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A "NooN State" is a kind of Schrödinger cat state: it consists of N photons, all of which are doing the same thing. They are either all in one polarization, or they are all in the opposite polarization. Or maybe they are either all going one direction, or they are all going in a different direction. The photons are a bit like the cells in Schrödinger's cat, which are either all alive, or all dead. A large number of microscopic objects behaving in concert. The quantum part is that they are in a superposition of these two possibilities, experiencing both at the same time.

Optical Parametric Oscillator for making NooN states.

Apart from their intrinsic interest, NooN states very interesting for quantum metrology. Compared to any other state of N photons, the NooN state gives the most phase information in an interferometer. This is related to the uncertainty principle for number and phase. Here the phase is the phase of the interferometer, and the number is the difference in number of photons in the two paths. A NooN state maximizes the uncertainty in number: they are either all in one path, or all in the other, maximum separation between these possibilities. And that allows for minimum uncertainty in phase.

We have recently made the first NooN states with atom-resonant photons. This allowed us to make the first ultra-sensitive optical measurements on atoms. The results are published here:

Wolfgramm et al. Nature Photonics 2012 (requires subscription) Wolfgramm et al. arXiv:1212.4729 (free)