  The transmission of Cooper pairs between two weakly coupled superconductors
produces a superfluid current and a phase difference; the celebrated Josephson
effect. Because of time-reversal and parity symmetries, there is no Josephson
current without a phase difference between two superconductors. Reciprocally,
when those two symmetries are broken, an anomalous supercurrent can exist in
the absence of phase bias or, equivalently, an anomalous phase shift
$\varphi_0$ can exist in the absence of a superfluid current. We report on the
observation of an anomalous phase shift $\varphi_0$ in hybrid Josephson
junctions fabricated with the topological insulator Bi$_2$Se$_3$ submitted to
an in-plane magnetic field. This anomalous phase shift $\varphi_0$ is observed
directly through measurements of the current-phase relationship in a Josephson
interferometer. This result provides a direct measurement of the spin-orbit
coupling strength and open new possibilities for phase-controlled Josephson
devices made from materials with strong spin-orbit coupling.
