Knowledge by Direct Measurement versus Inference from Steering

Abstract

If Alice and Bob start out with an entangled state $|\Psi_{AB}\rangle$, Bob may update his state to $|\varphi_B\rangle$ either by performing a suitable measurement himself, or by receiving the information that a measurement by Alice has steered that state. While Bob's update on his state is identical, his update on Alice's state differs: if Bob has performed the measurement, he has steered the state $|\chi_{\leftarrow}(\varphi)\rangle_A$ of Alice; if Alice has made the measurement, to steer $|\varphi\rangle_B$ on Bob she must have found a different state $|\chi_{\rightarrow}(\varphi)\rangle_A$. Based on this observation, a consequence of the well-known `Hardy's ladder', we show that information from direct measurement must trump inference from steering. The erroneous belief that both paths should lead to identical conclusions can be traced to the usual prejudice that measurements should reveal a pre-existing state of affairs. We also prove a technical result on Hardy's ladder: the minimum overlap between the steered and the steering state is $2\sqrt{p_{0}p_{n-1}}/(p_0+p_{n-1})$, where $p_0$ and $p_{n-1}$ are the smallest (non-zero) and the largest Schmidt coefficients of $|\Psi\rangle_{AB}$.