  Quantum physics allows for unconditionally secure communication through
insecure communication channels. The achievable rates of quantum-secured
communication are fundamentally limited by the laws of quantum physics and in
particular by the properties of entanglement. For a lossy communication line,
this implies that the secret-key generation rate vanishes at least
exponentially with the communication distance. We show that this fundamental
limitation can be violated in a realistic scenario where the eavesdropper can
store quantum information for only a finite, yet arbitrarily long, time. We
consider communication through a lossy bononic channel (modeling linear loss in
optical fibers) and we show that it is in principle possible to achieve a
constant rate of key generation of one bit per optical mode over arbitrarily
long communication distances.
