Separable states improve protocols with restricted randomness

Abstract: It is known from Bell's theorem that quantum predictions for some entangled states cannot be mimicked using local hidden variable (LHV) models. From a computer science perspective, LHV models may be interpreted as classical computers operating on a potentially infinite number of correlated bits originating from a common source. As such, Bell inequality violations achieved through entangled states are able to characterise the quantum advantage of certain tasks, so long as the task itself imposes no restriction on the availability of correlated bits. However, if the number of shared bits is limited, additional constraints are placed on the possible LHV models and separable, i.e. disentangled, states may become a useful resource. Bell violations are therefore no longer necessary to achieve a quantum advantage. Here we show that in particular, separable states may improve the so-called random access codes, which is a class of communication problems where one party tries to read a portion of the data held by another distant party in presence of finite shared randomness and limited classical communication. We also show how the bias of classical bits can be used to avoid wrong answers in order to achieve the optimal classical protocol and how the advantage of quantum protocols is linked to quantum discord.