  Quantum key distribution with continuous variables already uses advantageous
high-speed single-mode homodyne detection with low electronic noise at room
temperature. Together with continuous-variable information encoding to
nonclassical states, the distance for secure key transmission through lossy
channels can approach 300 km in current optical fibers. Such protocols tolerate
higher channel noise and also limited data processing efficiency compared to
coherent-state protocols. The secret key rate can be further increased by
increasing the system clock rates, and, further, by a suitable
frequency-mode-multiplexing of optical transmission channels. However, the
multiplexed modes couple together in the source or any other part of the
protocol. Therefore, multiplexed communication will experience crosstalk and
the gain can be minuscule. Advantageously, homodyne detectors allow solving
this crosstalk problem by proper data processing. It is a potential advantage
over protocols with single-photon detectors, which do not enable similar data
processing techniques. We demonstrate the positive outcome of this methodology
on the experimentally characterized frequency-multiplexed entangled source of
femtosecond optical pulses with natural crosstalk between eight entangled pairs
of modes. As the main result, we predict almost 15-fold higher secret key rate.
This experimental test and analysis of frequency-multiplexed entanglement
source opens the way for the field implementation of high-capacity quantum key
distribution with continuous variables.
