  Ultrafast chemical reactions are difficult to simulate because they involve
entangled, many-body wavefunctions whose computational complexity grows rapidly
with molecular size. In photochemistry, the breakdown of the Born-Oppenheimer
approximation further complicates the problem by entangling nuclear and
electronic degrees of freedom. Here, we show that analog quantum simulators can
efficiently simulate molecular dynamics using commonly available bosonic modes
to represent molecular vibrations. Our approach can be implemented in any
device with a qudit controllably coupled to bosonic oscillators and with
quantum hardware resources that scale linearly with molecular size, and offers
significant resource savings compared to digital quantum simulation algorithms.
Advantages of our approach include a time resolution orders of magnitude better
than ultrafast spectroscopy, the ability to simulate large molecules with
limited hardware using a Suzuki-Trotter expansion, and the ability to implement
realistic system-bath interactions with only one additional interaction per
mode. Our approach can be implemented with current technology; e.g., the
conical intersection in pyrazine can be simulated using a single trapped ion.
Therefore, we expect our method will enable classically intractable chemical
dynamics simulations in the near term.
