  The control of chemical reactions is a recurring theme in physics and
chemistry. Traditionally, chemical reactions have been investigated by tuning
thermodynamic parameters, such as temperature or pressure. More recently,
physical methods such as laser or magnetic field control have emerged to
provide completely new experimental possibilities, in particular in the realm
of cold collisions. The control of reaction pathways is also a critical
component to implement molecular quantum information processing. For these
undertakings, single particles provide a clean and well-controlled experimental
system. Here, we report on the experimental tuning of the exchange reaction
rates of a single trapped ion with ultracold neutral atoms by exerting control
over both their quantum states. We observe the influence of the hyperfine
interaction on chemical reaction rates and branching ratios, and monitor the
kinematics of the reaction products. These investigations advance chemistry
with single trapped particles towards achieving quantum-limited control of
chemical reactions and indicate limits for buffer gas cooling of single ion
clocks.
