  We present a theoretical and experimental study of a controllable
decoherence-assisted quantum key distribution scheme. Our method is based on
the possibility of introducing controllable decoherence to polarization qubits
using the spatial degree of freedom of light. We show that our method reduces
the amount of information that an eavesdropper can obtain in the BB84 protocol
under the entangling probe attack. We demonstrate experimentally that Alice and
Bob can agree on a scheme to that gives low values of the quantum bit error
rate, despite the presence of a large amount of decoherence in the transmission
channel of the BB84 protocol.
