arXiv subjects

Shubhanshu Karoliya

Publications and source records attributed to Shubhanshu Karoliya.

4 recordsLinked to original sources

arXiv2026

Majorana modes in graphene strips: polarization, wavefunctions, disorder, and Andreev states

Topologically protected Majorana zero modes (MZMs) have attracted intense interest due to their potential application in fault-tolerant quantum computation (TQC). Graphene nanoribbons, with tunable edge terminations and compatibility with planar device architectures, offer a promising alternative to semiconductor nanowires. Here we present a comprehensive theoretical study of finite graphene strips with armchair, zigzag, and nearly square geometries, proximitized by an s-wave superconductor and subject to Rashba spin-orbit coupling, Zeeman fields, and disorder. Using exact diagonalization of the Bogoliubov-de Gennes tight-binding Hamiltonian, we analyze Majorana polarization, low-energy spectra, and real-space wavefunctions to identify the non-trivial topological phases supporting MZMs and distinguish them from from partially separated Andreev bound states (psABS) or the quasi-Majoranas. We systematically chart the robustness of these modes across geometries and disorder regimes, finding that armchair strips with short zigzag edges provide the most stable platform. Our results unify polarization diagnostics with spatial wavefunction analysis and disorder effects, yielding concrete design guidelines for graphene-based topological superconductors.

cond-mat.mes-hall↗

arXiv2026

Strain-controlled crossover between Majorana and Andreev bound states in disordered superconductor-semiconductor heterostructures

The unambiguous identification of topological Majorana-bound states (MBSs) in superconducting hybrid systems is hindered by trivial low-energy excitations, especially partially separated Andreev bound states (psABSs), which can mimic Majorana signatures. Here we show that spatially nonuniform strain offers a systematic route to control and interconvert these low-energy states. Using tight-binding Bogoliubov--de Gennes simulations, we study one-dimensional semiconductor nanowires and graphene nanoribbons with superconductivity, Rashba spin-orbit coupling, Zeeman fields, and disorder. We find that even weak strain can qualitatively reshape the low-energy spectrum by modifying effective band parameters and redistributing wavefunction weight. In nanowires, strain tunes the spatial overlap of Majorana components and shifts the topological phase boundary, enabling controlled crossovers between trivial states, psABSs, and topological MBSs. In graphene nanoribbons, where multiband effects and edge states produce a dense, hybridized low-energy spectrum, strain suppresses subband mixing, lifts degeneracies, and stabilizes boundary-localized modes. In both platforms, we identify regimes where disorder-induced psABSs are converted into well-separated and robust MBSs through strain-enhanced nonlocality. We further develop an analytical framework based on a position-dependent topological mass and strain-driven domain-wall motion, which captures the physical mechanism of these crossovers and yields a real-space criterion for the emergence and stability of Majorana modes. Our results establish strain as an effective tuning parameter for distinguishing and stabilizing topological MBSs in realistic disordered systems, and suggest an experimentally relevant pathway toward improved control and identification of Majorana modes in complex hybrid structures relevant to topological quantum computation.

cond-mat.mes-hall↗

arXiv2026

A Preparation Nonstationarity Loophole in Superconducting-Qubit Bell Tests

Bell or Clauser-Horne-Shimony-Holt (CHSH) tests on superconducting quantum processors are commonly interpreted under the assumption that repeated circuit executions sample a single, stationary preparation ensemble. Here we show that this assumption can be violated on contemporary hardware, with direct implications for the interpretation of observed Bell violations. We introduce an ensemble-divergence framework in which slow temporal drift of the preparation process induces context-dependent effective ensembles, even when measurement independence and locality are preserved. This leads to a relaxed Bell bound $|S| \le 2 + 6δ_{\mathrm{ens}}$, where $δ_{\mathrm{ens}}$ quantifies preparation nonstationarity. Because $δ_{\mathrm{ens}}$ is not directly observable, we develop an operational witness $δ_{\mathrm{op}}$ based on bin-resolved outcome statistics for fixed measurement channels. Using Pauli-axis measurements on IBM superconducting processors, we observe statistically significant operational drift that persists after full two-qubit readout mitigation, ruling out measurement artifacts. In contrast, drift extracted from CHSH-optimal measurements is eliminated by mitigation, demonstrating that such settings are unsuitable for diagnosing preparation nonstationarity. We further show that the observed Bell violations imply only modest ensemble divergences, comparable in scale to those required in Hall-type measurement-dependence models, but arising here solely from preparation drift combined with experimental scheduling. Our results identify a preparation-dependent loophole relevant to Bell tests on noisy intermediate-scale quantum devices and highlight the necessity of drift-aware protocols for reliable quantum certification.

quant-ph↗

arXiv2025

Majorana polarization in disordered heterostructures

Recent studies propose Majorana polarization (MP) as a tool for identifying topological Majorana bound states (MBS). We analyze MP in two systems: a one-dimensional (1d) semiconducting nanowire and a quasi-1d system, both with Rashba spin-orbit coupling, proximity-induced superconductivity, and disorder. While MP reflects topological features, it does not always distinguish true MBS from partially separated Andreev bound states (psABS) or quasi-Majorana modes. True MBS are expected to satisfy $\mathcal{P}^*_\mathrm{left} \cdot \mathcal{P}_\mathrm{right} \sim -1$, but two additional criteria - a topological bandgap and edge-localized wavefunctions - are also necessary for their identification and use in topological quantum computation (TQC). We highlight cases where the polarization product condition holds without meeting these extra requirements. Our conclusions remain robust irrespective of the chosen definition of Majorana polarization, whether based on chiral or the particle-hole framework. Robust MBS identification for TQC demands combining MP with energy spectra and wavefunction localization, especially in disordered systems.

cond-mat.mes-hall↗