  We study the security of two-way quantum cryptography at different
wavelengths of the electromagnetic spectrum, from the optical range down to the
microwave range. In particular, we consider a two-way quantum communication
protocol where Gaussian-modulated thermal states are subject to random Gaussian
displacements and finally homodyned. We show how its security threshold (in
reverse reconciliation) is extremely robust with respect to the preparation
noise and able to outperform the security thresholds of one-way protocols at
any wavelength. As a result, improved security distances are now accessible for
implementing quantum key distribution at the very challenging regime of
infrared frequencies.
