We study theoretically the generation of photon pairs by spontaneous four-wave mixing (SFWM) in photonic crystal optical fiber. We show that it is possible to engineer two-photon states with specific spectral correlation ("entanglement") properties suitable for quantum information processing applications. We focus on the case exhibiting no spectral correlations in the two-photon component of the state, which we call factorability, and which allows heralding of single-photon pure-state wave packets without the need for spectral post filtering. We show that spontaneous four wave mixing exhibits a remarkable flexibility, permitting a wider class of two-photon states, including ultra-broadband, highly-anticorrelated states.

We experimentally control the spectral structure of photon pairs created via spontaneous four-wave mixing in microstructured fibers. By fabricating fibers with designed dispersion, one can manipulate the photons' wavelengths, joint spectrum, and, thus, entanglement. As an example, we produce photon pairs with no spectral correlations, allowing direct heralding of single photons in pure-state wave packets without filtering. We achieve an experimental purity of (85.9+/-1.6)%, while theoretical analysis and preliminary tests suggest that 94.5% purity is possible with a much longer fiber.

We study both experimentally and theoretically the generation of photon pairs by spontaneous four-wave mixing (SFWM) in standard birefringent optical fibers. The ability to produce a range of two-photon spectral states, from highly correlated (entangled) to completely factorable, by means of cross-polarized birefringent phase matching, is explored. A simple model is developed to predict the spectral state of the photon pair which shows how this can be adjusted by choosing the appropriate pump bandwidth, fiber length and birefringence. Spontaneous Raman scattering is modeled to determine the tradeoff between SFWM and background Raman noise, and the predicted results are shown to agree with experimental data.

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into a single-mode fiber of 85%. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as is required for large-scale entanglement generation

We study theoretically the joint spectral properties of photon-pairs produced through spontaneous four-wave mixing (SFWM) with two spectrally distinct pump pulses in optical fibers. We show that, due to the group velocity difference between the pulses, the signature of the interaction can be significantly different from spontaneous parametric down-conversion or SFWM with a single pump pulse. Specifically, we study the case where temporal walk-off between the pumps enables a gradual turn-on and turn-off of the interaction. By utilizing this property, we develop a new approach towards tailoring the spectral correlations within the generated photon pairs, demonstrating the ability to produce factorable photon-pair states, and hence heralded single photons in a pure wave-packet. We show that the use of two pumps is advantageous over single-pump SFWM approaches towards this goal: the usage of the dual-pump configuration enables, in principle, the creation of completely factorable states without any spectral filtering, even in media for which single-pump SFWM tailoring techniques are unsatisfactory, such as standard polarization-maintaining fiber.

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We demonstrate a fiber-based source of polarization-entangled photon pairs at visible wavelengths suitable for integration with local quantum processing schemes. The photons are created through birefringent phase-matching in spontaneous four-wave mixing inside a Sagnac interferometer. We address entanglement degradation due to temporal distinguishability of the photons to enable the generation of a spectrally unfiltered polarization-entangled photon-pair state with 95.86 ± 0.10% fidelity to a maximally entangled Bell state, evaluated with a tomographic state reconstruction without applying any corrections or background subtractions. Owing to the large birefringence of the fiber, photons are created far detuned from the pump, where Raman contamination is negligible. This source's spatial mode and ability to produce spectrally uncorrelated photons make it suitable for implementing quantum information protocols over free-space and fiber-based networks.

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