Quantum universal detectors
Giacomo Mauro D’Ariano, Paolo Perinotti, Massimiliano Federico Sacchi · 2003
PACS. 03.65.Ta – Foundations of quantum mechanics; measurement theory. PACS. 03.65.Wj – State reconstruction, quantum tomography. Abstract. – We address the problem of achieving a universal measuring apparatus that allows to estimate the expectation value of any arbitrary operator by changing only the data processing of the outcomes. The “universal detector ” performs a joint measurement on the system and on a suitably prepared ancilla. We characterize such universal detectors, and show how they can be obtained either via Bell measurements or via local measurements and classical communication between system and ancilla. Quantum technology is nowadays turning from the stage of experimental setup design to that of quantum system engineering. The aim is to produce tools for communication, infor-mation processing, and storage, which rely on the principles of quantum mechanics, with the chance of achieving much higher speeds and capacities than those of classical devices. In this scenario, a new kind of quantum lab can be devised, in which universality and programmability are crucial features, with different tasks achieved by a basic set of devices. A “universal detector ” would allow the estimation of ensemble averages of arbitrary op-