  We propose a two-way secure-communication protocol in which Alice uses an
amplified spontaneous emission source while Bob employs binary phase-shift
keying and an optical amplifier. Against an eavesdropper who captures all the
light lost in fibers linking Alice and Bob, this protocol is capable of 3.5
Gbps quantum-secured direct communication at 50 km range. If Alice augments her
terminal with a spontaneous parametric downconverter and both Alice and Bob add
channel monitors, they can realize 2 Gbps quantum key distribution at that
range against an eavesdropper who injects her own light into Bob's terminal.
Compared with prevailing quantum key distribution methods, this protocol has
the potential to significantly increase secure key rates at long distances by
employing many ultrabroadband photons per key bit to mitigate channel loss.
