beam impinges on a ridge waveguide microcavity with two symmetrical angles of incidence $+{\theta }_{\mathrm{deg}}$ and $-{\theta }_{\mathrm{deg}}$, through a Fresnel biprism, and entangled pairs of counterpropagating photons are emitted and collected at both facets. The waveguide is illuminated on its whole length ($L=1.8\mathrm{mm}$).Reuse & Permissions

Figure 2

Tunability curves. Simulated signal and idler tuning curves as a function of the angle of incidence of the pump beam, for ${\lambda }_p=759\mathrm{nm}$. Polarization entangled photons can be generated by simultaneously pumping interaction 1 at $+{\theta }_{\mathrm{deg}}$ and interaction 2 at $-{\theta }_{\mathrm{deg}}$.Reuse & Permissions

Figure 3

Photon correlation measurements. Time-coincidence detection histogram of signal and idler photons having respective polarizations $HV$ and $HH$. Most of the coincidences are produced by the entangled state, whereas a small part of them stems from noise. Since noise is polarization independent, its contribution to $HV$ coincidences can be inferred from the amount of $HH$ coincidences: the signal to noise ratio of coincidence events is thus given by the ratio between the areas of the $HV$ and $HH$ histograms.Reuse & Permissions

Figure 4

Polarization state tomography. Real and imaginary part of the density matrix $\rho$ of the two-photon state, as reconstructed from quantum-state tomography.Reuse & Permissions

Figure 5

Dependence of the net concurrence on the pump angle and the spatial overlap between the two pumping beams. The contour plot results from numerical simulation, while the gray ellipses correspond to the experimental results. The fact that the net concurrence saturates to a value of 0.84 when $\delta z=0$ and the pump angle is equal to ${\theta }_{\mathrm{deg}}=0.35°$ is due to the particular shape of the pumping beams [26].Reuse & Permissions