Speaker
Description
Device-independent (DI) tests draw conclusions about nature solely from observed correlations, without trusting the devices used. Originating in Bell's work on nonlocality, this paradigm has become central to foundational studies of quantum theory and practical applications (e.g., cryptography). More recently, DI tests of indefinite causal order (ICO) have extended this approach to more exotic quantum structures where correlations may violate the assumption of a definite causal order. However, unlike Bell tests where loopholes can be closed, tests of ICO face a fundamental challenge that has been largely overlooked: the closed-labs (CL) assumption. This requires that each party interacts with a shared signal-mediating system at most once, preventing two-way signalling mimicking ICO correlations. Yet CL is inherently device-dependent and cannot be directly verified without revealing a definite spacetime structure, thereby destroying any causal indefiniteness present. In this talk, I introduce a framework that embeds a validation of the CL assumption directly within a DI test of ICO. By adding a minimal set of components, we transform CL from a trust assumption into an operationally testable feature of the lab infrastructure. Placing beamsplitters with detectors at the device input and output ports allows the inter-lab signalling structure to be probabilistically probed. The resulting detection statistics bound the number of system-device interactions per run, enabling an operational test of CL. Our work highlights a fundamental structural difference between DI tests of ICO and those of nonlocality: certifying ICO necessitates additional operational assumptions not required in the nonlocality setting. By making these assumptions explicit and testable, our framework shows that their associated limitations can be overcome. Our results enable rigorous certification of ICO suitable for practical applications such as cryptography, and open avenues for foundational experiments probing quantum causal structures, which may in turn offer insights for quantum gravity.