  We introduce a general definition of a quantum committor in order to clarify
reaction mechanisms and facilitate control in processes where coherent effects
are important. With a quantum committor, we generalize the notion of a
transition state to quantum superpositions and quantify the effect of
interference on the progress of the reaction. The formalism is applicable to
any linear quantum master equation supporting metastability for which absorbing
boundary conditions designating the reactant and product states can be applied.
We use this formalism to determine the dependence of the quantum transition
state on coherences in a polaritonic system and optimize the initialization
state of a conical intersection model to control reactive outcomes, achieving
yields of the desired state approaching 100%. In addition to providing a
practical tool, the quantum committor provides a conceptual framework for
understanding reactions in cases when classical intuitions fail.
