  Topological insulators (TIs) have attracted immense interest because they
host helical surface states. Protected by time-reversal symmetry, they are
robust to non-magnetic disorder. When superconductivity is induced in these
helical states, they are predicted to emulate p-wave pairing symmetry, with
Majorana states bound to vortices. Majorana bound states possess non-Abelian
exchange statistics which can be probed through interferometry. Here, we take a
significant step towards Majorana interferometry by observing pronounced
Fabry-Perot oscillations in a TI sandwiched between a superconducting and
normal lead. For energies below the superconducting gap, we observe a doubling
in the frequency of the oscillations, arising from the additional phase
accumulated from Andreev reflection. When a magnetic field is applied
perpendicular to the TI surface, a number of very sharp and gate-tunable
conductance peaks appear at or near zero energy, which has consequences for
interpreting spectroscopic probes of Majorana fermions. Our results demonstrate
that TIs are a promising platform for exploring phase-coherent transport in a
solid-state system.
