  In 1929, Hermann Weyl derived the massless solutions from the Dirac equation
- the relativistic wave equation for electrons. Neutrinos were thought, for
decades, to be Weyl fermions until the discovery of the neutrino mass.
Moreover, it has been suggested that low energy excitations in condensed matter
can be the solutions to the Weyl Hamiltonian. Recently, photons have also been
proposed to emerge as Weyl particles inside photonic crystals. In all cases,
two linear dispersion bands in the three-dimensional (3D) momentum space
intersect at a single degenerate point - the Weyl point. Remarkably, these Weyl
points are monopoles of Berry flux with topological charges defined by the
Chern numbers. These topological invariants enable materials containing Weyl
points to exhibit a wide variety of novel phenomena including surface Fermi
arcs, chiral anomaly, negative magnetoresistance, nonlocal transport, quantum
anomalous Hall effect, unconventional superconductivity[15] and others [16,
17]. Nevertheless, Weyl points are yet to be experimentally observed in nature.
In this work, we report on precisely such an observation in an
inversion-breaking 3D double-gyroid photonic crystal without breaking
time-reversal symmetry.
