  We propose and analyze theoretically an experimental setup for detecting the
elusive Majorana particle in semiconductor-superconductor heterostructures. The
experimental system consists of one-dimensional semiconductor wire with strong
spin-orbit Rashba interaction embedded into a superconducting quantum
interference device. We show that the energy spectra of the Andreev bound
states at the junction are qualitatively different in topologically trivial
(i.e., not containing any Majorana) and nontrivial phases having an even and
odd number of crossings at zero energy, respectively. The measurement of the
supercurrent through the junction allows one to discern topologically distinct
phases and observe a topological phase transition by simply changing the
in-plane magnetic field or the gate voltage. The observation of this phase
transition will be a direct demonstration of the existence of Majorana
particles.
