Generation of a superposition of odd photon number states for quantum   information networks
  We report on the experimental observation of quantum-network-compatible light described by a non-positive Wigner function. The state is generated by photon subtraction from a squeezed vacuum state produced by a continuous wave optical parametric amplifier. Ideally, the state is a coherent superposition of odd photon number states, closely resembling a superposition of weak coherent states (a Schroedinger cat), with the leading contribution from a single photon state in the low parametric gain limit. Light is generated in a nearly perfect spatial mode with a Fourier-limited frequency bandwidth which matches well atomic quantum memory requirements. The source is also characterized by high spectral brightness with about 10,000 and more events per second routinely generated. The generated state of light is the ultimate input state for testing quantum memories, quantum repeaters and linear optics quantum computers. 
J. S. Neergaard-Nielsen (1 and 3), B. Melholt Nielsen (1 and 3), C.   Hettich (1 and 3), K. Moelmer (2 and 3) and E. S. Polz...