| This paper reviews some aspects of the development and application of quantum information and quantum cryptography, which is going to play an important role in the field of cryptography and the future worldwide quantum communication net.We make a Free-Space Quantum Cryptography system coding with single photon polarization states. Alice sends the single photon pulses with synchronizing light pulses, the receiver, Bob, detects the photons by our detection system with SPCM using the synchronizing electronic pulses to trigger. Then Alice and Bob get the sifted cryptographic keys by reconciling their keys through publicly discussion. After Error Correction and Privacy Amplification, the final secure quantum keys are acquired. The article also gives the analysis of existed eavesdropping strategies.Avalanche Photo-Diode has been widely used in the field of weak light detection, single photon detection and many kinds of photo-count detection. Nevertheless, the non-ideal characters of Single-Photon-Detection-Module give a challenge to Quantum Cryptography. Firstly, we measured the photo-statistic characteristic of the photon emission of silicon APD under the breakdown condition, studied experimentally its temporal character and fluorescence spectrum. Temporal decay process of photons emitting from avalanche photodiodes is demonstrated and it is found that there is a higher probability of photons emission after steep rise edge of the reverse voltage. To a Si avalanche photo-diode (HAMAMATSU, S3884), the photons emission have a broad spectral distribution from 500nm to 1100nm with two peaks at 750 nm and 994 nm, separately. We also find that the photons emission show super-Poissonian distribution. The current of the APD is the most important determinant of the photons emission. In order to reduce the background noise and dark counts in Bob's detection, we present a...
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