Demonstration of logical quantum phase estimation for X-ray absorption spectra

TL;DR

This paper demonstrates the use of Fourier-based quantum phase estimation to calculate X-ray absorption spectra, including implementation on a trapped-ion quantum computer with noise mitigation techniques, advancing quantum spectroscopy methods.

Contribution

It introduces a practical quantum phase estimation approach for XAS spectra, including noise reduction strategies and implementation on current quantum hardware.

Findings

QPE accurately reproduces XAS spectra in noiseless simulations.

Post-processing reduces statistical error in spectral calculations.

Quantum error detection improves spectra quality on noisy hardware.

Abstract

In this study, we employed Fourier-based quantum phase estimation (QPE) to calculate X-ray absorption spectroscopy (XAS) spectra. The primary focus of this study is the calculation of the XAS spectra of transition metal $L_{2,3}$-edges, which are dominated by strong correlation effects. First, the Fe $L_{2,3}$-edge X-ray absorption near-edge structure of FePO$_4$ is calculated using a noiseless simulator. The present computation involves a comparison of three types of input states: a uniform superposition state, optimal entangled input state, and Slater function state. Subsequently, we investigated the resolution error of the QPE and statistical error attributed to the measurements. It was revealed that post-processing to introduce Lorentzian broadening reduces the statistical error, which becomes a significant problem for a large number of qubits. Subsequently, we implemented QPE on a trapped-ion quantum computer, encompassing three orbitals within the active space. To this end, we implemented QPE using dynamic circuits to reduce ancilla qubits and [[k+2, k, 2]] quantum error detection code to mitigate the quantum noise inherent in current quantum computers. As a result, it was demonstrated that hardware noise was reduced, and spectra close to the noiseless ones were obtained.