High sensitivity optical Faraday-magnetometry with intracavity   electromagnetically induced transparency
  We suggest a multiatom cavity quantum electrodynamics system for the weak magnetic field detection based on Faraday rotation with intracavity electromagnetically induced transparency. Our study demonstrates that the collective coupling between the cavity modes and the atomic ensemble can be used to improve the sensitivity. With single probe photon input, the sensitivity is inversely proportional to the number of atoms, and the sensitivity with 0.7(5)~nT/$\sqrt{\rm Hz}$ could be attained. With multiphoton measurement, our numerical calculations show that the magnetic field sensitivity can be improved to 4.7(9)~fT/$\sqrt{\rm Hz}$. 
Qiaolin Zhang, Hui Sun, Shuangli Fan, and Hong Guo
quant-ph