  Topological superconductors can host Majorana quasiparticles which supersede
the fermion/boson dichotomy and offer a pathway to fault tolerant quantum
computation. In one-dimensional systems zero-energy Majorana states are bound
to the ends of the topologically superconducting regions. An experimental
signature of a Majorana bound state is a conductance peak at zero source-drain
voltage bias in a tunneling experiment. Here, we identify the bulk topological
phase in a semiconductor nanowire coupled to a conventional superconductor. We
map out its phase diagram through the dependence of zero-bias peak on the
chemical potential and magnetic field. Our findings are consistent with
calculations for a finite-length topological nanowire. Knowledge of the phase
diagram makes it possible to predictably tune nanowire segments in and out of
the topological phase, thus controlling the positions and couplings of multiple
Majorana bound states. This ability is a prerequisite for Majorana braiding, an
experiment in which Majorana quantum states are exchanged in order to both
demonstrate their non-abelian character and realize topological quantum bits.
