Quantum cryptography has been in practical interests that can ensure an unconditional security for the communication system. The quantum key distribution system has been analyzed to show the factors that are important to a successful quantum cryptography. After introducing two of the quantum key distribution systems including two Mach-Zehnder interferometers with phase modulation and the Faraday mirrors to compensates automatically any birefringence effects and polarization dependent loss, a new system is proposed that the rotating photons are used to transmit the quantum bits.