  Digital signatures guarantee the authorship of electronic communications.
Currently used "classical" signature schemes rely on unproven computational
assumptions for security, while quantum signatures rely only on the laws of
quantum mechanics. Previous quantum signature schemes have used unambiguous
quantum measurements. Such measurements, however, sometimes give no result,
reducing the efficiency of the protocol. Here, we instead use heterodyne
detection, which always gives a result, although there is always some
uncertainty. We experimentally demonstrate feasibility in a real environment by
distributing signature states through a noisy 1.6km free-space channel. Our
results show that continuous-variable heterodyne detection improves the
signature rate for this type of scheme and therefore represents an interesting
direction in the search for practical quantum signature schemes.
