  We consider the security of continuous-variable quantum key distribution
using thermal (or noisy) Gaussian resource states. Specifically, we analyze
this against collective Gaussian attacks using direct and reverse
reconciliation where both protocols use either homodyne or heterodyne
detection. We show that in the case of direct reconciliation with heterodyne
detection, an improved robustness to channel noise is achieved when large
amounts of preparation noise is added, as compared to the case when no
preparation noise is added. We also consider the theoretical limit of infinite
preparation noise and show a secure key can still be achieved in this limit
provided the channel noise is less than the preparation noise. Finally, we
consider the security of quantum key distribution at various electromagnetic
wavelengths and derive an upper bound related to an entanglement-breaking
eavesdropping attack and discuss the feasibility of microwave quantum key
distribution.
