Preparation Of Entangled Optical Fields With High Entanglement Degree And Its Applications In Quantum Secret Sharing

Quantum information science is a cross discipline,which is developing rapidly in recent years.A variety of quantum resources have been used in information transmission and information processing to improve security,processing speed and calculating accuracy for breaking through limitations of classical physics.Quantum information science spans a wide variety of topics,including quantum communication,quantum computation,quantum metrology and so on.In quantum communication technology,various quantum protocols such as quantum teleportation,quantum dense coding,quantum key distribution,quantum secret sharing and quantum entanglement swapping have been theoretically and experimentally studied and are developing toward practical applications.These investigations exhibit the great potential of quantum information science and technology.Quantum entanglement is the most essential quantum resource.Continuous variable(cv)entangled state of light are the basic requirements for implementing various cv quantum information protocols.Meanwhile,the quality of quantum information directly depends on the quality of used entangled states of light.As well known,the fidelity of quantum teleportation,processing speed of information and security of communication etc.are finally determined by the entanglement degree of quantum states.Thus,the improvement of entanglement degree for entangled optical fields is significant for developing cv quantum technology.During my Ph.D study,direct method(improvement of the NOPA system structure)and indirect method(coherent feedback control)were studied theoretically and experimentally to improve entanglement degree.Then,using the obtained EPR optical entangled state,we generated the bound entangled state of light which can be applied in quantum information networks and experimentally achieve a(k,n)threshold QSS(Quantum Secret Sharing)protocol.The main contents of the thesis:1.We designed and built an efficient non-degenerate optical parametric amplifier(NOPA)with a wedged nonlinear type-II crystal and experimentally obtained the cv optical entangled state of 8.4 dB by improving the transmissivity of the output coupler for idler(signal)mode.The triple-resonance of pump,signal and idler modes in a NOPA is realized and the threshold power of the NOPA is decreased efficiently.The cv entanglement of 8.4dB is the hightest level obtained by a single optical cavity so far and the generation system of optical entangled states is compact,thus it is favorable for applications in real world.2.The phase-sensitive operations and the coherent feedback control are two main approaches to control quantum optical fields.Because there is no measurement element in coherent feedback control(CFC),i.e.no the access noise,the coherent feedback control is an excellent approach to implement the manipulation of noise-sensitive optical squeezed or entangled states.We theoretically designed and experimentally demonstrate a CFC-NOPA linearly optical system for generation and manipulation of optical entangled states.Not only the entanglement between the output optical modes was enhanced but also the threshold pump power of NOPA was significantly reduced.The experimental results are well agreement with the theoretical analysis.3.Secret sharing is a kind of basic technology in quantum communication,which is a key protol to realize information sharing among several players securely.In our experiment,the dealer modulates a classical secret string on sub-modes of EPR optical entangled states used to generate four-partite bound entanglement(BE)state and distributes these sub-modes to four players,which are placed at four space separated nodes in a quantum network.We experimentally demonstrate a new(k,n)threshold QSS protocol with which the dealer can freely manipulate the threshold number of k by simply controlling the strength of modulated signals.The creative works are as follows:1.Using a wedged KTP crystal instead of a traditional perpendicularly cut KTP crystal,we minimized the introcavity losses and obtained the highest entanglement of optical modes with a single optical cavity.2.We theoretically proposed a scheme of manipulating EPR entangled states of light by means of coherent feedback and experimentally proved its feasibility.3.A controlled(k,n)threshold quantum secret sharing(QSS)protocol based on applying cv four partite bound entanglement of optical field was experimentally realized firstly by our group.