Abstract:
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Small-scale quantum data networks are envisioned to be deployed in the near future, increasing the need for efficient estimation and validation protocols for network components. The statistical determination of one or more parameters of a quantum channel is called quantum channel identification (QCI). QCI is accomplished by passing probes in prepared quantum states through the channel and then estimating the parameter(s) from the measured channel outputs. Quantum Fisher information (QFI) is a means to assess and compare different probing schemes. We use QFI to study a probing scheme in which the channel is put in indefinite causal order (ICO) with copies of itself, focusing our investigation on the state preservation probability in a standard noisy channel model, the quantum depolarizing channel. In this setting we are able to obtain the QFI in closed analytic form, and we find that ICO-assisted probing yields greater information than does the comparable scheme with definite ordering and that the information gained is consistently greater when the channel ordering is more indefinite. Our approach extends to the study of different types of channel orderings.
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