Robust entanglement-based magnetic field sensor beyond the standard   quantum limit
  Recently, there have been significant developments in entanglement-based quantum metrology. However, entanglement is fragile against experimental imperfections, and quantum sensing to beat the standard quantum limit in scaling has not yet been achieved in realistic systems. Here, we show that it is possible to overcome such restrictions so that one can sense a magnetic field with an accuracy beyond the standard quantum limit even under the effect of decoherence, by using a realistic entangled state that can be easily created even with current technology. Our scheme could pave the way for the realizations of practical entanglement-based magnetic field sensors. 
Tohru Tanaka, Paul Knott, Yuichiro Matsuzaki, Shane Dooley, Hiroshi   Yamaguchi, William J. Munro, Shiro Saito
quant-ph