Quantum System Identification with Limited Resources

The aim of quantum system identification is to estimate the ingredients inside a black box, in which some quantum-mechanical unitary process takes place, by just looking at its input-output behavior. Here we establish a basic and general framework for quantum system identification, that allows us to classify how much knowledge about the quantum system is attainable, in principle, from a given experimental setup. Unlike usual quantum metrology, our theory applies to cases where the set of observables are not topographically complete, and where knowledge about the system can only be retrieved indirectly. When the topology of the system is known, the framework enables us to establish a general criterion for the estimability of the coupling constants in its Hamiltonian. Reference: http://arxiv.org/abs/1104.0583