  We report the experimental observation of tunable, non-reciprocal quantum
transport of a Bose-Einstein condensate in a momentum lattice. By implementing
a dissipative Aharonov-Bohm (AB) ring in momentum space and sending atoms
through it, we demonstrate a directional atom flow by measuring the momentum
distribution of the condensate at different times. While the dissipative AB
ring is characterized by the synthetic magnetic flux through the ring and the
laser-induced loss on it, both the propagation direction and transport rate of
the atom flow sensitively depend on these highly tunable parameters. We
demonstrate that the non-reciprocity originates from the interplay of the
synthetic magnetic flux and the laser-induced loss, which simultaneously breaks
the inversion and the time-reversal symmetries. Our results open up the avenue
for investigating non-reciprocal dynamics in cold atoms, and highlight the
dissipative AB ring as a flexible building element for applications in quantum
simulation and quantum information.
