Witnessing non-classicality of a quantum oscillator state by coupling it   to a qubit
  We propose a new witness operation for the non-classical character of a harmonic oscillator state. The method does not require state reconstruction. For all harmonic oscillator states that are classical, a bound is established for the evolution of a qubit which is coupled to the oscillator. Any violation of the bound can be rigorously attributed to the non-classical character of the initial oscillator state. 
S. Agarwal and J. H. Eberly
quant-ph