  Almost all novel observable phenomena in quantum optics are related to the
quantum coherence. The coherence here is determined by the relative phase
inside a state. Unfortunately, so far all the relevant experimental results in
quantum optics are insensitive to the phase information of the coherent state.
Lack of phase information may cause serious consequences in many problems in
quantum optics. For example, an ensemble of two mode squeezed states is a
classical ensemble if the phase in each state is totally random; but it is a
non-classical ensemble if the phase in each state is fixed. As a timly
application, verification of this type of phase information in an ensemble of
two mode squeezed states from the conventional laser is crucial to the validity
of the continuous variavable quantum teleportation(CVQT) experiment. Here we
give a simple scheme to distinguish two different ensemble of states: the
Rudolph-Sanders ensemble, by which each squeezed states emitted has a uniform
distribution from $0-2\pi$ on the phase value; and the van Enk-Fuchs ensemble,
which emmits identical states with a fixed(but unknown) phase for every state.
We believe our proposal can help to give a clear picture on whether the
existing two mode squeezed states so far are indeed non-classical states which
can be used as the entanglement resource.

Quantum Physics