Exploiting many-body localization for scalable variational quantum simulation
1HK Institute of Quantum Science $\&$ Technology, The University of Hong Kong, Hong Kong, China
2Dahlem Center for Complex Quantum Systems, Freie Universität Berlin, 14195 Berlin, Germany
3Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA
4Department of Computer Science, University of Wisconsin-Madison, Madison, WI 53706, USA
5Theory of Quantum Matter Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa 904-0412, Japan
6Theoretical Sciences Visiting Program (TSVP), Okinawa Institute of Science and Technology Graduate University, Onna, 904-0495, Japan
| Published: | 2025-12-12, volume 9, page 1942 |
| Editor: | Marco Cerezo |
| Eprint: | arXiv:2404.17560v6 |
| Doi: | https://doi.org/10.22331/q-2025-12-12-1942 |
| Citation: | Quantum 9, 1942 (2025). |
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Abstract
Variational quantum algorithms (VQAs) represent a promising pathway toward achieving practical quantum advantage on near-term hardware. Despite this promise, for generic, expressive ansätze, their scalability is critically hindered by barren plateaus—regimes of exponentially vanishing gradients. We demonstrate that initializing a hardware-efficient, Floquet-structured ansatz within the many-body localized (MBL) phase mitigates barren plateaus and enhances algorithmic trainability. Through analysis of the inverse participation ratio, entanglement entropy, and a novel low-weight stabilizer Rényi entropy, we characterize a distinct MBL--thermalization transition. Below a critical kick strength, the circuit avoids forming a unitary 2-design, exhibits robust area-law entanglement, and maintains non-vanishing gradients. Leveraging this MBL regime facilitates the efficient variational preparation of ground states for several model Hamiltonians with significantly reduced computational resources. Crucially, experiments on a 127-qubit superconducting processor provide evidence for the preservation of trainable gradients in the MBL phase for a kicked Heisenberg chain, validating our approach on contemporary noisy hardware. Our findings position MBL-based initialization as a viable strategy for developing scalable VQAs and motivate broader integration of localization into quantum algorithm design.

Featured image: Exploiting the MBL phase for scalable quantum optimization. The top panel illustrates the transition controlled by the kick strength W: weak kicks keep the circuit in a many-body localized (MBL) phase with area-law entanglement, while strong kicks drive it into a thermal phase with volume-law entanglement and barren plateaus. The bottom panel depicts the corresponding optimization landscape. A low-complexity trial state captures essential structure of the target ground state; initializing the Floquet circuit within the MBL regime preserves this physical information, ensuring that the optimizer starts in a gradient-rich region and can efficiently converge to the solution.
Popular summary
We turn a phenomenon from many-body physics into a practical cure. We design a hardware-efficient, periodically driven (“Floquet”) ansatz whose behaviour is controlled by a single “kick strength.” For weak kicks, the circuit lives in a many-body localized (MBL) regime: quantum information stays local, entanglement follows an area law and the dynamics avoid scrambling into effectively random states. In this localized regime we find that gradients remain sizable, so the VQA is actually trainable. As the kick strength increases, the same circuit crosses sharply into a thermalizing phase with volume-law entanglement, rapid approach to unitary 2-design behaviour and the onset of barren plateaus.
We diagnose this localization–thermalization transition using the inverse participation ratio and entanglement entropy, and we introduce a new low-weight stabilizer Rényi entropy that reveals higher-order randomness using only local Pauli measurements. We use the MBL regime only for initialization; subsequent optimization moves the circuit away from strict localization while still avoiding the fully random 2-design regime where plateaus dominate. Initializing Variational Quantum Eigensolver (VQE) runs inside the MBL regime enables efficient preparation of ground states for several model Hamiltonians, substantially outperforming standard random initialization. Experiments on a 127-qubit superconducting processor observe the predicted restoration of gradients in a kicked Heisenberg chain of up to 31 qubits. Together, these results establish localization-based initialization as a robust and scalable route to training VQAs.
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