Floodlight quantum key distribution: A practical route to gigabit-per-second secret-key rates

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 [M. Lucamarini et al., Opt. Express 21, 24550 (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.

Figure 1

Quantum channel setup for FL-QKD under frequency-domain collective attack. ASE: amplified spontaneous emission source. SPDC: spontaneous parametric downconverter. BPSK: binary phase-shift keying. LO: local oscillator.

Figure 2

Realization of Eve's optimum frequency-domain collective attack. SPDC: spontaneous parametric downconverter. Eve's SPDC signal beam (shown) is coupled to Bob through a beam splitter, while her SPDC idler beam (not shown) is retained for use in her receiver.

Figure 3

(a) Lower bound on Alice and Bob's secret-key rate and Alice's optimum signal brightness when Eve mounts her optimum frequency-domain collective attack with injection fraction $f_E=0.01$. (b) Lower bound on Alice and Bob's secret-key rate vs $f_E$ for a 50-km fiber link with all other parameters as in (a).

Figure 4

(a) Alice's transmitted photons per bit (ppb) and Bob's received ppb when Eve mounts her optimum frequency-domain collective attack with injection fraction $f_E=0.01$. (b) Upper bounds on Eve's optimum frequency-domain collective attack, passive attack, and active attack Holevo informations per mode—along with her entanglement-assisted capacity—as a function of Alice's signal brightness, $N_S$, for a 50-km one-way path length assuming $f_E=0.01$.

Figure 5

Schematic of Eve's $K$-mode collective attack used to upper bound her Holevo information rate. BPSK: binary phase-shift keying. $G_B$ amplifier gain. The dashed wavy line represents an entanglement that purifies the state of the ${\widehat{a}}_{S_m}$ mode.