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A secure communication network comprising:a sender node for randomly selecting a first coherent light pulse sequence encoded with a random bit sequence and a second coherent light pulse sequence containing a superposition of quantum states and transmitting the randomly selected sequences of the first and second sequences over an optical communication link anda receiver node connected to the optical communication link for receiving the randomly selected transmitted sequence,said receiver node determining whether or not the received second light pulse sequence is destroyed and producing a key from the received random bit sequence encoded in the first light pulse sequence if the second light pulse sequence is not destroyed by an unauthorised interception. The homodyne detector may include a local light oscillator, phase control circuitry for controlling the local light source so that the local light oscillator produces first and second local light oscillations having a phase difference of 90 degrees therebetween, and a beamsplitter for receiving light from the optical communication link and mixing the first coherent light pulse sequence with the first local light oscillations and mixing the second coherent light pulse sequence with the second local light oscillation.ฤก. At a receiver site, a homodyne detector receives the transmitted light pulse sequence and detects a random bit sequence and a superposition of quantum states. The outputs of the modulator and the transducer are multiplexed and transmitted over an optical communication link. At a sender site of a secure communication network, a first coherent light pulse sequence is phase modulated with a random bit sequence by a phase modulator, and a second coherent light pulse sequence synchronised to the first coherent light pulse sequence is transformed by an optical transducer to a superposition of coherent states.