Loophole-Free Test of Einstein-Podolsky-Rosen Steering with One Bit of Faster-than-Light Communication

Abstract

In 1935, Einstein, Podolsky and Rosen (EPR) put forward their famous paradox to declare the quantum mechanical description of physical reality incomplete [1]. Their argument relied on the remarkable feature of bipartite quantum entanglement, that the choice of measurement by one party (Alice) seems to affect, instantaneously, the type of state held by the second party (Bob). In the same year Schrödinger called the phenomenon “steering” [2], and discussed the possibility of using arbitrarily many types of measurement. More recently, Schrödinger's generalization was formalized as EPR steering: the experimental violation of the asymmetric model comprising a local hidden state quantum model for Bob's system and a local hidden variable model to generate Alice's measurement results [3]. From the perspective of Schrödinger, who was convinced of the completeness and correctness of the quantum state as a description of a local system, EPR steering implied a genuinely superluminal effect [2]. This raises the foundational question of how much faster-than-light (FTL) classical communication would be required for Alice to steer Bob's state, in the absence of entanglement. It has been shown theoretically that an infinite amount of classical communication from (untrusted) Alice to (trusted) Bob is necessary to simulate all EPR-steering correlations for any pure entangled two-qubit state [4], [5]. For any experimentally accessible correlations, the required amount of classical communication will be finite, and moreover is a measure of the strength of EPR steering. Here we report on the first demonstration of EPR-steering correlations that require more than one bit of FTL communication to simulate classically, in a loophole-free experiment.