Erwin Schrödinger Center for Quantum Science & Technology (ESQ)

For minimum-error channel discrimination tasks that involve only unitary channels, they show that sequential strategies may outperform the parallel ones. Additionally, they show that general strategies that involve indefinite causal order are also advantageous for this task. However, for the task of discriminating a uniformly distributed set of unitary channels that forms a group, they show that parallel strategies are, indeed, optimal, even when compared to general strategies. They also show that strategies based on the quantum switch cannot outperform sequential strategies in the discrimination of unitary channels. Finally, they derive an absolute upper bound for the maximal probability of successfully discriminating any set of unitary channels with any number of copies for the most general strategies that are suitable for channel discrimination. Their bound is tight since it is saturated by sets of unitary channels forming a group k-design.

For more information see:

https://aip.scitation.org/doi/10.1063/5.0075919

https://www.iqoqi-vienna.at/

https://mtcq.github.io/