  We present a quantum key distribution (QKD) protocol based on long lived
coherent states prepared on superconducting rings with a mesoscopic Josephson
junction (dc-SQUIDs). This enables storage of the prepared states for long
durations before actually performing the key distribution. Our on-demand QKD
protocol is closely related to the coherent state based continuous variable
quantum key distribution protocol. A detailed analysis of preparation,
evolution and different measurement schemes that are required to be implemented
on dc-SQUIDs to carry out the QKD is provided. We present two variants of the
protocol, one requiring time stamping of states and offering a higher key rate
and the other without time stamping and a lower key rate. This is a step
towards having non-photon based QKD protocols which will be eventually
desirable as photon states cannot be stored for long and therefore the key
distribution has to be implemented immediately after photon exchange has
occurred. Our protocol offers an innovative scheme to perform QKD and can be
realized using current experimental techniques.
