Reassessing the advantage of indefinite causal orders for quantum metrology
arxiv(2023)
摘要
The quantum switch, the canonical example of a process with indefinite causal
order, has been claimed to provide various advantages over processes with
definite causal orders for some particular tasks in the field of quantum
metrology. In this work, we argue that some of these advantages in fact do not
hold if a fairer comparison is made. To this end, we consider a framework that
allows for a proper comparison between the performance, quantified by the
quantum Fisher information, of different classes of indefinite causal order
processes and that of causal strategies on a given metrological task. More
generally, by considering the recently proposed classes of circuits with
classical or quantum control of the causal order, we come up with different
examples where processes with indefinite causal order offer (or not) an
advantage over processes with definite causal order, qualifying the interest of
indefinite causal order regarding quantum metrology. As it turns out, for a
range of examples, the class of quantum circuits with quantum control of causal
order, which are known to be physically realizable, is shown to provide a
strict advantage over causally ordered quantum circuits as well as over the
class of quantum circuits with causal superposition. Thus, the consideration of
this class provides new evidence that indefinite causal order strategies can
strictly outperform definite causal order strategies in quantum metrology.
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