  The existence of inequivalent valleys K and K' in the momentum space of
two-dimensional hexagonal lattices provides a new electronic degree of freedom,
the manipulation of which can potentially lead to new types of electronics, in
analogy to the role played by electron spin. In materials with broken inversion
symmetry, such as an electrically gated bilayer graphene, the momentum-space
Berry curvature $\Omega$ carries opposite sign in the K and K' valleys. A sign
reversal of $\Omega$ along an internal boundary of the sheet gives rise to
counter-propagating one-dimensional conducting modes encoded with opposite
valley indices. These metallic states are topologically protected against
backscattering in the absence of valley-mixing scattering, and thus can carry
current ballistically. In bilayer graphene, the reversal of $\Omega$ can occur
at the domain wall of AB and BA stacked domains, or at the line junction of two
oppositely gated regions. The latter approach can provide a scalable platform
to implement valleytronic operations such as valves and waveguides, but is
technically challenging to realize. Here we fabricate a dual-split-gate
structure in bilayer graphene and demonstrate transport evidence of the
predicted metallic states. They possess a mean free path of up to a few hundred
nanometers in the absence of a magnet field. The application of perpendicular
magnetic field suppresses backscattering significantly and enables a
400-nanometer-long junction to exhibit conductance close to the ballistic limit
of 4 $e^2/h$ at 8 Tesla. Our experiment paves the path to the realization of
gate-controlled ballistic valley transport and the development of valleytronic
applications in atomically thin materials.
