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 discuss recent advances in phase-sensitive amplification technology and review its application to the regeneration of multi-level phase-encoded signals.

We experimentally demonstrate frequency translation of a nonclassical optical field via the Bragg scattering four-wave mixing process in a photonic crystal fiber (PCF). The high nonlinearity and the ability to control dispersion in PCF enable efficient translation between photon channels within the visible to-near-infrared spectral range, useful in quantum networks. Heralded single photons at 683 nm were translated to 659 nm with an efficiency of 28.6 ± 2.2 percent. Second-order correlation measurements on the 683-nm and 659-nm fields yielded g (2) 683 (0) = 0.21±0.02 and g (2) 659 (0) = 0.19±0.05 respectively, showing the nonclassical nature of both fields.PACS numbers: 42.50.Ex, 42.65.KyAs more advanced quantum-information applications and systems are created, it is likely that the sharing of quantum information between remote devices will be desirable, and a quantum network will be needed [1]. A good candidate to transfer such information is the single photon, which is relatively robust against loss or decoherence, allowing transfer of entanglement between remote locations. Such photons can travel long distances through optical fibers, which function optimally in particular wavelength ("telecom") ranges. Quantum frequency translation (QFT), in which a photon at one central frequency is annihilated and another photon at a different central frequency is created (see Figure 1b)), serves three important purposes in quantum networks-It allows quantum devices (memories and processors) that operate at different optical frequencies to communicate via a quantum channel; It allows low-loss, long-distance exchange over fiber between two quantum devices that operate at frequencies other than the telecom ones; It allows converting photons to frequencies where the optimal detectors operate. To be useful in quantum networks, QFT must 1) allow flexible choices of photon frequencies within the visible and near IR, 2) preserve the nature of the original state other than its central frequency, including any entanglement with other systems, and 3) must not introduce additional unwanted "noise" photons. QFT in optical fiber is predicted to satisfy all of these requirements [2]. We present the first demonstration of QFT in optical fiber, and show that it preserves the nonclassical nature of the single-photon field being translated.Frequency conversion in second-order χ (2) nonlinear optical media such as crystals via three-wave mixing has been studied in much depth, especially for coherent-state fields [3]. Sum frequency generation allows a weak field, when combined with a strong pump field, to be translated to a higher frequency (upconversion). Difference frequency generation allows for the creation of the con- FIG. 1. (color online) a) The modulation interaction process. Two Pump 1 photons are annihilated while two sideband photons (signal and idler), equally spaced in frequency from the pump by ∆ωMI, are created. Up(down) arrows indicate creation(destruction). b) The Bragg-scattering quantum frequency translation process. Photo...

Optical frequency conversion by four-wave mixing (Bragg scattering) in a fiber is considered. If the frequencies and polarizations of the waves are chosen judiciously, Bragg scattering enables the translation of individual and entangled states, without the noise pollution associated with parametric amplification (modulation instability or phase conjugation), and with reduced noise pollution associated with stimulated Raman scattering.

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