  The phase conjugation of an unknown Gaussian state cannot be realized
perfectly by any physical process. A semi-classical argument is used to derive
a tight lower bound on the noise that must be introduced by an approximate
phase conjugation operation. A universal transformation achieving the optimal
imperfect phase conjugation is then presented, which is the continuous
counterpart of the universal-NOT transformation for quantum bits. As a
consequence, it is also shown that more information can be encoded into a pair
of conjugate Gaussian states than using twice the same state.
