# Causality and Entanglement in Quantum Field Theory

**Nov 11, 2019 at 2:00 PM - 3:00 PM**

204 Physics Building

Causality and Entanglement in Quantum Field Theory

by: Aiyalam Balachandran

abstract: Some years ago, Rafael Sorkin argued that standard quantum measurement theory and relativistic causality are not compatible because of the existence of entangled quantum states . He showed this by devising experiments using entangled states which transmit information to spacelike distances. This talk elaborates on this result in the framework of Algebraic Quantum Field Theory ( AQFT). The observables in AQFT are localised in finite spacetime regions O and form von Neumann algebras A (O) . There are no normal states, that is, density matrices, in A(O). If A(O)’ is the causal complement of A(O) (i.e. consisting of elements commuting with those of A(O) ), then the algebra generated by A(O) and A(O)’ does have density matrices. But they are all entangled across A (O) and A(O)’ and cause causality problems. Such entanglement can be avoided by thickening O to a region containing O and assuming the ’split property’ which the talk explains.Then unentangled density matrices do exist. Projection operators of local observables, while commuting for space like separations, are not orthogonal. Experiments in O involve these projectors. But they do not seem to give causality problems.

Contact: Judah Unmuth-Yockey