Surpassing the Standard Quantum Limit in an Atom Interferometer with   Four-mode Entanglement Produced from Four-Wave Mixing
  We theoretically investigate a scheme for atom interferometry that surpasses the standard quantum limit. A four-wave mixing scheme similar to the recent experiment performed by Pertot et al. \cite{pertot} is used to generate sub-shot noise correlations between two modes. These two modes are then interfered with the remaining two modes in such a way as to surpass the standard quantum limit, whilst utilising all of the available atoms. Our scheme can be viewed as using two correlated interferometers. That is, the signal from each interferometer when looked at individually is classical, but there are correlations between the two interferometers that allow for the standard quantum limit to be surpassed. 
S. A. Haine, and A. J. Ferris
quant-ph cond-mat.quant-gas