  The spin-polarized surface states in topological insulators offer unique
transport characteristics which make them distinguishable from trivial
conductors. Due to the topological protection, these states are gapless over
the whole surface of the material. Here, we detect the surface states in the
topological insulator BiSbTeSe$_{2}$ by electrical means using a non-local
transport configuration. We unambiguously probe the spin-momentum locking of
the topologically protected surface states by spin-sensitive electrical
read-out using ferromagnetic Co/Al$_2$O$_3$ electrodes. We show that the
non-local measurement allows to probe the surface currents flowing along the
whole surface, i.e. from the top along the side to the bottom surface and back
to the top surface along the opposite side. This is in contrast to local
transport configurations where only the surface states of the one face being in
contact to the electrodes can be measured. Our results furthermore exclude the
contribution of the bulk to the non-local transport at low temperatures.
Increasing the temperature, on the other hand, increases the interaction
between bulk and surface states, which shortens the non-local current path
along the surface and hence leads to a complete disappearance of the non-local
signal at around 20 K. All this demonstrates that the non-local signal at low
temperatures is solely due to the topologically protected surface states.
