Indefinite causal order strategy nor adaptive strategy does not improve the estimation of group action

Masahito Hayashi (Jan 17 2025).

Abstract: We consider estimation of unknown unitary operation when the set of possible unitary operations is given by a projective unitary representation of a compact group. We show that indefinite causal order strategy nor adaptive strategy does not improve the performance of this estimation when error function satisfies group covariance. That is, the optimal parallel strategy gives the optimal performance even under indefinite causal order strategy and adaptive strategy.

Arxiv: https://arxiv.org/abs/2501.09312