  We seek the first indications that a nanoelectromechanical system (NEMS) is
entering the quantum domain as its mass and temperature are decreased. We find
them by studying the transition from classical to quantum behavior of a driven
nonlinear Duffing resonator. Numerical solutions of the equations of motion,
operating in the bistable regime of the resonator, demonstrate that the quantum
Wigner function gradually deviates from the corresponding classical phase-space
probability density. These clear differences that develop due to nonlinearity
can serve as experimental evidence, in the near future, that NEMS resonators
are entering the quantum domain.
