Talks

In quantum information, entanglement has often been viewed as a resource. But in this talk, I will look at (pure bipartite) entanglement through the lens of superselection rules. The idea is that it requires quantum communication not only to create entanglement, but also to destroy it in a way that doesn't leak information to the environment. As a result, when communication is scarce, superpositions of different numbers of EPR pairs can be difficult to obtain. This constraint is not a strict superselection rule, but rather an approximate version that gives rigorous bounds on achievable fidelities. After describing the general phenomenon, I will show how it relates to communication complexity, information theory and fairy tales about princesses. This talk is based on 0803.3066, 0909.1557, 0912.5537 and other unpublished work.

In this talk, I will demonstrate that correlations inconsistent with any locally causal description can be a generic feature of measurements on entangled quantum states. Specifically, spatially-separated parties who perform local measurements on a maximally-entangled state using randomly chosen measurement bases can, with significant probability, generate nonclassical correlations that violate a Bell inequality. For n parties using a Greenberger-Horne-Zeilinger state, this probability of violation rapidly tends to unity as the number of parties increases. Moreover, even with both a randomly chosen two-qubit pure state and randomly chosen measurement bases, a violation can be found about 10% of the time. Amongst other applications, our work provides a feasible alternative for the demonstration of Bell inequality violation without a shared reference frame.