  It is well known that the charge current in a conductor is proportional to
the applied electric field. This famous relation, known as Ohm's law, is the
result of relaxation of the current due to charge carriers undergoing
collisions, predominantly with impurities and lattice vibrations in the
material. The field of spintronics, where the spin of the electron is
manipulated rather than its charge, has recently also led to interest in spin
currents. Contrary to charge currents, these spin currents can be subject to
strong relaxation due to collisions between different spin species, a
phenomenon known as spin drag. This effect has been observed for electrons in
semi-conductors\cite{Weber} and for cold fermionic atoms, where in both cases
it is reduced at low temperatures due to the fermionic nature of the particles.
Here, we perform a transport experiment using ultra-cold bosonic atoms and
observe spin drag for bosons for the first time. By lowering the temperature we
find that spin drag for bosons is enhanced in the quantum regime due to Bose
stimulation, which is in agreement with recent theoretical predictions. Our
work on bosonic transport shows that this field may be as rich as transport in
solid-state physics and may lead to the development of advanced devices in
atomtronics.
