Quantum cryptography for WDM networks: encryption with coherent states and key generation with fiber based entanglement

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New telecommunication techniques utilizing distinctive quantum properties, e.g., measurement uncertainties and entanglement, extend the capabilities of existing systems. Quantum cryptography, as an example, provides physical layer security enforced by fundamental physical laws, while modern cryptographic techniques rely on assumptions of intractability of certain mathematical problems with limited computational power. Rapid growth of the Internet leading to global telecommunications puts heavy demands on information security. A novel keyed direct data encryption technique based on the fundamental and irreducible quantum noise of laser light is shown to be compatible with the existing high-speed optical communications infrastructure. With this technique, line encryption for OC-12 (622 Mbps) SONET data over 250 km in a wavelength-divisionmultiplexing network is demonstrated with fully streaming data. Nonlocal correlation is employed in applications including cryptographic key generation whose practical realizations require telecom-band photon counting and entangled photon-pair generation. Existing telecom-band avalanche-photodiode based photoncounting techniques suffer from large detection noise at high operation rates. New techniques such as ultrashort gating and synchronous sampling at the onset of avalanches are introduced for suppressing the detection noise at high operation speeds. Photon counting at record speeds (25 MHz) that employing an avalanche photodiode is developed, demonstrated and deployed in the experimental studies. Telecom-band correlated photons can be directly created inside optical fibers through its X(3) nonlinearity. This technique brings practical advantages such as the easy compatibility with fiber-optic systems, excellent spatial modal purity, and potential high-speed operation. As a practical development of this technique, a novel Faraday-mirror based ultra-stable scheme for generating polarization entangled photon-pairs is proposed and demonstrated. Fiber-based polarization entangled photon-pairs are experimentally analyzed and characterized with various single-photon detectors. In an effort to satisfy the cryptographic objective of key generation, a keyed protocol of entanglement-based key generation is experimentally studied in wavelength-division-multiplexing lines wherein classical optical signals co-exist with entangled photons. Telecom-band correlated photon-pair generation at 9.95 GHz is also demonstrated for the first time. Such high operation rate demonstrates the feasibility of future high speed quantum communications

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  • 06/01/2018
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