Quantum key distribution (QKD) enables two remote parties to grow a shared
key which they can use for unconditionally secure communication [1]. The
applicable distance of a QKD protocol depends on the loss and the excess noise
of the connecting quantum channel [2-10]. Several QKD schemes based on coherent
states and continuous variable (CV) measurements are resilient to high loss in
the channel, but strongly affected by small amounts of channel excess noise
[2-6]. Here we propose and experimentally address a CV QKD protocol which uses
fragile squeezed states combined with a large coherent modulation to greatly
enhance the robustness to channel noise. As a proof of principle we
experimentally demonstrate that the resulting QKD protocol can tolerate more
noise than the benchmark set by the ideal CV coherent state protocol. Our
scheme represents a very promising avenue for extending the distance for which
secure communication is possible.Comment: 8 pages, 5 figure
