Every nonsignaling channel is common-cause realizable

In this work we show that the set of nonsignaling resources of a locally tomographic generalized probabilistic theory (GPT), such as quantum and classical theory, coincides with its set of GPT-common-cause realizable resources, where the common causes come from an associated GPT. From a causal perspective, this result provides a reason for, in the study of resource theories of common-cause processes, taking the nonsignaling channels as the resources of the enveloping theory. This answers a critical open question in Schmidt et al. [Quantum 5, 419 (2021)]. An immediate corollary of our result is that every nonsignaling assemblage is realizable in a GPT, answering in the affirmative the question posed in Cavalcanti et al. [npj Quantum Inf. 8, 76 (2022)].