Macroscopic quantum state analyzed particle by particle.

Macroscopic quantum phenomena, e.g., superconductivity and squeezing, are believed to result from entanglement of macroscopic numbers of particles.

Interferometric measurement of the biphoton wave function.

Interference between an unknown two-photon state (a "biphoton") and the two-photon component of a reference state gives a phase-sensitive arrival-time distribution containing full information about the biphoton temporal wave function. Using a coherent state as a reference, we observe this interference and reconstruct the wave function of single-mode biphotons from a low-intensity narrow band squeezed vacuum state.

Atom-resonant heralded single photons by interaction-free measurement.

We demonstrate the generation of rubidium-resonant heralded single photons for quantum memories. Photon pairs are created by cavity-enhanced down-conversion and narrowed in bandwidth to 7 MHz with a novel atom-based filter operating by "interaction-free measurement" principles. At least 94% of the heralded photons are atom-resonant as demonstrated by a direct absorption measurement with rubidium vapor.