Controllable generation of highly nonclassical states from nearly pure squeezed vacua

We present controllable generation of various kinds of highly nonclassical states of light, including the single photon state and superposition states of mesoscopically distinct components. The high nonclassicality of the generated states is measured by the negativity of the Wigner function, which is largest ever observed to our knowledge. Our scheme is based on photon subtraction from a nearly pure squeezed vacuum, generated from an optical parametric oscillator with a periodically-poled KTiOPO$_4$ crystal as a nonlinear medium. This is an important step to realize basic elements of universal quantum gates, and to serve as a highly nonclassical input probe for spectroscopy and the study of quantum memory.