Searcharxiv⌕ Search
Home/Archive/arXiv

arXiv · 2610.31393

Qubits Are Not Enough: Deflation and Measurement Limits in Variational Quantum Modal Analysis

Bikalpa Gautam·Akil Raj Subedi
View original source ↗Read source document ↗Export metadata ↓

Abstract

Amplitude encoding stores an N-degree-of-freedom structural mode using ceiling(log2 N) qubits. This article develops an evaluation framework for variational quantum eigensolvers in finite-element structural dynamics. A Cholesky transformation maps the generalized eigenproblem to a symmetric mass-orthonormal Hamiltonian, ensuring Euclidean quantum-state orthogonality equals structural mass orthogonality. For higher modes, the deflation penalty condition is sharpened to a necessary-and-sufficient threshold, with a Davis-Kahan bound quantifying effects of approximate projectors. Numerical studies separate mapping correctness, ansatz expressibility, preprocessing, Pauli growth, finite-shot uncertainty, and damage localization. For 128-DOF chains, consistent-mass whitening increases transformed-operator density from 0.023 to 0.308 and Pauli terms from 576 to 2950. Exact state-vector experiments show qubit count and circuit expressibility are distinct. At 10^5 shots per Pauli term, first-mode frequency RMS error is 2.45%. In an oracle-state study, 10% stiffness loss is localized in 84.5% of trials. Results sharpen the deflation condition, quantify resource bottlenecks, and claim no speedup.

Explore related subjects

quant-ph

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Bikalpa Gautam, Akil Raj Subedi. 2026-10-22. Qubits Are Not Enough: Deflation and Measurement Limits in Variational Quantum Modal Analysis. https://arxiv.org/abs/2610.31393

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Computational reach of machine learning models is independent of untrained quantum resources

This paper proves that if an untrained quantum circuit is used as a resource in a machine learning model, then on average no resource quantum circuit is better than any other that can achieve the same set of computational effects. The paper also proves a supporting theorem that even if certain mild idealisations are omitted, the average quantum advantage remains negligible at best. These results cast serious doubt on several proposals to use untrained quantum circuits in machine learning models: at best such claims should be substantiated empirically, as this paper proves there is no a priori theoretical reason to suppose that introducing an untrained quantum circuit will increase performance.

quant-ph↗
arXiv2026

Operator quantum singular value transformation: Spectral mapping, design and topological invariants

Quantum singular value transformation (QSVT) provides a unifying framework for quantum algorithms by applying the polynomials of quantum signal processing (QSP) to block-encoded matrices. We connect this framework with spectral analysis, design and topological protection in condensed matter physics. Extending QSVT beyond matrices to possibly infinite-dimensional operators (operator QSVT), we establish the following. First, it transforms every block of the operator by the QSP polynomials; no singular value decomposition is needed. Second, a spectral mapping theorem determines the full spectrum of the QSVT operator, including continuous spectra, from the embedded operator through the QSP polynomial, allowing spectral analysis without translation invariance. Third, the spectral mapping turns QSP into spectral design: the QSP phases can stretch, fold and separate the bands and place the eigenvalues. Finally, the kernel sectors that conventional QSVT usually sets aside carry a topological index, invariant under the choice of QSP phases, local perturbations and gap-preserving deformations, which guarantees topologically protected states. These results unify two quantum walks: Szegedy's walk from quantum algorithms and the split-step walk from condensed matter physics. Their kernel sectors encode the graph's Euler characteristic and the difference of bulk winding numbers, respectively. For the split-step walk, QSP breaks chiral symmetry but not the topological protection, and can move the protected state into a continuous band to form a bound state in the continuum (BIC).

quant-ph↗
arXiv2026

Quantum theory of molecular orientation

We formulate a quantum phase space for rotational and nuclear-spin states of rigid molecules. For each nuclear spin isomer, we re-derive the isomer's admissible angular momentum states from molecular geometry and nuclear-spin data, introduce its angular position states using quantization theory, and develop a generalized Fourier transform converting between the two. We classify molecules into three types -- asymmetric, rotationally symmetric, and perrotationally symmetric -- with the last type having no macroscopic analogue due to nuclear-spin statistics constraints. We discuss two general features in perrotationally symmetric state spaces that are Hamiltonian-independent and induced solely by symmetry and spin statistics. First, we quantify when and how an isomer's state space is completely rotation-spin entangled, meaning that it does not admit any separable states. Second, we identify isomers whose position states house an internal pseudo-spin or "fiber" degree of freedom, and the fiber's Berry phase or matrix after adiabatic changes in position yields naturally robust operations, akin to braiding anyonic quasiparticles or realizing fault-tolerant quantum gates. We outline how the fiber can be used as a quantum error-correcting code and discuss scenarios where these features can be experimentally probed

quant-ph↗