Poled Fiber-based Source of Polarization-entangled Photon Pairs

Entanglement is a property of quantum mechanics that confounds both physicists and laymen alike, but has found practical applications in secure communications and metrology. In this thesis, I develop and characterize a telecom-band (~ 1550 nm) fiber-based source of polarization-entangled photon pairs. However, unlike other fiber-based sources, where the third-order nonlinear optical process of spontaneous four-wave mixing is exploited, the second-order process of spontaneous parametric downconversion (SPDC) is utilized here. In particular, a type-II phase-matched SPDC process in the fiber is utilized for the direct generation of high-quality (fidelity > 96%) broadband (>100 nm) entangled photon pairs. Neither temporal compensation nor interferometric methods are employed in the production of the high-quality entangled photon pairs. Conventional step-index silica fibers do not possess a second-order optical nonlinearity due to their amorphous structure and inversion symmetry. However, through the process of thermal poling, that symmetry can be broken, creating a non-zero second-order nonlinearity. The fiber-based sources described in this thesis have been thermally-poled. The bulk of this thesis involves the development of techniques to measure the spectral and polarization-entangled natures of the downconverted photon pairs generated from poled fibers. I detail the schemes and strategies for observing the entanglement, as well as the creation of new tools to achieve that goal. Of particular interest is a spectrographic method involving a dispersive medium (20 km of SMF-28 single-mode fiber) that allows us to measure the broadband spectrum of our downconverted photons by mapping the spectral information onto the temporal domain. Spectral (non-)overlap of the two phase-matchings that give rise to the polarization-entanglement is investigated using this technique. Lastly, to demonstrate the utility of our high-quality and broadband entangled source, a poled fiber is used in an entanglement-based quantum key distribution experiment. The broadband nature of the entangled photons is exploited so that multiple users can be served simultaneously. Distributing the entangled photons over 40 km of fiber, a sifted (secure) key generation rate of more than 30 bits/sec (20 bits/sec) is obtained.Ph.D