2024/09/27 by Kristjánsson, Hlér, Odake, Tatsuki, Yoshida, Satoshi +4 · 3 citations
#FOS: Physical sciences #Quantum Physics (quant-ph)
paper · doi:10.48550/arxiv.2409.18420
Quantum theory is consistent with a computational model permitting black-box operations to be applied in an indefinite causal order, going beyond the standard circuit model of computation. The quantum switch -- the simplest such example -- has been shown to provide numerous information-processing advantages. Here, we prove that the action of the quantum switch on two n-qubit quantum channels cannot be simulated deterministically and exactly by any causally ordered quantum circuit that uses M calls to one channel and one call to the other, if M ≤ max(2, 2n-1). This demonstrates an exponential separation in quantum query complexity of indefinite causal order compared to standard quantum circuits.