Exact Excitation Spectra and Spectral Gaps of the Frustrated Spin-1/2 $J_1$-$J_2$ Model via Dual Spin-Sector Variational Quantum Eigensolvers

Abstract

Finding excited states and excitation gaps in frustrated spin systems using Variational Quantum Eigensolvers (VQE) is complicated by state-ordering inversions. In the frustrated spin-1/2 $J_1$-$J_2$ Heisenberg model, standard single-sector algorithms like Variational Quantum Deflation (VQD) in $S_z=0$ fail when $J_2/J_1 < 0.25$. In this regime, the lowest-lying excitation in $S_z=0$ is a Triplet ($S=1$), which lies below the Excited Singlet ($S=0$). Standard deflation projects out the ground singlet and converges onto the Triplet state, misidentifying it as the Excited Singlet. Here, we present a Dual Spin-Sector VQE framework that resolves these state-ordering inversions across 1D spin chains and 2D rectangular lattices. By first optimizing the ground Triplet state in the $S_z=1$ sector (where singlet states cannot exist), mapping it into $S_z=0$ via the total spin-lowering operator $S^- = \sum_i S_i^-$, and evaluating a dual-sector penalty cost function, we isolate the Excited Singlet ($E_S$) without variational ambiguity. Numerical state-vector simulations confirm exact convergence to machine precision with unit overlap fidelity. The method captures key physical features, including the gapless phase, the BKT critical point at $J_2/J_1 \approx 0.2411$, the Majumdar-Ghosh dimer point ($J_2/J_1 = 0.50$), and the 2D non-magnetic frustrated regime ($J_2/J_1 \approx 0.40 - 0.60$). Finite-shot sampling simulations and gate-scaling analytics up to $N=100$ qubits demonstrate the resource efficiency and shot resilience of the algorithm for near-term quantum execution.