Existence of processes violating causal inequalities on time-delocalised subsystems

Abstract

It has been shown that it is theoretically possible for there to exist quantum and classical processes in which the operations performed by separate parties do not occur in a well-defined causal order. A central question is whether and how such processes can be realised in practice. In order to provide a rigorous framework for the notion that certain such processes have a realisation in standard quantum theory, the concept of time-delocalised quantum subsystem has been introduced. In this paper, we show that realisations on time-delocalised subsystems exist for all unitary extensions of tripartite processes. This class contains processes that violate causal inequalities, i.e., that can generate correlations that witness the incompatibility with definite causal order in a device-independent manner, and whose realisability has been a central open problem. We consider a known example of such a tripartite classical process that has a unitary extension, and study its realisation on time-delocalised subsystems. We then discuss this finding with regard to the assumptions that underlie causal inequalities, and argue that they are indeed a meaningful concept to show the absence of a definite causal order between the variables of interest.

Device-independent certification of indefinite causal order in the quantum switch

“…For some causally nonseparable processes, such device-independent certification is possible through the violation of so-called causal inequalities 3 , 6 , 36 – 39 ; however, the physicality of these processes is still unclear 39 – 42 . The quantum switch, on the other hand—the only causally nonseparable process to have been studied experimentally—has been shown not to violate any such inequality 5 , 6 , a result that was recently extended to the broader class of quantum circuits with quantum control of causal order 43 , 44 .…”

Trading Causal Order for Locality

“…To be sure, in such a situation, the communication between the labs would still be classical but the physical conditions for achieving this communication would be outside the realm of possibilities afforded by general-relativistic spacetimes. The latter possibility could have interesting implications for how one might interpret time-delocalized realizations [39] of the AF-BW process [40].…”