Probe readout and quantum-limited measurements

We consider the readout process on a probe made of N qubits (two-level systems) in a quantum single-parameter estimation scheme. The parameter-dependent evolution of the probe as well as the measurements that are done for readout are assumed to be fixed. We find the optimal initial states of the probe that will saturate the quantum Cramer-Rao bound under these conditions. The relation between the optimal state of the N-qubit probe and that of the one-qubit probe is obtained for both entangling and nonentangling dynamics for the probe qubits. We study the limitations placed on the optimal initial state of the probe and on the achievable measurement uncertainty by the restrictions on the readout procedure. DOI: 10.1103/PhysRevA.87.022330