An international research team has developed a new quantum-classical framework for computing quantum many-body structure and dynamics. The findings were published as a Letter in Physical Review C.

Ab initio many-body calculations serve as a fundamental tool for understanding strongly correlated quantum systems, with broad applications in quantum chemistry and nuclear physics. However, the Hilbert space of such systems grows exponentially with the number of particles, creating a major bottleneck for classical computers.

Quantum computing is regarded as a promising technology for overcoming this "exponential wall". Nevertheless, most existing quantum algorithms are restricted to determining the structural information such as the complete bound-state spectrum and total angular momentum. In addition, traditional encoding schemes often require prohibitive circuit-compilation overhead, which severely limits their practical application to realistic nuclear many-body problems.

To address this challenge, a theoretical physics team at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), together with collaborators, developed a universal quantum-classical framework for calculating Green's functions and spectral functions of nuclear many-body systems.

The team introduced a new quantum encoding scheme for multi-fermion Hamiltonians that bypasses the heavy compilation overhead of traditional methods. This encoding scheme preserves the Hamiltonian's intrinsic symmetries while requiring only a low gate count. Employing this encoding scheme, the team proposed a new quantum-classical framework.

To validate the effectiveness of the framework, the researchers applied the quantum algorithm to a nuclear many-body Hamiltonian with realistic nucleon-nucleon interactions and obtained the full bound-state spectrum of oxygen-20, together with the corresponding total angular momentum quantum numbers. Quantum-simulation results obtained on a classical simulator showed good agreement with classical full-configuration interaction calculations and were also consistent with available experimental data.

Featuring both universality and scalability, this framework paves the way for ab initio investigations of nuclear structure and dynamics on future fault-tolerant quantum computers, and can be directly extended to study strongly correlated many-body problems across various research fields.

This work was carried out jointly by IMP, the Guangdong Laboratory of Advanced Energy Science and Technology, Iowa State University, and Lawrence Berkeley National Laboratory.

DOI: https://doi.org/10.1103/1spg-5ld3

Figure. (a) Schematic illustration of the quantum-classical hybrid algorithm framework. (b) Excitation energies and total angular momentum J values of the energy eigenstates of oxygen-20 calculated based on this framework, and their comparison with classical full configuration interaction results and experimental values. (c-f) Schematic illustrations of spectral-function scans of oxygen-20 under different total angular momentum projections and energy resolutions. (Image from IMP)