Generalized Limits for Single-Parameter Quantum Estimation
Sergio Boixo, Steven T. Flammia, Carlton M. Caves, J. M. Geremia · Physical Review Letters · 2007
We develop generalized bounds for quantum single-parameter estimation problems for which the coupling to the parameter is described by intrinsic multisystem interactions. For a Hamiltonian with k-system parameter-sensitive terms, the quantum limit scales as 1/Nk, where N is the number of systems. These quantum limits remain valid when the Hamiltonian is augmented by any parameter-independent interaction among the systems and when adaptive measurements via parameter-independent coupling to ancillas are allowed.