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  • Open Access

Imaginary-time-enhanced feedback-based quantum algorithms for ground-state preparation

Thanh Nguyen Van Long1,2, Lan Nguyen Tran1,2, and Le Bin Ho3,4,*

  • *Contact author: ho.bin.le.e3@tohoku.ac.jp
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Phys. Rev. Research 8, 043121 – Published 2 November, 2026

DOI: https://doi.org/10.1103/cbcv-fnm2

Abstract

Preparing ground states of strongly correlated quantum systems is a central goal in quantum simulation and optimization. The feedback-based quantum algorithm (FALQON) provides an attractive alternative to variational methods with a fully quantum feedback rule, but it fails in the presence of spectral degeneracies, where the feedback signal collapses and the evolution cannot reach the ground state. Using the Fermi-Hubbard model on lattices up to 3×3, we show that this breakdown appears at half filling on the 2×2 lattice and extends to both half-filled and doped configurations on the 3×3 lattice. We then introduce an imaginary-time-enhanced FALQON (iFALQON) scheme, which inserts short imaginary-time evolution steps into the feedback loop. This proposed method suppresses excited-state components, escapes degenerate subspaces, restores monotonic energy descent, and improves ground-state convergence across the tested fillings. We further benchmark its performance against existing methods, showing improved results for iFALQON, demonstrating the potential of hybrid real-imaginary-time feedback dynamics for ground-state preparation in strongly correlated quantum systems.

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Phys. Rev. Research 8, 043121– Published 2 November, 2026
  • Received 16 December 2025
  • Revised 25 January 2026
  • Accepted 21 October 2026
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