Long-distance Continuous Variable Quantum Key Distribution With Einstein-Podolsky-Rosen(EPR)-entangled State

The global information networks system has been established with the rapidly development of science and technology which make the information easily accessible and vulnerable.The quantum cryptography is based on quantum mechanics and classical cryptography which using quantum states to carry quantum information and one time pad to realize the unconditional security of the two legal parties.The first quantum cryptography protocol,quantum key distribution(QKD)is proposed by Charles H.Bennett and Gilles Brassard in 1984(BB84 protocol),then quantum key distribution(QKD)protocol becomes one of the highlights research in the world.The past three decades have witnessed rapid progress of QKD both in theory and experiment.At present,discrete variable quantum key distribution have bottlenecked,the secure communication distance up to 404 km with low-loss fiber.The continuous variable quantum key distribution(CVQKD)can supplement the weakness of the DVQKD.The advantage of the CVQKD is effectively detection,easily accessible device,and compatible with the fiber communication and so on.If combined the advantage of CVQKD and DVQKD protocol,the more rapid and longer communication distance quantm networks will be established.Quantum entanglement is the essential resource for CVQKD.Thanks to a well-established high quality quantum entanglement source,quantum key distribution have been rapidly developing in recent years and construct the more rapid and longer quantum communication network.In this thesis,the core objective of the job is to prepare a high quality two-color continuous variable entanglement states and long-distance continuous variable quantum key distribution with EPR state.Compared with discrete variables in terms of quantum distribution,continuous variable quantum key distribution(CVQKD)has a higher efficiency and lower cost measurement equipment,higher secure key rate and more compatible with the existing classical optical fiber communication network.While there are some deficiencies,such as local beam participation,more complex data processing and shorter secure communication distance.If we can combine the advantage between CVQKD and DVQKD,one can maximize the distance and secure key rate.Quantum entangled state is one of the most important resources to stimulate the development of the CVQKD.For example,CVQKD with EPR state is more tolerant to channel excess noise,channel loss and limited post-processing efficiency than the coherent state-based protocols.Innovation in this thesis are:1.By injecting the seed beam through the output coupler instead of the high-reflectivity cavity mirror,a high degree of two-mode quantum entanglement can still be yielded regardless of the relative phase at a large range of idler-to-signal ratio.The results we presented can provide useful guidance for the preparation of two-color continuous variable quantum entanglement from a NOPO with an injected seed.2.By analyze the relevant factors,such as the total efficiency of the optical system and the fluctuation of the phase-locking,we improved and optimized the experimental system and successfully increased the two-color CV entanglement from 3.3dB to 6dB which is the best result using this design to our knowledge.3.The system utilize the modulated signal fields injected into non-degenerate optical parametric amplifier(NOPA)generating a pair of bright EPR entangled beams,which is a promising candidate for suppressing the degradation of quantum entanglement which is sensitively to environment influences and simplifying the secure communication system.4.Investigating the effect of depolarized GAWBS in single mode fibers on the excess noise in fiber-based CVQKD and calculating the lower bound for the extinction ratio of the pulsed light in time-multiplexing CVQKD protocol.The results will be useful to the design of a high speed(GHz)CVQKD system where the depolarized GAWBS noise will be nontrivial and relevant to the excess noise.5.Long-distance continuous variable quantum key distribution with entangled state is realized with raw key for 50 km secure communication distance.Analyzing the experimental parameters,the excess noise from phase-locked and GAWBS and results.