  A topological insulator is characterized by spin-momentum locking on its
boundary. The spin momentum locking on the surface of a three dimensional
topological insulator leads to the existence of a non-trivial Berry's phase
which leads to exotic transport phenomena on topological surfaces. Using
spin-sensitive probes (Mott polarimetry), we observe the spin-momentum coupling
and uncover the chiral nature of surface electrons in TlBiSe2. We demonstrate
that the surface electrons in TlBiSe2 collectively carry a quantum Berry's
phase of $\pi$ and a definite chirality ($\eta$ = -1, left-handed) associated
with its spin-texture or vortex-structure on the Fermi surface on both the top
and the bottom surfaces. Our experimental results for the first time not only
prove the existence of Z2 topological-order in the bulk but also reveal the
existence of helical quasiparticle modes on the topological surface.
Spin-texture calculations would be reported elsewhere.
