Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Demonstration of a two-dimensional PT-symmetric crystal: Bulk dynamics,
  topology, and edge states   In 1998, Carl Bender challenged the perceived wisdom of quantum mechanics
that the Hamiltonian operator describing any quantum mechanical system has to
be Hermitian. He showed that Hamiltonians that are invariant under combined
parity-time (PT) symmetry transformations likewise can exhibit real eigenvalue
spectra. These findings had a particularly profound impact in the field of
photonics, where PT-symmetric potential landscapes can be implemented by
appropriately distributing gain and loss. Following this approach, several
hallmark features of PT symmetry were shown, such as the existence of
non-orthogonal eigenmodes, non-reciprocal light evolution, diffusive coherent
transport, and to study their implications in settings including PT-symmetric
lasers and topological phase transitions. Similarly, PT-symmetry has enriched
other research fields ranging from PT-symmetric atomic diffusion,
superconducting wires, and PT-symmetric electronic circuits. Nevertheless, to
this date, all experimental implementations of PT-symmetric systems have been
restricted to one dimension, which is mostly due to limitations in the
technologies at hand for realizing appropriate non-Hermitian potential
landscapes. In this work, we report on the experimental realization and
characterization of a two-dimensional PT-symmetric system by means of photonic
waveguide lattices with judiciously designed refractive index landscape with
alternating loss. A key result of our work is the demonstration of a
non-Hermitian two-dimensional topological phase transition that coincides with
the emergence of mid-gap edge states. Our findings pave the grounds for future
investigations exploring the full potential of PT-symmetric photonics in higher
dimensions. Moreover, our approach may even hold the key for realizing
two-dimensional PT-symmetry also in other systems beyond photonics, such as
matter waves and electronics.

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The categories of this paper are: hep-ex
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The categories of this paper are: econ
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The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
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The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
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The categories of this paper are: solv-int
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The categories of this paper are: adap-org
29
The categories of this paper are: funct-an
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The categories of this paper are: astro-ph
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The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th
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The categories of this paper are: math
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The categories of this paper are: bayes-an
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The categories of this paper are: math-ph
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The categories of this paper are: stat