Colloquia

We have developed a means of producing entangled pairs of photons, using the process of spontaneous parametric downconversion in a novel two-crystal geometry. The quality of the source has enabled us to produce states of unparalleled purity, while the brightness has permitted an extreme violation of Bell's inequalities. Furthermore, the source is tunable, and we have been able to produce for the first time non-maximally entangled states, and states of arbitrary purity. The result is the capability to produce (almost) any two-photon quantum (polarization) state. Such states have application to such problems in quantum information as quantum cryptography, quantum teleportation, and quantum cooking.