A method of data transmission, and more particularly a secure method of data transmission. The method comprises generating and sending a classical data stream and quantum data stream from a source to a destination. The step of generating and sending a classical data stream from the source to the destination comprises encrypting the data stream with the repeated use of a once-seen pad. Observing the quantum data stream received at the destination, may indicate eavesdropping and if so, the classical data stream is modified and sent from the source to the destination, including stopping the repeated use of the once-seen pad. The same once-seen pad may be used a number of times provided no third party has seen data encrypted by the once-seen pad.