  Coherent control of reactants remains a longstanding challenge in quantum
chemistry. In particular, we have studied laser-induced molecular formation
(photoassociation) in a Raman-dressed spin-orbit-coupled 87Rb Bose-Einstein
condensate, whose spin quantum state is a superposition of multiple bare spin
components. In contrast to the notably different photoassociation-induced
fractional atom losses observed for the bare spin components of a statistical
mixture, a superposition state with a comparable spin composition displays the
same fractional loss on every spin component. We interpret this as the
superposition state itself undergoing photoassociation. For superposition
states induced by a large Raman coupling and zero Raman detuning, we observe a
nearly complete suppression of the photoassociation rate. This suppression is
consistent with a model based upon quantum destructive interference between two
photoassociation pathways for colliding atoms with different spin combinations.
This model also explains the measured dependence of the photoassociation rate
on the Raman detuning at a moderate Raman coupling. Our work thus suggests that
preparing atoms in quantum superpositions may represent a powerful new
technique to coherently control photochemical reactions.
