1. A receiver in a quantum key distribution (QKD) system, the receiver comprising: a quantum optical unit configured to receive quantum signals from a transmitter of the QKD system, modulate the quantum signals based on a basis sequence, and output detection information by detecting the modulated quantum signals; and a signal processor configured to generate a raw key by using the detection information and the basis sequence based on a double buffering scheme, wherein the signal processor comprises: a first memory and a second memory configured to store the detection information and the basis sequence; a raw key extractor configured to extract the raw key; and at least one raw key memory configured to store the raw key, wherein the raw key extractor is configured to control a storage of the detection information and the basis sequence in one of the first and second memories, and to store the detection information and the basis sequence in the other of the first and second memories when a preset period has ended, wherein the preset period is shorter than a preset raw key generation time which is determined based on an operating environment of the QKD system.