  The channel loss incurred in long-distance transmission places a significant
burden on quantum key distribution (QKD) systems: they must defeat a passive
eavesdropper who detects all the light lost in the quantum channel and does so
without disturbing the light that reaches the intended destination. The current
QKD implementation with the highest long-distance secret-key rate meets this
challenge by transmitting no more than one photon per bit [Opt. Express 21,
24550-24565 (2013)]. As a result, it cannot achieve the Gbps secret-key rate
needed for one-time pad encryption of large data files unless an impractically
large amount of multiplexing is employed. We introduce floodlight QKD (FL-QKD),
which floods the quantum channel with a high number of photons per bit
distributed over a much greater number of optical modes. FL-QKD offers security
against the optimum frequency-domain collective attack by transmitting less
than one photon per mode and using photon-coincidence channel monitoring, and
it is completely immune to passive eavesdropping. More importantly, FL-QKD is
capable of a 2 Gbps secret-key rate over a 50 km fiber link, without any
multiplexing, using available equipment, i.e., no new technology need be
developed. FL-QKD achieves this extraordinary secret-key rate by virtue of its
unprecedented secret-key efficiency, in bits per channel use, which exceeds
those of state-of-the-art systems by two orders of magnitude.
