The Classical-to-Quantum Transition with Broadband Four-Wave Mixing
  A key question of quantum optics is how nonclassical bi-photon correlations at low power evolve into classical coherence at high-power. Direct observation of the crossover from quantum to classical behavior is desirable, but difficult due to the lack of adequate experimental techniques that cover the ultra-wide dynamic range in photon flux from the single photon regime to the classical level. We investigate bi-photon correlations within the spectrum of light generated by broadband four-wave mixing (FWM) over a \emph{large dynamic range of $\sim80dB$ in photon flux} across the classical-to-quantum transition using a two-photon interference effect that distinguishes between classical and quantum behavior. We explore the quantum-classical nature of the light by observing the interference contrast dependence on internal loss and demonstrate quantum collapse and revival of the interference when the FWM gain in the fiber becomes imaginary. 
Rafi Z. Vered, Yaakov Shaked, Yelena Ben-Or, Michael Rosenbluh and Avi   Pe'er
physics.optics quant-ph