Squeezed quantum fluctuations and macroscopic quantum coherence

Preservation of a macroscopic superposition quantum state is achieved in the presence of dissipation when the quantum fluctuations of the heat bath are squeezed. An example is given for a single mode of the electromagnetic field interacting with a squeezed bath. We show that a squeezed bath is more efficient than a thermal or vacuum bath for optical quadrature-phase measurements, and may also be used to prepare states with low quantum noise in one quadrature phase.