  In chemical reactions, the interplay between coherent evolution and
dissipation is central to determining key properties such as the rate and
yield. Of particular interest are cases where two potential energy surfaces
cross at features known as conical intersections (CIs), resulting in
nonadiabatic dynamics that may promote ultrafast and highly efficient reactions
when rovibrational damping is present. A prominent chemical reaction that
involves a CI is the cis-trans isomerization reaction in rhodopsin, which is
crucial to vision. CIs in real molecular systems are typically investigated via
optical pump-probe spectroscopy, which has demanding spectral bandwidth and
temporal resolution requirements, and where precise control of the environment
is challenging. A complementary approach for understanding chemical reactions
is to use quantum simulators that can provide access to a wider range of
observables, though thus far combining strongly interacting linear
(rovibrational) and nonlinear (electronic) degrees of freedom with engineered
dissipation has yet to be demonstrated. Here, we create a tunable CI in a
hybrid qubit-oscillator circuit QED processor and simultaneously track both a
reactive wave-packet and electronic qubit in the time-domain. We identify
dephasing of the electronic qubit as the mechanism that drives wave-packet
branching along the reactive coordinate in our model. Furthermore, we directly
observe enhanced branching when the wave-packet passes through the CI. Thus,
the forces that influence a chemical reaction can be viewed as an effective
measurement induced dephasing rate that depends on the position of the
wave-packet relative to the CI. Our results set the groundwork for more complex
simulations of chemical dynamics, offering deeper insight into the role of
dissipation in determining macroscopic quantities of interest such as the
quantum yield of a chemical reaction.
