  Secure quantum key distribution with multimode Gaussian entangled states and
multimode homodyne detectors is proposed. In general the multimode character of
both the sources of entanglement and the homodyne detectors can cause a
security break even for a perfect channel when trusted parties are unaware of
the detection structure. Taking into account the multimode structure and
potential leakage of information from a homodyne detector reduces the loss of
security to some extent. We suggest the symmetrization of the multimode sources
of entanglement as an efficient method allowing us to fully recover the
security irrespectively to multimode structure of the homodyne detectors.
Further, we demonstrate that by increasing the number of the fluctuating but
similar source modes the multimode protocol stabilizes the security of the
quantum key distribution. The result opens the pathway towards quantum key
distribution with multimode sources and detectors.
