From Event: SPIE Optical Engineering + Applications, 2017
Let us consider the experimental setup where SPDC source generates photon pairs which are subsequently coupled to single-mode fibers (SMFs). We assume that there are three parties involved: 1) Alice, possessing the photon pair source, detection system and the fiber connecting them, 2) Bob, who monitors the output of the second, long-distance fiber and 3) Eve, who can perform the most general collective attacks in order to acquire information which Alice and Bob wish to transfer. Typically, in fiber-based communication the chromatic dispersion is considered to be an obstacle, limiting the maximal distance at which information carrier can be securely transmitted. This phenomenon forces the trusted parties to define longer detection windows to avoid losing signal photons and increases the amount of detection noise that is being registered.
We consider standard BB84 quantum key distribution protocol, based on the SPDC source located in between Alice and Bob. The parameters of standard realistic telecommunication fibers (SMF28e+) are take into account. The source emits photon which apart of being entangled in polarization degree of freedom are entangled in spectral domain. This is the key feature which allows one to reduce detection noise by manipulating the spectral correlation between the produced photons. In this way the maximal security distance can be increased by around 10%.
Piotr L. Kolenderski, Karolina Sedziak, Mikolaj Lasota, and Andrzej Gajewski, "On photonic spectral entanglement improving quantum communication (Conference Presentation)," Proc. SPIE 10409, Quantum Communications and Quantum Imaging XV, 104090F (Presented at SPIE Optical Engineering + Applications: August 06, 2017; Published: 19 September 2017); https://doi.org/10.1117/12.2274730.5581336803001.
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