Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct Measurement of a Non-Hermitian Topological Invariant in a Hybrid
  Light-Matter System   Topology is central to understanding and engineering materials that display
robust physical phenomena immune to imperfections. Different topological phases
of matter are characterised by topological invariants. In energy-conserving
(Hermitian) systems, these invariants are determined by the winding of
eigenstates in momentum space. In non-Hermitian systems, a novel topological
invariant is predicted to emerge from the winding of the complex eigenenergies.
Here, we directly measure the non-Hermitian topological invariant arising from
exceptional points in the momentum-resolved spectrum of exciton polaritons.
These are hybrid light-matter quasiparticles formed by photons strongly coupled
to electron-hole pairs (excitons) in a halide perovskite semiconductor at room
temperature. We experimentally map out both the real (energy) and imaginary
(linewidth) parts of the spectrum near the exceptional points and extract the
novel topological invariant - fractional spectral winding. Our work represents
an essential step towards realisation of non-Hermitian topological phases in a
condensed matter system.

7
The categories of this paper are: hep-th
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
28
The categories of this paper are: alg-geom
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
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The categories of this paper are: math
26
The categories of this paper are: bayes-an
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin