Predicting molecular vibronic spectra using time-domain analog quantum simulation

Spectroscopy is one of the most accurate probes of the molecular world. However, predicting molecular spectra accurately is computationally difficult because of the presence of entanglement between electronic and nuclear degrees of freedom. Although quantum computers promise to reduce this computational cost, existing quantum approaches rely on combining signals from individual eigenstates, an approach whose cost grows exponentially with molecule size. Here, we introduce a method for scalable analog quantum simulation of molecular spectroscopy: by performing simulations in the time domain, the number of required measurements depends on the desired spectral range and resolution, not molecular size. Our approach can treat more complicated molecular models than previous ones, requires fewer approximations, and can be extended to open quantum systems with minimal overhead. We present a direct mapping of the underlying problem of time-domain simulation of molecular spectra to the degrees of freedom and control fields available in a trapped-ion quantum simulator. We experimentally demonstrate our algorithm on a trapped-ion device, exploiting both intrinsic electronic and motional degrees of freedom, showing excellent quantitative agreement for a single-mode vibronic photoelectron spectrum of SO2.

Automated summary

Spectroscopy is a powerful tool for studying molecular structures, but its accurate prediction is computationally challenging due to entanglement between electronic and nuclear degrees of freedom. Quantum computers have the potential to solve this problem, but existing quantum approaches are limited by the exponential cost of combining signals from individual eigenstates. In this study, we propose a scalable analog quantum simulation method that performs simulations in the time domain, reducing the computational cost to the desired spectral range and resolution, rather than the molecule size. Our approach can handle complex molecular models with fewer approximations and can be extended to open quantum systems with minimal overhead. We demonstrate the effectiveness of our algorithm using a trapped-ion quantum simulator to simulate the vibronic photoelectron spectrum of SO2.