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Symmetry

Aspects of Symmetry in Nuclear Quantum Computing Variational Algorithms

P. D. Stevenson, Chandan Sarma, Lloyd La Ronde, Serkan Akkoyun, Bhoomika Maheshwari, Piet Van Isacker

Published Oct 15, 2026• 0 citations
Publisher

Abstract

Variational Quantum Eigensolvers (VQEs) have become a standard tool for use in preparing eigenstates in quantum simulations of many-body systems. In the case of the nuclear shell model, various adaptations of the VQE have been shown to be able to produce ground and excited states of nuclei. The VQE algorithm works by seeking minima in a cost function, which is often the expectation value of the energy. For a well-formulated problem, symmetries that are respected by the Hamiltonian should appear in the final converged solution provided by the VQE, even if those symmetries are broken by the wave function ansatz. We demonstrate explicit appearance of the particle–hole symmetry, and explicit recovery of angular momentum conservation due to rotational symmetry, during VQE runs in the case of the nuclear shell model. We conduct analyses of how the symmetry convergence appears in different ansatz formulations, and suggest that information obtained could be used in future VQE design.

DOI
10.3390/sym18101716

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Variational Quantum Eigensolvers (VQEs) have become a standard tool for use in preparing eigenstates in quantum simulations of many-body systems. In the case of the nuclear shell model, various adaptations of the VQE have been shown to be able to produce ground and excited states of nuclei. The VQE algorithm works by seeking minima in a cost function, which is often the expectation value of the energy. For a well-formulated problem, symmetries that are respected by the Hamiltonian should appear in the final converged solution provided by the VQE, even if those symmetries are broken by the wave function ansatz. We demonstrate explicit appearance of the particle–hole symmetry, and explicit recovery of angular momentum conservation due to rotational symmetry, during VQE runs in the case of the nuclear shell model. We conduct analyses of how the symmetry convergence appears in different ansatz formulations, and suggest that information obtained could be used in future VQE design.
Abstract This study presents a simulated quantum computing approach for the investigation into the shell-model energy levels of58Ni through the application of the variational quantum eigensolver (VQE) method in combination with a problem-specific ansatz. The primary objective is to achieve a fully accurate low-lying energy spectrum of58Ni. The chosen isotope,58Ni is particularly interesting in nuclear physics through its role in astrophysical reactions while also being a simple but non-trivial nucleus for shell-model study, it being two particles outside a closed shell. Our ansatz, along with the VQE method are shown to be able to reproduce exact energy values for the ground state and first and second excited states. We compare a classical shell model code, the values obtained by diagonalization of the Hamiltonian after qubit mapping, and a noiseless simulated ansatz+VQE simulation. The exact agreement between classical and qubit-mapped diagonalization shows the correctness of our method, and the high accuracy of the simulation means that the ansatz is suitable to allow a full reconstruction of the full nuclear wave function.
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Quantum computing of the 6^66Li nucleus via ordered unitary coupled cluster
The variational quantum eigensolver (VQE) is an algorithm to compute ground and excited state energy of quantum many-body systems. A key component of the algorithm and an active research area is the construction of a parametrized trial wave function—a so-called variational ansatz. The wave function parametrization should be expressive enough, i.e., represent the true eigenstate of a quantum system for some choice of parameter values. On the other hand, it should be trainable, i.e., the number of parameters should not grow exponentially with the size of the system. Here, we apply VQE to the problem of finding ground and excited state energies of the odd-odd nucleus Li6 . We study the effects of ordering fermionic excitation operators in the unitary coupled clusters ansatz on the VQE algorithm convergence by using only operators preserving the Jz quantum number. The accuracy is improved by two orders of magnitude in the case of descending order. We first compute optimal ansatz parameter values using a classical state-vector simulator with arbitrary measurement accuracy and then use those values to evaluate energy eigenstates of Li6 on a superconducting quantum chip from IBM. We post-process the results by using error mitigation techniques and are able to reproduce the exact energy with an error of 3.8% and 0.1% for the ground state and for the first excited state of Li6 , respectively.
We demonstrate the importance of symmetries in Variational Quantum Eigensolver (VQE) algorithms to prepare the ground or specific low-lying states of quantum Hamiltonians. We examine two spin problems, one with random all-to-all couplings inspired by neutrino flavor evolution in supernovae, and the standard Heisenberg spin Hamiltonian on a 4×34 \times 34×3 lattice. The neutrino Hamiltonian has the total spin JJJ and third component JzJ_{\rm{z}}Jz​ as its only symmetries. The Heisenberg model has these symmetries plus translational invariance and reflection symmetry. We demonstrate that the convergence of variational methods is dramatically improved by keeping all symmetries. In both cases a nearly exact solution can be obtained in cases where standard unconstrained variational algorithms fail. Since variational algorithms can use standard Trotter steps as part of the optimization, allowing additional correlations that obey all the symmetries of the Hamiltonian will speed convergence of variational algorithms. This will lead to faster convergence than standard projection algorithms.
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In this work, we compare three qubit-mapping strategies to study the structure of the nuclear ground state within the shell model description employing the Variational Quantum Eigensolver (VQE) approach. Although the initial point for different mappings is a Hamiltonian matrix in many-body particle basis or Slater determinant (SD) basis, the structure of the trial wavefunction and resource counts are different for each mapping. These three mappings are tested for a mid ppp-shell nucleus 10^{10}10B and compared the quantum resources required to find the ground state for each mapping. Further, we extend the qubit-efficient mapping to study the ground state of one more mid ppp-shell nucleus 12^{12}12C. We run circuits up to 26-qubits representing their ground states on a noisy simulator (IBM's FakeFez backend) and quantum hardware (ibm_fezibm\_fezibm_fez). The best post-error mitigated results from the hardware for 10^{10}10B ground state is obtained following SD to qubit mapping with a percent error of 0.21 \%. The percent errors for the same state following cSD and pnSD mapping are 3.37 and 8.88 \%, respectively. On the other hand, following the cSD mapping, the post-error mitigated ground state energy of 12^{12}12C is 6.82 \% away from the exact result. We further evaluate the fidelity of the VQE wavefunctions obtained from hardware with respect to the shell model wavefunctions for the cSD mapping. This cSD mapping can be useful for scaling the VQE algorithm for complex nuclei across different mass regions in terms of qubit efficiency.