Experimental demonstration of high two-photon time-energy entanglement

We report on the experimental demonstration of high energy-time entanglement in two-photon states created in the process of spontaneous parametric down-conversion. We show that the classical variance product, which we violate by three orders of magnitude, actually represents a lower bound estimate of the number of information eigenmodes K. Explicit measurements estimate K to be greater than 100, with theoretical estimates predicting a value of as high as 1×10⁶. These results provide incentive for the practical feasibility of large bandwidth quantum information processing, particularly in cryptography over large distances.