Dear all,
Yelena Guryanova from IQOQI, Vienna, is visiting us this week (until
Thursday lunch time).
Yelena works on thermodynamics, nonlocality and quantum foundations. She
will give the following seminar talk tomorrow (Wednesday) at 3pm, in
room HIT E41.1.
Best,
Lídia
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*_Exploring process matrices via the formalism of multi-time states_**_
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When describing or observing a physical phenomenon, it is generally
assumed that there exists an underlying causal order to the events. The
belief is that events which occur later in time can be influenced by
events occurring at earlier times, but not the other way around.
Recently, however there has been an intensified interest in the role
that quantum mechanics has to play when describing causal order in
physical phenomena. Principles fundamental and unique to quantum
mechanics can be exploited in such a way that they appear to result in
descriptions of processes with indefinite causal structures.
In this talk we look at two formalisms. The first is that of multi-time
states, first introduced by Aharanov and colleagues, who developed a
time-symmetric formulation of quantum theory. Here, a quantum system at
several different times is described by associating a quantum state to
each time. The second work we look at is the process matrix formalism,
introduced by Oreshkov, Costa, and Brukner. Here, the idea of the
'process matrix' was introduced as a possible generalisation of quantum
states where the causal structure of events can be partially relaxed.
Here, operations in local laboratories are described by quantum theory
without assuming a global causal structure.
At first glance, the two approaches appear to be completely different
and unrelated. We show that every process matrix is shown to correspond
to a two-time state (the converse link however does not hold in
general). We also provide an extension of a result which characterises
process matrices. The connection presented here allows us to apply new
methods and concepts to the issue of physics with indefinite causal
structure, open new questions and hopefully create a platform of
discussion about the issues of time and causality in quantum theory.