Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Noncausal Page-Wootters circuits   One of the most fundamental open problems in physics is the unification of
general relativity and quantum theory to a theory of quantum gravity. An aspect
that might become relevant in such a theory is that the dynamical nature of
causal structure present in general relativity displays quantum uncertainty.
This may lead to a phenomenon known as indefinite or quantum causal structure,
as captured by the process matrix formalism. Due to the generality of that
framework, however, for many process matrices there is no clear physical
interpretation. A popular approach towards a quantum theory of gravity is the
Page-Wootters formalism, which associates to time a Hilbert space structure
similar to spatial position. By explicitly introducing a quantum clock, it
allows to describe time-evolution of systems via correlations between this
clock and said systems encoded in history states. In this paper we combine the
process matrix framework with a generalization of the Page-Wootters formalism
in which one considers several agents, each with their own discrete quantum
clock. We describe how to extract process matrices from scenarios involving
such agents with quantum clocks, and analyze their properties. The description
via a history state with multiple clocks imposes constraints on the
implementation of process matrices and on the perspectives of the agents as
described via causal reference frames. While it allows for scenarios where
different definite causal orders are coherently controlled, we explain why
certain noncausal processes might not be implementable within this setting.

34
The categories of this paper are: chem-ph
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th