  Quantum entanglement has been actively sought for in optomechanical and
electromechanical systems. The simplest such system is a mechanical oscillator
interacting with a coherent beam, while the oscillator also suffers from
thermal decoherence. For this system, we show that quantum entanglement is
always present between the oscillator and continuous outgoing fields -- even
when the environmental temperature is high and the oscillator is highly
classical. Such universal entanglement is also shown to be able to survive more
than one oscillation cycle if characteristic frequency of the optomechanical
interaction is larger than that of the thermal noise. Furthermore, we derive
the effective optical mode that is maximally entangled with the oscillator,
which will be useful for future quantum computing and encoding information into
mechanical degrees of freedom.
