Observation of Bloch oscillations with a threshold

We demonstrate experimentally Bloch oscillations, which occur above a certain threshold value of the effective potential gradient in lattices with specially modulated coupling between the neighboring sites. We formulate the general conditions for this phenomenon, arising due to the competition between the tilting and broadening of the transmission band, and explain why no threshold was present in any previous observations. Our experiments are performed in inhomogeneous photonic lattices, which represent the process of quantum two-mode squeezing in Fock space, underpinning a fundamental quantum-classical correspondence.