Abstract
Our common understanding of the physical world deeply relies on the notion that events are ordered with respect to some time parameter, with past events serving as causes for future ones. Nonetheless, it was recently found that it is possible to formulate quantum mechanics without any reference to a global time or causal structure. The resulting framework includes new kinds of quantum resources that allow performing tasks—in particular, the violation of causal inequalities—which are impossible for events ordered according to a global causal order. However, no physical implementation of such resources is known. Here we show that a recently demonstrated resource for quantum computation—the quantum switch—is a genuine example of ‘indefinite causal order’. We do this by introducing a new tool—the causal witness—which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order. We show however that the quantum switch does not violate any causal inequality.
Highlights
It is commonly assumed that information is processed through a series of operations which are performed according to a specific order
We show that the protocol of [7] can be reformulated as a causal witness which detects the causal nonseparability of the quantum switch
The process matrix formalism was originally conceived as a rather speculative extension of quantum mechanics to possibly include the indefinite causal structures expected in a quantized theory of gravity [10]
Summary
Our common understanding of the physical world deeply relies on the notion that events are ordered author(s) and the title of the work, journal citation with respect to some time parameter, with past events serving as causes for future ones. The quantum switch—is a genuine example of ‘indefinite causal order’. We do this by introducing a new tool—the causal witness—which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order. We show that the quantum switch does not violate any causal inequality
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