Simulating X-ray absorption spectroscopy of battery materials on a quantum computer

TL;DR

The paper tackles the challenge of simulating X-ray absorption spectra for battery materials using quantum computers, exploiting the ultralocal nature of near-edge XAS to keep problem sizes small. It presents three quantum algorithms—frequency-domain Green's function, quantum phase estimation sampling, and a time-domain Monte Carlo method—and discusses how to isolate core-excited states via CVS or filtering. Resource estimates for a CAS(22e,18o) Li2MnO3 cluster suggest substantial but potentially feasible requirements on early fault-tolerant hardware, with the time-domain approach offering hardware-friendly advantages. The work demonstrates a pathway toward ab initio XAS simulations to fingerprint oxidation states and guide design of Li-excess cathodes, potentially accelerating materials development for high-capacity batteries.

Abstract

X-ray absorption spectroscopy is a crucial experimental technique for elucidating the mechanisms of structural degradation in battery materials. However, extracting information from the measured spectrum is challenging without high-quality simulations. In this work, we propose simulating near-edge X-ray absorption spectra as a promising application for quantum computing. It is attractive due to the ultralocal nature of X-ray absorption that significantly reduces the sizes of problems to be simulated, and because of the classical hardness of simulating spectra. We describe three quantum algorithms to compute the X-ray absorption spectrum and provide their asymptotic cost. One of these is a Monte-Carlo based time-domain algorithm, which is cost-friendly to early fault-tolerant quantum computers. We then apply the framework to an industrially relevant example, a CAS(22e,18o) active space for an O-Mn cluster in a Li-excess battery cathode, showing that practically useful simulations could be obtained with much fewer qubits and gates than ground-state energy estimation of the same material.

Figures (6)

• Figure 1: In order to overcome poor recharge performance of Li-excess batteries, it is necessary to understand the redox processes occurring in the cathode. XAS is the key experimental approach to gaining this information, but simulations are required to interpret the results. Quantum computing can help with this challenge by providing accurate simulations of X-ray spectra that are inaccessible to classical simulation methods. Through spectral fingerprinting, these simulations yield information such as oxidation states, that can point the way towards better cathode materials and battery designs.
• Figure 2: Quantum circuit implementing the frequency-domain Green's function approach to the simulation of X-ray absorption spectra. Having prepared the normalized initial state $\mathbf{m}\left|I\right\rangle/\|\mathbf{m}\left|I\right\rangle\|$, we use $d+1$ ancillas to perform QPE with a controlled rotation to invert the resolvent $(H - \omega + i\eta)^{-1}$. A global Hadamard test then yields the imaginary part of the Green's function $G(\omega)$ per choice of the gate $S^{\dagger}$ on the Hadamard ancilla as shown in the circuit.
• Figure 3: Quantum circuit implementing the QPE-based approach to X-ray absorption spectrum simulation. Performing QPE on the initial state $\mathbf{m}\left|I\right\rangle/\|\mathbf{m}\left|I\right\rangle\|$ directly reconstructs the absorption spectrum $\propto\text{Im}\,G(\omega)$.
• Figure 4: Quantum circuit implementing the time-domain Monte Carlo sampling based algorithm for X-ray absorption. Starting with the normalized initial state $\mathbf{m}\left|I\right\rangle/\|\mathbf{m}\left|I\right\rangle\|$, single-ancilla-controlled time evolution yields the real (imaginary) part of the Green's function in the time domain $G(t)$ via a Hadamard test for $W = I$ ($W = S^{\dagger}$). The evolution times are determined through Monte Carlo sampling: see the text for details.
• Figure 5: Illustration of the core-excited state problem. There are numerous low-lying excited states featuring mostly valence electron excitations: however, they appear on the same footing as the core-excited states which define the X-ray absorption spectrum.