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A "circular type" interferometric system for quantum key distribution is proposed. The system, which adopts a Sagnac loop, is highly stable and simple. The stability derives from its self-alignment and compensation for birefringence and the system's simplicity allows it to achieve an extremely fast bit rate. Moreover, it is easily applicable to multiparty setup. Key creation with 0.1 photon per pulse at a rate of 1.2 kHz with a 5.4% quantum bit-error rate over a 200-m fiber was realized.