arXiv subjects

Sven Benjamin Kožić

Publications and source records attributed to Sven Benjamin Kožić.

5 recordsLinked to original sources

arXiv2026

Exploring the Effect of Basis Rotation on NQS Performance

Neural Quantum States (NQS) are powerful variational representations of quantum many-body wavefunctions, yet their performance depends sensitively on the chosen basis. Using an exactly solvable one-dimensional Ising model, we show that local basis rotations leave the minimization landscape unchanged while relocating the exact ground state in parameter space. This provides a controlled framework to disentangle representational limitations from optimization-induced trainability effects. This geometric displacement, quantified through information-geometric measures, can steer optimization of shallow architectures toward saddle points and high-curvature regions. As a result, low energy errors may coexist with an incorrect wavefunction structure. By comparing energy and infidelity optimization within the same variational architectures, we show that optimization failure can persist even when the rotated target state remains representable. Our results identify a geometric mechanism contributing to basis dependence in NQS and motivate landscape-aware variational design.

quant-ph↗

arXiv2025

Resource complexity of Symmetry Protected Topological phases

We pursue the identification of quantum resources carried by topological order, by evaluating quantum magic, quantified through the rank-$2$ Stabilizer Rényi entropy $\mathcal{M}_2$, in one-dimensional systems hosting symmetry-protected topological phases (SPTP). Focusing on models with an exact duality between an SPTP and a trivial one, namely the dimerized XX and the Cluster-Ising chains, we show that dual points exhibit identical amounts of magic, even thought they belong to distinct topological sectors. A subextensive asymmetry arises only under open boundary conditions, where edge effects break the duality, but this correction is non-topological and depends on microscopic parameters. These results stand in contrast to the case of topological frustration, where delocalized excitations enhance the magic logarithmically with system size. They also complement recent analyses in the literature, showing that the total magic is largely insensitive to the presence of topological order, hence suggesting that topological order is not necessarily a genuine computational resource.

quant-ph↗

arXiv2025

A new rung on the ladder: exploring topological frustration towards two dimensions

Topological frustration arises when boundary conditions impose geometric frustration in a quantum system, creating delocalized defects in the ground states and profoundly altering the low-energy properties. While previous studies have been concerned with one-dimensional systems, showing that the ground state structure can be described in terms of quasiparticle excitations, the two-dimensional setting remains unexplored. We address this gap by studying a three-legged antiferromagnetic quantum Ising ladder on a torus using tensor network methods, where topological frustration is induced by an odd number of spins along both spatial directions. Our results reveal the first instance in which topological frustration shifts the position of the quantum critical point. By studying the entanglement structure, we find that the ground state can be characterized as hosting three delocalized quasiparticles. This work builds the quasiparticle picture of topological frustration toward higher dimensions and more complex systems than those considered so far.

cond-mat.str-el↗

arXiv2025

Computing Quantum Resources using Tensor Cross Interpolation

Quantum information quantifiers are indispensable tools for analyzing strongly correlated systems. Consequently, developing efficient and robust numerical methods for their computation is crucial. We propose a general procedure based on the family of Tensor Cross Interpolation (TCI) algorithms to address this challenge in a fully general framework, independent of the system or the quantifier under consideration. To substantiate our approach, we compute the non-stabilizerness Rényi entropy (SRE) and Relative Entropy of Coherence (REC) considering the 1D and 2D ferromagnetic Ising models with minimal modifications to the numerical procedure. This method not only demonstrates its versatility, but also provides a generic framework for exploring other quantum information quantifiers in complex systems.

quant-ph↗

arXiv2024

Interplay between local and non-local frustration in the 1D ANNNI chain I -- The even case

We consider the effects of the competition between different sources of frustration in 1D spin chains through the analysis of the paradigmatic ANNNI model, which possesses an extensive amount of frustration of local origin due to the competition between nearest and next-to-nearest neighbor interactions. An additional, non-extensive amount of topological frustration can be added by applying suitable boundary conditions, and we show that this seemingly subdominant contribution significantly affects the model. Choosing periodic boundary conditions with an {\it even} number of sites not divisible by 4 and using the entanglement entropy as a probe, we demonstrate that in one of the model's phases, the ground state can be characterized as hosting two (almost) independent excitations. Thus, not only do we show an intriguing interplay between different types of frustration, but also manage to propose a non-trivial quasi-particle interpretation for it.

cond-mat.str-el↗