The elusive Heisenberg limit in quantum enhanced metrology
  We provide efficient and intuitive tools for deriving bounds on achievable precision in quantum enhanced metrology based on the geometry of quantum channels and semi-definite programming. We show that when decoherence is taken into account, the maximal possible quantum enhancement amounts generically to a constant factor rather than quadratic improvement. We apply these tools to derive bounds for models of decoherence relevant for metrological applications including: dephasing,depolarization, spontaneous emission and photon loss. 
Rafal Demkowicz-Dobrzanski, Jan Kolodynski, Madalin Guta
quant-ph