Hybrid QPE-Ansatz Strategy for Reliable Excited-State Variational Quantum Deflation

TL;DR

The paper presents a symmetry-preserving ansatz and a shallow quantum phase estimation routine combined into a scheme for improved excited-state calculations on NISQ devices, effectively reducing spin contamination.

Contribution

It introduces a modular, ancilla-assisted symmetry screening method compatible with various VQE-based excited-state algorithms for NISQ quantum computers.

Findings

Enhanced separation of singlet and triplet states in LiH and BeH₂.

The screening method reduces spin contamination without costly total spin evaluations.

Applicable to other conserved quantities beyond spin.

Abstract

We introduce a spin $z$-component ($S_{z}$) conserving symmetry-preserving ansatz and a shallow quantum phase estimation (QPE) routine of spin $x$ ($S_x$), and combine them into a spin-filtering variational quantum deflation (sfVQD) scheme for noisy intermediate-scale quantum (NISQ) computing era excited state calculations. The scheme encodes the spin information into a small ancilla register through controlled rotations under $\mathrm{exp} (i\theta\hat{S}_{x})$ with only modest circuit overhead. The encoded information is then utilized to suppress spin contamination by screening, avoiding costly explicit evaluation on the total spin $\langle\hat{S}^{2}\rangle$. Because the screening module operates independently of the variational ansatz, it can also be employed with other excited-state calculation schemes based on variational quantum eigensolvers. As a demonstration, we apply sfVQD to LiH and BeH$_2$ with varying geometries to show markedly improved separation of singlet and triplet manifolds over conventional VQD without QPE-derived screening. These results suggest that ancilla-assisted symmetry screening provides a modular and NISQ-compatible route to securing excited state calculations of physically meaningful properties. We discuss how our scheme may naturally be extended to computing other conserved quantities.