On the Improvement of the Degree of Polarization Entanglement in the Double-Crystal Scheme of Spontaneous Parametric Down-Conversion

A method has been proposed to improve the degree of polarization entanglement of biphotons obtained by means of a double-crystal scheme. The effect of deviation of the polarization of spontaneous parametric down-converted radiation on the degree of polarization entanglement of biphotons has been examined theoretically and experimentally. The dependence of the concurrence as a measure of entanglement on the difference between the pump phases in the nonlinear crystals of the double-crystal scheme has been studied experimentally, and the improvement of the concurrence by a factor of 2.7 has been achieved at the optimal phase difference.