Recent experiments and theories have suggested that strong spin-orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the socalled topological insulator, which can show macroscopic quantum-entanglement effects 1-7 . Such systems feature twodimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe interacting quarks 8-15 . It has been proposed that a topological insulator 2 with a single Dirac cone interfaced with a superconductor can form the most elementary unit for performing fault-tolerant quantum computation 14 . Here we present an angle-resolved photoemission spectroscopy study that reveals the first observation of such a topological state of matter featuring a single surface Dirac cone realized in the naturally occurring Bi 2 Se 3 class of materials. Our results, supported by our theoretical calculations, demonstrate that undoped Bi 2 Se 3 can serve as the parent matrix compound for the long-sought topological device where in-plane carrier transport would have a purely quantum topological origin. Our study further suggests that the undoped compound reached via n-to-p doping should show topological transport phenomena even at room temperature.It has been experimentally shown that spin-orbit coupling can lead to new phases of quantum matter with highly nontrivial collective quantum effects . Two such phases are the quantum spin Hall insulator 4 and the strong topological insulator , both realized in the vicinity of a Dirac point but yet quite distinct from graphene . The strong-topological-insulator phase contains surface states (SSs) with novel electromagnetic properties . It is currently believed that the Bi 1-x Sb x insulating alloys realize the only known topological-insulator phase in the vicinity of a three-dimensional Dirac point 5 , which can in principle be used to study topological electromagnetic and interface superconducting properties . However, a particular challenge for the topological-insulator Bi 1-x Sb x system is that the bulk gap is small and the material contains alloying disorder, which makes it difficult to gate for the manipulation and control of charge carriers to realize a device. The topological insulator Bi 1-x Sb x features five surface bands, of which only one carries the topological quantum number 6 . Therefore, there is an extensive world-wide search for topological phases in stoichiometric materials with no alloying disorder, with a larger gap and with fewer yet still odd-numbered SSs that may