  Interference of spin-up and spin-down eigenstates depicts spin rotation of
electrons, which is a fundamental concept of quantum mechanics and accepts
technological challenges for the electrical spin manipulation. Here, we
visualize this coherent spin physics through laser spin- and angle-resolved
photoemission spectroscopy on a spin-orbital entangled surface-state of a
topological insulator. It is unambiguously revealed that the linearly polarized
laser can simultaneously excite spin-up and spin-down states and these quantum
spin-basis are coherently superposed in photoelectron states. The superposition
and the resulting spin rotation is arbitrary manipulated by the direction of
the laser field. Moreover, the full observation of the spin rotation displays
the phase of the quantum states. This presents a new facet of
laser-photoemission technique for investigation of quantum spin physics opening
new possibilities in the field of quantum spintronic applications.
