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True Quantum Precision and Unique Optimal Probes in presence of Decoherence.

Sergey Knysh, NASA Ames Research Center

(Session 13 : Saturday from 5:30pm - 6:00pm)

Quantum instruments derived from composite systems allow greater measurement precision than their classical counterparts due to coherences maintained between the N component elements; spins, atoms or photons. Typical decoherence that plagues real-world devices can be dephasing, particle loss, thermal excitation and relaxation. All these adversely affect precision (mean squared error), whether one is measuring time or phase, or even the noise amplitude itself. We develop a novel technique that uncovers the uniquely optimal probe states of the N `qubits' alongside new tight bounds on precision under local and collective mechanisms of these noise types above.   For large quantum ensembles (where numerical techniques fail), the problem reduces by analogy  to finding the ground state of a 1-D particle in a potential well, with the shape of the well dictated by the type and strength of decoherence. Under collective dephasing alone we find that optimal estimation of phase and noise parameter can be effected simultaneously, utilizing the same optimal probe and measurement scheme. The formalism is applied to real-world devices such as the Mach-Zehnder interferometer and atom clocks.