Dissertation > Mathematical sciences and chemical > Physics > Theoretical Physics > Quantum theory

Some Theoreticalresearch on Continuous Variable Quantum Key Distribution and Surface Plasmon Polaritons

Author ZuoZhiXin
Tutor YuLi
School Beijing University of Posts and Telecommunications
Course Physical Electronics
Keywords continuous variable quantum key distribution lowdensity parity check code surface plasmon polaritons Femto-secondpulse Bragg grating
CLC O413
Type PhD thesis
Year 2012
Downloads 114
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The security of most classical key distribution is based on the assumption of computational complexity. Differently, according to quantum information theory, quantum key distribution (QKD) can obtain theoretically unconditional security. Early QKD systems use single photon as information carrier. However, it is difficult to prepare reliable single-photon sources and high-detect-efficiency photon detectors at present. In contrast, QKD based on continuous-variable (CVQKD) eliminates the requirement for single photon technology. In CVQKD scheme, stable reliable light source can be obtained easily, and the detection system is efficient. Besides, CVQKD is compatible with classical optical communication. Nowadays, CVQKD has attracted more attention in quantum information. Because principle and technology of traditional optic are limited by diffraction limit, it’s difficult to realize optical information transmission and processing at nanometer scale with traditional optic.Surface Plasmon allows light to pass through small regions, so it is possible to integrate plasmonics, electronic, and conventional dielectric photonic devices on the same chip at nanometer scale without interference between them. These unique features of surface plasmon could lead to SP-based photonic devices that are much more compact than those currently achievable with optical refraction technologies. Nonlinear optical devices based on subwavelength metallic structures have been proposed to actively control plasmonic signals by nonlinear material. Compared with the usual all-optical devices based on various types of optical nonlinearities, these new nonlinear optical devices have the advantage of smaller size and stronger nonlinear effects because of electromagnetic field confinement and enhancement in metallic structures.This paper consists of continuous variable quantum key distribution and surface plasmon polaritons.1. An efficient and practical post-processing technique based on reverse reconciliation for continuous variable quantum key distribution is proposed and simulated with low-density parity check (LDPC) codes. MultiLevel Coding/MultiStage Decoding, which fully utilizes optimization technique such as vector quantization and iterative decoding and the optimal channel coding most close to the Shannon limit, was used to realize efficient reverse reconciliation algorithm. Simulation results showed that the proposed method can improve the secure key distribution rate to2.2kb/s and the coding efficiency to0.89over20km in single-mode optical fiber. Moreover, there still is room for much improvement.2. Femto-second Gauss pulse evolution along symmetric metal/nonlinear dielectric/metal surface plasmon waveguide with length of560nm is studied using finite-difference time-domain (FDTD) method. Linear and nonlinear dielectrics are chosen to be the core layers whose thicknesses are set to be20nm,50nm100nm and200nm respectively. The results show that for the linear case, the strong coupling of SPP mode can lead to pulse tailing phenomenon and spectrum compression due to waveguide resonance. For the nonlinear case, the output pulse is broadened, and the fluctuation is more complex than the input pulse and can induce the spectrum splitting as well. The smaller the thickness of core layer, the more distinct the pulse distortion is, which may be due to the combined effects of dispersion, waveguide resonance and self-phase modulation (SPM).3. Transmittance of Bragg grating based on metal/insulator/metal surface plasmon polaritons waveguide is studied with finite-difference time-domain method.Band gap appears in the transmission spectrum.By changing structural parameters, the center and width can be adjusted.If a defect is introduced, a sharp transmission peak will appear.Highlights of the dissertation are as following:1.LDPC code whose performance could be extremely close to the Shannon limit is applied in the reconciliation for continuous variable quantum key distribution.Using Mackay’s random-constructing method, we find relative ideal LDPC codes, and realize secure key distribution rate of2.2kb/s over20km in single-mode optical fiber.2.By calculate the output pulse of metal/nonlinear dielectric/metal surface plasmon waveguide, the mechanism of dispersion, waveguide resonance self-phase modulation is analyzed. Here, the dispersion does not play a key role in the pulse distortion because the propagation distance is too short, and the waveguide resonance should be responsible primarily for the phenomenon. SPM is responsible for the broadening of output pulse spectrum.3.Using composite Lorentz model of metal, we calculate the transmittance of Bragg grating based on metal/insulator/metal surface plasmon polaritons waveguide.Band gap appears in the transmission spectrum.Introducing a defect and changing the central insulator, the law of the defect transmission peak can be obtained.

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