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Photons are ideal carriers for quantum information as they can have a longcoherence time and can be transmitted over long distances. These properties area consequence of their weak interactions within a nearly linear medium. Tocreate and manipulate nonclassical states of light, however, one requires astrong, nonlinear interaction at the single photon level. One approach togenerate suitable interactions is to couple photons to atoms, as in the strongcoupling regime of cavity QED systems. In these systems, however, one onlyindirectly controls the quantum state of the light by manipulating the atoms. Adirect photon-photon interaction occurs in so-called Kerr media, whichtypically induce only weak nonlinearity at the cost of significant loss. Sofar, it has not been possible to reach the single-photon Kerr regime, where theinteraction strength between individual photons exceeds the loss rate. Here,using a 3D circuit QED architecture, we engineer an artificial Kerr mediumwhich enters this regime and allows the observation of new quantum effects. Werealize a Gedankenexperiment proposed by Yurke and Stoler, in which thecollapse and revival of a coherent state can be observed. This time evolutionis a consequence of the quantization of the light field in the cavity and thenonlinear interaction between individual photons. During this evolutionnon-classical superpositions of coherent states, i.e. multi-componentSchroedinger cat states, are formed. We visualize this evolution by measuringthe Husimi Q-function and confirm the non-classical properties of thesetransient states by Wigner tomography. The single-photon Kerr effect could beemployed in QND measurement of photons, single photon generation, autonomousquantum feedback schemes and quantum logic operations.

Observation of quantum state collapse and revival due to the single-photon Kerr effect