Preparation of excited states for nuclear dynamics on a quantum computer

We study two different methods to prepare excited states on a quantum computer, a key initial step to study nuclear dynamics within linear-response theory. The first method uses unitary evolution for a short time T=O(1-F) to approximate the action of an excitation operator Ô with fidelity F and success probability P≈1-F. The second method probabilistically applies the excitation operator using the linear combination of unitaries (LCU) algorithm. We benchmark these techniques on emulated and real quantum devices, using a toy model for thermal neutron-proton capture. Despite its larger-memory footprint, the LCU-based method is efficient even on current generation noisy devices and can be implemented at a lower gate cost than a naive analysis would suggest. These findings show that quantum techniques designed to achieve good asymptotic scaling on fault-tolerant quantum devices might also provide practical benefits on devices with limited connectivity and gate fidelity.