  Cd3As2 is a three-dimensional topological Dirac semimetal with connected
Fermi-arc surface states. It has been suggested that topological
superconductivity can be achieved in the nontrivial surface states of
topological materials by utilizing the superconductor proximity effect. Here we
report observations of both {\pi} and 4{\pi} periodic supercurrents in
aluminum-Cd3As2-aluminum Josephson junctions. The {\pi} period is manifested by
both the magnetic-field dependence of the critical supercurrent and the
appearance of half-integer Shapiro steps in the a.c. Josephson effect. Our
macroscopic theory suggests that the {\pi} period arises from interference
between the induced bulk superconductivity and the induced Fermi arc surface
superconductivity. The 4{\pi} period is manifested by the missing first Shapiro
steps and is expected for topological superconductivity.
