  In the heterostructure composed of a topological insulator sandwiched by two
s-wave superconductors, the time reversal invariant topological superconducting
phase, possessing helical Majorana edge modes, is found to exist when the two
s-wave superconductors form a Josephson junction with a pi phase shift. Based
on such a heterostructure, a helical Dirac-Majorana interferometer is proposed
to directly measure a unique transport signature of the helical Majorana modes.
Furthermore, we envision how our proposal can be realized on the basis of
existing materials such as Bi2Se3 or Bi2Te3 thin films.
