Causal Asymmetry in a Quantum World

ABSTRACT: Causal asymmetry is one of the great surprises in predictive modelling: The memory required to predict differs from the memory required to retrodict. That is, many processes appear simpler in one temporal direction versus the other. Here, we analyze a process that is causally asymmetric when modelled classically, but whose minimal quantum models exhibit no such asymmetry. More generally, we construct quantum models guaranteed to be smaller than the classical counterparts in either temporal direction. These quantum models can exhibit unbounded advantage. In light of these results, we conjecture that temporal asymmetry arises from casting quantum phenomena in terms of classical information.