  Supersolids are states of matter that spontaneously break two continuous
symmetries: translational invariance due to the appearance of a crystal
structure and phase invariance due to phase locking of single-particle wave
functions, responsible for superfluid phenomena. While originally predicted to
be present in solid helium, ultracold quantum gases provided a first platform
to observe supersolids, with particular success coming from dipolar atoms.
Phase locking in dipolar supersolids has been probed through e.g. measurements
of the phase coherence and gapless Goldstone modes, but quantized vortices, a
hydrodynamic fingerprint of superfluidity, have not yet been observed. Here,
with the prerequisite pieces at our disposal, namely a method to generate
vortices in dipolar gases and supersolids with two-dimensional crystalline
order, we report on the theoretical investigation and experimental observation
of vortices in the supersolid phase. Our work reveals a fundamental difference
in vortex seeding dynamics between unmodulated and modulated quantum fluids.
This opens the door to study the hydrodynamic properties of exotic quantum
systems with multiple spontaneously broken symmetries, in disparate domains
such as quantum crystals and neutron stars.
