  In electronic solids with strong spin-orbit interactions (SOIs), the spin and
orbital degrees of freedom of an electron are quantum mechanically entangled,
which may result in an exotic multipolar order instead of a conventional
dipolar order such as a magnetic order. Such a higher-degree order is called
"hidden order" because of difficulties in experimental detection. Moreover, the
number of candidate compounds is limited, especially rare in d electron
systems, in which an interplay between SOIs and Coulomb interactions is
expected to cause rich physics. Here, we employ state-of-the-art synchrotron
X-ray diffraction techniques on a high-quality single crystal to probe subtle
symmetry breaking induced by a multipolar order. We unequivocally demonstrate
that the double-perovskite Ba2MgReO6 exhibits successive transitions to
quadrupolar and then dipolar orders upon cooling, which is consistent with a
theory considering SOIs. Our findings are a significant step towards
understanding the intriguing physics of multipoles realized by
spin-orbit-entangled 5d electrons.
