  Measurement techniques based upon the Hall effect are invaluable tools in
condensed matter physics. When an electric current flows perpendicular to a
magnetic field, a Hall voltage develops in the direction transverse to both the
current and the field. In semiconductors, this behaviour is routinely used to
measure the density and charge of the current carriers (electrons in conduction
bands or holes in valence bands) -- internal properties of the system that are
not accessible from measurements of the conventional resistance. For strongly
interacting electron systems, whose behaviour can be very different from the
free electron gas, the Hall effect's sensitivity to internal properties makes
it a powerful tool; indeed, the quantum Hall effects are named after the tool
by which they are most distinctly measured instead of the physics from which
the phenomena originate. Here we report the first observation of a Hall effect
in an ultracold gas of neutral atoms, revealed by measuring a Bose-Einstein
condensate's transport properties perpendicular to a synthetic magnetic field.
Our observations in this vortex-free superfluid are in good agreement with
hydrodynamic predictions, demonstrating that the system's global
irrotationality influences this superfluid Hall signal.
