Quantum parameter estimation on coherently superposed noisy channels

A generic qubit unitary operator affected by quantum noise is duplicated and inserted in a coherently superposed channel, superposing two paths offered to probe across the noisy unitary, driven control qubit. characterization performed of transformation realized channel on joint state probe-control pair. The then specifically analyzed for fundamental metrological task phase estimation with performance assessed Fisher information, classical or quantum. comparison made conventional techniques also switched indefinite causal order recently investigated similar estimation. In analysis here, first important observation that although it never directly interacts being estimated, can nevertheless be measured alone effective estimation, while discarding unitary. This property present but inaccessible techniques. optimal measurement here characterized general conditions. second plays an essential role coupling remains operative at very strong noise, even fully depolarizing whereas become inoperative these results extend capabilities controlled channels which represent novel devices exploitable signal information processing.