Locked Subharmonic Oscillations in the Entanglement Spectrum of a Periodically Driven Topological Chain

Quantum Physics [Submitted on 8 Apr 2026] Locked Subharmonic Oscillations in the Entanglement Spectrum of a Periodically Driven Topological Chain Rishabh Jha Periodically driven quantum systems can exhibit subharmonic response, usually characterized through physical observables and often discussed in interacting settings. Here we show that a sharp subharmonic signature already appears in the entanglement spectrum of a number-conserving free-fermion system. We study a two-step driven Su-Schrieffer-Heeger chain whose Floquet operator supports symmetry-protected edge modes at quasienergies 0 and \pi. When the initial state is a coherent superposition of these two edge sectors, the subsystem correlation matrix alternates between two stroboscopic structures, and an overlap-tracked single-particle entanglement level distills a robust period-doubling response with Fourier weight concentrated at half the drive frequency. By contrast, diagonal edge densities remain flat by sublattice symmetry, while an off-diagonal edge-bond observable provides the corresponding linear one-body comparator. The effect disappears both when the initial state is replaced by a stroboscopically stationary Floquet eigenstate built from the same topological mode content, and when the system is placed in the topologically trivial phase where no edge modes exist. Altogether, these establish zero-\pi Floquet topology as a necessary condition and coherent nonequilibrium preparation as the additional sufficient ingredient. Our results identify entanglement spectroscopy as a sharp subsystem-resolved probe of Floquet topological coherence. Comments: 5+25 pages, 2+6 figures, 0+3 tables Subjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech) Cite as: arXiv:2604.07442 [quant-ph]   (or arXiv:2604.07442v1 [quant-ph] for this version)   https://doi.org/10.48550/arXiv.2604.07442 Focus to learn more Submission history From: Rishabh Jha [view email] [v1] Wed, 8 Apr 2026 18:00:01 UTC (3,220 KB) Access Paper: HTML (experimental) view license Current browse context: quant-ph < prev   |   next > new | recent | 2026-04 Change to browse by: cond-mat cond-mat.stat-mech References & Citations INSPIRE HEP NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)