Sources of Polarization-Entangled Photons

Polarization-entangled photons are essential resources for many quantum information science applications, such as quantum key distribution and linear optics quantum computing. Compact, high-flux sources of high-quality polarization-entangled photons are desirable for implementing these applications. In designing our entanglement sources we take advantage of advances in nonlinear crystals and utilize some of the standard techniques in nonlinear optics. The method of quasi-phase matching in periodically-poled potassium titanyl phosphate (PPKTP) and periodically-poled lithium niobate (PPLN) enables efficient downconversion at user-specified wavelengths, and collinearly propagating geometry in PPKTP and PPLN allows efficient collection of output photons. We have implemented a continuous-wave (cw) PPKTP polarization Sagnac interferometer as a spectrally bright source of polarization-entangled photons yielding 10,000 detected pairs/s/mW of pump power in a 1-nm bandwidth with a quantum-interference visibility of 98%. A pulsed version of the PPKTP Sagnac source is under development for use in free-space quantum key distribution. We are also working on a wavelength-nondegenerate PPLN Sagnac source that is expected to show a ten-fold increase in pair generation efficiency that can be used to characterize high-speed single-photon counters.