Polarization-entangled mode-locked photons from cavity-enhanced spontaneous parametric down-conversion

We report the generation of polarization-entangled photons by cavity-enhanced spontaneous parametric down-conversion. The bandwidth of entangled photons is reduced to that of the optical parometric oscillator and the conversion rate is enhanced by cavity. The time interval between the conjugate two photon is prolongated so that it can be directly measured by a common detector. The time correlation function of the entangled-photons pair shows a time-oscillatory characteristic. To demonstrate the entanglement, we experimentally obtain two-photon quantum interference for the polarization variable.

Figure 1

Schematic for the generation of polarization entanglement in cavity-enhanced SPDC. M1–M4, cavity mirrors; WP, compound waveplate; KN1 and KN2, ${\mathrm{KbNO}}_3$ crystals, their optic axes aligned in mutually perpendicular planes.

Figure 2

Schematic of experimental setup. KN1(2), ${\mathrm{KbNO}}_3$ crystal; WP, compound zero-order waveplate; BS, $50∕50$ beam splitter; IF, interference filter; P1-2, polarizer; DM, dichroic mirror.

Figure 3

The correlation function of two photons from cavity enhanced SPDC. The round-trip length of OPO is 558 mm. The line is the fitted curve of the the datas to the equation derived from Eq. (5)

Figure 4

Measurements of the polarization entanglement. The polarization analysis of P2 was varied, while that of P1 was at $-45°$. The rate at detector D2 (squares, right axis) is essentially constant, i.e., the photons are individually nearly unpolarized, while the coincidence rate (circles, left axis) displays the expected quantum-mechanical correlations. The solid curve is the best fit.