  We propose a scheme for unambiguous state comparison (USC) of two unknown
squeezed vacuum states of an electromagnetic field. Our setup is based on
linear optical elements and photon-number detectors, and achieves optimal USC
in an ideal case of unit quantum efficiency. In realistic conditions, i.e., for
non-unit quantum efficiency of photodetectors, we evaluate the probability of
getting an ambiguous result as well as the reliability of the scheme, thus
showing its robustness in comparison to previous proposals.
