Classical digital signatures are commonly used in e-mail, electronic financial transactions and other forms of electronic
communications to ensure that messages have not been tampered with in transit, and that messages are transferrable. The
security of commonly used classical digital signature schemes relies on the computational difficulty of inverting certain
mathematical functions. However, at present, there are no such one-way functions which have been proven to be hard to
invert. With enough computational resources certain implementations of classical public key cryptosystems can be, and
have been, broken with current technology. It is nevertheless possible to construct information-theoretically secure
signature schemes, including quantum digital signature schemes. Quantum signature schemes can be made information theoretically
secure based on the laws of quantum mechanics, while classical comparable protocols require additional
resources such as secret communication and a trusted authority.
Early demonstrations of quantum digital signatures required quantum memory, rendering them impractical at present.
Our present implementation is based on a protocol that does not require quantum memory. It also uses the new technique
of unambiguous quantum state elimination, Here we report experimental results for a test-bed system, recorded with a
variety of different operating parameters, along with a discussion of aspects of the system security.