Observing the transition from quantum to classical energy correlations with photon pairs

The exact role of entanglement in various quantum metrology schemes is still subject to debates. This is why it would be interesting to be able to experimentally control the relative amount of quantum and classical correlations. Here, we demonstrate a method to tune energy correlations between two photons from a pair emitted by spontaneous parametric downconversion. Decoherence in the energy basis is achieved by applying random spectral phases on the photons. As a consequence, a diverging temporal second-order correlation function is observed and is explained by a mixture between an energy entangled pure state and a fully classically correlated mixed state. Such source of tunable energy entangled photon pairs could be used to demonstrate quantum advantages in future energy-time sensing schemes.