Abstract

Entangled states are central in the evolving field of quantum information. Creating them has been challenging, however; the present best method for two particles is spontaneous parametric downconversion. A “pump” photon in a nonlinear optical crystal may split into two highly correlated daughter photons, satisfying energy and momentum conservation (a.k.a., phase matching), in turn leading to energy-time and momentum entanglements, respectively. Producing polarization-entangled photons has thus far been more challenging. A type-II phase-matching configuration recently permitted tests of Bell’s inequalities, with rather good success.¹ However, even then, only the photon pairs emitted along two special directions possessed the desired correlations. And there seems to be no mechanism for tuning the degree of entanglement, short of selecting a different pair of special directions.²

© 1998 Optical Society of America