The present specification provides a method and a device, the method, performed by a device, for transmitting a first pulse train in a quantum communication system being characterized by: receiving a random access (RA) preamble from another device; transmitting an RA response (RAR) to the other device as a response to the RA preamble; performing a radio resource control (RRC) connection process with the other device; receiving data from the other device; and decoding the data on the basis of key information, wherein a first pulse train is generated in a laser diode of the device, and the key information is determined on the basis that the first pulse train passes through both a first path and a second path and is then transmitted from the device to the other device via a quantum channel.