Research On Some Key Problems Of The Experimental System In Quantum Communication

As a result of integrating quantum mechanics and information theory, quantum communication is also a hot research topic for the information science. It can be categorized into two branches by the carrier of information, the discrete quantum communication and continuous variable quantum communication. Among discrete quantum communication, based on fiber network, Quantum Key Distribution (QKD) has firstly been applied for the actual goal. Continuous variable quantum communication is becoming a focus in the information field, and many important outcomes have been obtained.In the second chapter, some fundamental quantum mechanics concepts and theories involved with quantum communications are introduced, such as qubit, state superposition principle, uncertainty principle, no-cloning theorem and entanglement.Some correlative problems existing in QKD system for the single-photon transmission in optical fiber have been researched in the third chapter. First, operation procedures of three basic protocols including BB84, B92 and Ekert91, based on the polarization and phase coding, have been elaborated. Next, we have expatiated on the 'Plug and Play' QKD scheme for the single-photon communication in optical fiber. Then properties of some key elements and work scheduling are thoroughly discussed, thus we have set down parameters of the drive circuit of an experimental system. Security and code rate must be considered all the while for an actual QKD system, and all of Eve's possible attack strategies have been demonstrated. Last, we have pointed out that the security relies on not only those quantum theorems but also effective technical means.Some problems involving with the single-photon detector have been treated in the forth chapter. Because the present single-photon detector must work on the 'Geiger' mode, which dead time should be set up to several micro-seconds to reduce error counts, the code rate is too low to suit practical applications for QKD system. A new idea is put up to design a quick single-photon detector. This quick detector is composed of a multi-port optic fiber splitter and many Avalanche Photo-diodes (APD). All of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal. The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector. The single-photon prompt detection can come true for high repetition-rate pulses. The optoelectronic integration for the optic fiber splitter, logic discriminating and deciding unit, optical switch, even APDs, can be adopted for the reliable detector module in the future.The quick detector can count on photons of a faint laser pulse, thus a new method has been proposed on physical detection to Eve's attacks. If Eve takes the 'PNS' attack strategy, the mean number of photons will decrease, while he employs 'intercept-resend' strategy, the poisson distribution of the number of photons must be disturbed.Some problems on developing an optical soltion source are researched in the fifth chapter for continuous variable quantum communicatios. The femto-second laser to use Cr⁴⁺:YAG crystal as active matter can provide soliton pulses with strong coherent characteristic for optic-fiber communication band.Cr⁴⁺:YAG crystal is a Kerr media. When Self-phase modulation (SPM) in time domain to an optical pulse balances with negative group velocity dispersion (GVD) effect, soliton mode-locking will occur within resonator, and femto-second order of magnitude soliton pulses can be obtained. Semiconductor saturable absorber mirror (SESAM) only stirs up this process and stabilize the soliton, but its longer relexation time will take no action to the width of a soliton pulse. We have designed a SESAM operating in 1520nm band. Mechanism for inducing the positive dispersion rests with the gain of a laser crystal. The formula of dispersion compensation is deduced for a pair of prisms by calculating fuse silca's dispersion according to Sellmeier expressions.Optium design of the optics structure for the femto-second laser aimed at Cr⁴⁺:YAG crystal is investigated by theoretical analysis and numerical calculation. So as to reduce the complexity, a thin focus lens is plugged into the resonator to replace the fold-mirror. The pair of prisms can affect the stability and astigmatism besides the dispersion compensation, so the optics transmission matrix is achieved to eliminate them. The stability diagrams are obtained for continue-wave-operation and mode-lock-operation with diverse parameters by computer programming, and the way for, selecting the work point is described. The un-match factor (F) of the astigmatism compensation is defined, and the area distribution diagram is painted according to it. The excellent astigmatism compensation can be possessed for running laser on the stable area with F<0.05. The match of pump and oscillating laser beams within the cavity will come true by changing the position of the focus lens.