  Quantum dynamics of the radical pair mechanism is a major driving force in
quantum biology, materials science, and spin chemistry. The rich quantum
physical underpinnings of the mechanism are determined by a coherent
oscillation (quantum beats) between the singlet and triplet spin states and
their interactions with the environment, which is challenging to experimentally
explore and computationally simulate. In this work, we take advantage of
quantum computers to simulate the Hamiltonian evolution and thermal relaxation
of two radical pair systems undergoing the quantum-beat phenomena. We study
radical pair systems with nontrivial hyperfine coupling interactions, namely,
9,10-octalin+/p-terphenyl-d14 and 2,3-dimethylbutane/p-terphenyl-d14 that have
one and two groups of magnetically equivalent nuclei, respectively. Thermal
relaxation dynamics in these systems are simulated using three methods: Kraus
channel representations, noise models on Qiskit Aer and the inherent qubit
noise present on the near-term quantum hardware. By leveraging the inherent
qubit noise, we are able to simulate noisy quantum beats in the two radical
pairs better than with any classical approximation or quantum simulator. While
classical simulations of paramagnetic relaxation grow errors and uncertainties
as a function of time, near-term quantum computers can match the experimental
data throughout its time evolution, showcasing their unique suitability and
future promise in simulating open quantum systems in chemistry.
