  Supersolidity is an intriguing concept. It combines the property of
superfluid flow with the long-range spatial periodicity of solids, two
properties which are often mutually exclusive. The original discussion of
quantum crystals and supersolidity focuses on solid Helium-4 where it was
predicted that vacancies could form dilute weakly interacting Bose-Einstein
condensates. In this system, direct observation of supersolidity has been
elusive. The concept of supersolidity was then generalized to include other
superfluid systems which break the translational symmetry of space. One of such
systems is a Bose-Einstein condensate with spin-orbit coupling which has a
supersolid stripe phase. Despite several recent studies of this system, the
stripe phase has not been observed. Here we report the direct observation of
the predicted density modulation of the stripe phase using Bragg reflection.
Our work establishes a system with unique symmetry breaking properties. Of
future interest is further spatial symmetry breaking through the introduction
of vortices, solitons, impurities or disorder.
