  Decoherence remains one of the most serious challenges to the implementation
of quantum technology. It appears as a result of the transformation over time
of a quantum superposition state into a classical mixture due to the quantum
system interacting with the environment. Since quantum systems are never
completely isolated from their environment, decoherence therefore cannot be
avoided in realistic situations. Decoherence has been extensively studied,
mostly theoretically, because it has many important implications in quantum
technology, such as in the fields of quantum information processing, quantum
communication and quantum computation. Here we report a novel experimental
scheme on the study of decoherence of a two-mode squeezed vacuum state via its
second harmonic generation signal. Our scheme can directly extract the
decoherence of the phase-sensitive quantum correlation $\langle
\hat{a}\hat{b}\rangle$ between two entangled modes $a$ and $b$. Such a
correlation is the most important characteristic of a two-mode squeezed state.
More importantly, this is an experimental study on the decoherence effect of a
squeezed vacuum state, which has been rarely investigated.
