[PhilPhys] Online seminar: Caslav Brukner

[Matthew Leifer]

Q+ hangouts are a series of online seminars about quantum foundations 
and quantum information that take place in Google+ hangouts.  I post the 
announcement to this list when the topic is relevant to quantum 
foundations.  To watch the seminars live, go to http://gplus.to/qplus at 
the appointed hour.  Here are the details of the next talk:

Date: Tuesday 24th July
Time: 2pm British Summer Time
Speaker: Caslav Brukner (University of Vienna)
Title: Quantum correlations with indefinite causal order

Abstract: In quantum physics it is standardly assumed that the 
background time or definite causal structure exists such that every 
operation is either in the future, in the past or space-like separated 
from any other operation. Consequently, the correlations between 
operations respect definite causal order: they are either signalling 
correlations for the time-like or no-signalling correlations for the 
space-like separated operations. We develop a framework that assumes 
only that operations in local laboratories are described by quantum 
mechanics (i.e. are completely-positive maps), but relax the assumption 
that they are causally connected. Remarkably, we find situations where 
two operations are neither causally ordered nor in a probabilistic 
mixture of definite causal orders, i.e. one cannot say that one 
operations is before or after the other. The correlations between the 
operations are shown to enable performing a communication task ("causal 
game") that is impossible if the operations are ordered according to a 
fixed background time.

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