We present a fiber based source of entangled photon-pairs in the 1550 nm telecom band that can be integrated into the existing fiber network and is well suited for quantum information processing. With this source we have demonstrated the generation, storage, and long-distance distribution of polarization entanglement in standard optical fiber. We have also investigated the origin of the large number of accidental coincidences in the experiments, which has been proved to be Raman scattering, and discussed how to suppress the Raman scattering to improve the quality of the fiber source.
We present the design and construction of a high-speed telecom-band (1.5 μm) single-photon counting system based on an InGaAs/InP avalanche photodiode (APD) operating in the gated Geiger mode. The detector can be gated at high speeds (we examine its performance up to 25 MHz) to maximize the counting rate in long-distance, telecom-band, fiber-optic quantum communication applications. Narrow gate pulses (250 ps full width at half maximum) are used to reduce the dark-count and the after-pulse probability. In order to count the avalanche events, we employ a high-speed comparator to sample the unfiltered and unamplified avalanche photocurrent. The APD and all the associated electronics are integrated onto a printed circuit board with a computer interface. In addition, we cool the APD to -27°C to reduce the dark-count probability.
We review on-going progress in the development of fiber-based
telecom-band entanglement sources. Two different schemes (a
Sagnac-loop scheme and a counter-propagating scheme) for
generating polarization entanglement are reviewed and the pros and
cons of each are summarized. A new scheme, called the double-loop
scheme is proposed, which is theoretically shown to be capable of
combining the benefits and avoiding the pitfalls of each previous
We present a quantum theory for generating two-photon states by means of four-wave mixing in optical fiber. We start with an interaction Hamiltonian that can correctly describe all nonlinear interactions among the four waves present in the fiber, namely, the frequency non-degenerate pumps, signal, and idler, including the terms responsible for self-phase modulation (SPM), cross-phase modulation (XPM), and four-photon scattering (FPS). The exact form of this Hamiltonian is obtained through comparison between the classical and quantum equations of motion. The two-photon state is then calculated by means of first-order perturbation theory. It turns out that only the FPM and the pump SPM terms contribute to the formation of the two-photon state. The entangled nature of this state is verified in a coincidence counting experiment. The results of the theoretical calculation agree well with experimental data.