  Advances in highly sensitive detection techniques for classical coherent
communication systems have reduced the received signal power requirements to a
few photons per bit. At this level one can take advantage of the quantum noise
to create secure communication, using continuous variable quantum key
distribution (CV-QKD). In this work therefore we embed CV-QKD signals within
classical signals and transmit classical data and secure keys simultaneously
over 25km of optical fibre. This is achieved by using a novel coherent
displacement state generator, which has the potential for being used in a wide
range of quantum optical experiments. This approach removes the need for
separate channels for quantum communication systems and allows reduced system
bandwidth for a given communications specification. This demonstration
therefore demonstrates a way of implementing direct quantum physical layer
security within a conventional classical communications system, offering a
major advance in term of practical and low cost implementation.
