arXiv subjects

Yoshito Watanabe

Publications and source records attributed to Yoshito Watanabe.

7 recordsLinked to original sources

arXiv2026

Floquet implementation of a 3d fermionic toric code with full logical code space

Floquet quantum error-correcting codes provide an operationally economical route to fault tolerance by dynamically generating stabilizer structures using only two-body Pauli measurements. But while it is well established that stabilizer codes in higher spatial dimensions gain additional levels of intrinsic robustness, higher-dimensional Floquet codes have hitherto been explored only in limited scope. Here we introduce a 3d generalization of a Floquet code whose instantaneous stabilizer group realizes a 3d fermionic toric code, while crucially preserving all three logical qubits throughout the entire measurement sequence. One central ingredient is the identification of a 3d lattice geometry that generalizes the features of the Kekul\'e lattice underlying the 2d Hastings-Haah code - specifically, a structure where deleting any one edge color yields a two-color subgraph that decomposes into short, closed loops rather than homologically nontrivial chains. This loop property avoids the collapse of logical information that plagues naive sequential two-color measurement schedules on many 3d lattices. Although, for our lattice geometry, a simple 3-round cycle that sequentially measures the three types of parity checks does not expose the full error syndrome set, we show that one can append a measurement sequence to extract the missing syndromes without disturbing the logical subspace. Beyond code design, 3d tricoordinated lattice geometries define a family of 3d monitored Kitaev models, in which random measurements of the non-commuting parity checks give rise to dynamically created entangled phases with nontrivial topology. In discussing the general structure of their underlying phase diagrams and, in particular, the existence of certain quantum critical points, we again make a connection to the general preservation of logical information in time-ordered Floquet protocols.

quant-ph↗

arXiv2025

Two-dimensional coherent spectroscopy of CoNb$_2$O$_6$

With recent advances in terahertz (THz) sources and detection, two-dimensional coherent spectroscopy (2DCS), which allows to probe nonlinear responses in a two-frequency plane, now reaches the meV regime relevant for quasiparticle excitations in magnetic materials. This opens a promising route to reveal many-body phenomena that evade linear-response probes. To date most experimental applications have focused on classical magnets, and a solid demonstration in a quantum magnet has yet to be established. Here we present a theoretical study of 2DCS in CoNb$_2$O$_6$, a quasi-one-dimensional Ising magnet that is believed to host fractionalized spinons which at low temperatures are confined by weak interchain coupling. Our analysis, which builds on an effective $S=1/2$ Hamiltonian is found to reveal unambiguous 2DCS signatures of spinon deconfinement above the low-temperature ordered phase. Using a four-spinon approximation, we track these 2DCS signatures by sequentially building a faithful microscopic model for CoNb$_2$O$_6$, starting from the exactly solvable one-dimensional transverse-field Ising model (1$d$ TFIM) and successively adding interactions to capture its key low-energy physics. In particular, adding a bond-dependent staggered YZ interaction to the 1$d$-TFIM already reproduces many key spectral features of the full material Hamiltonian. Within this TFIM+YZ model, we find a series of bound states, including a four-spinon bound state that is distinct from the familiar two-spinon bound states. We further find that introducing a confinement potential suppresses sharp spinon-echo features in the two-frequency space, which are thought to reflect an underlying continuum of fractionalized excitations. Our results provide concrete predictions and clear targets for future THz 2DCS experiments on CoNb$_2$O$_6$ and related quasi-one-dimensional quantum magnets.

cond-mat.str-el↗

arXiv2025

Symmetry-deformed toric codes and the quantum dimer model

Motivated by the recent introduction of a $U(1)$-symmetric toric code model, we investigate symmetry-based deformations of topological order by systematically deconstructing the Gauss-law-enforcing star terms of the toric code (TC) Hamiltonian. This "term-dropping" protocol introduces global symmetries that go beyond the alternative framework of "ungauging" topological order in symmetry-deformed models and gives rise to models such as the $U(1)$TC or $XY$TC. These models inherit (emergent) subsystem symmetries (from the original 1-form symmetry of the TC) that can give rise to (subextensive) ground-state degeneracies, which can still be organized by the eigenvalues of Wilson loop operators. However, we demonstrate that these models do not support topological or fracton order (as has been conjectured in the literature) due to the loss of (emergent) gauge symmetry. An extreme deformation of the TC is the quantum dimer model (QDM), which we discuss along the family of symmetry-deformed models from the perspective of subsystem symmetries, sublattice modulation, and quantum order-by-disorder mechanisms resulting in rich phase diagrams. For the QDM, this allows us to identify an emergent SO(2) symmetry for what appears to be a gapless ground state (by numerical standards) that is unstable to the formation of a plaquette valence bond solid upon sublattice modulation.

cond-mat.str-el↗

arXiv2024

Revealing Quadrupolar Excitations with Non-Linear Spectroscopy

Local moments with a spin $S>1/2$ can exhibit a rich variety of elementary quasiparticle excitations, such as quadrupolar excitations, that go beyond the dipolar magnons of conventional spin-$1/2$ systems. However, the experimental observation of such quadrupolar excitations is often challenging due to the dipolar selection rules of many linear response probes, rendering them invisible. Here we show that non-linear spectroscopy, in the form of two-dimensional coherent spectroscopy (2DCS), can be used to reveal quadrupolar excitations. Considering a family of spin-1 Heisenberg ferromagnets with single-ion easy-axis anisotropy as an example, we explicitly calculate their 2DCS signature by combining exact diagonalization and generalized spin wave theory. We further demonstrate that 2DCS can provide access to the quadrupolar weight of an excitation, analogous to how linear response provides access to the dipolar weight. Our work highlights the potential of non-linear spectroscopy as a powerful tool to diagnose multipolar excitations in quantum magnets.

cond-mat.str-el↗

arXiv2024

Exploring Two-dimensional Coherent Spectroscopy with Exact Diagonalization: Spinons and Confinement in 1D Quantum Magnets

Two-dimensional coherent spectroscopy (2DCS) with terahertz radiation offers a promising new avenue for the exploration of many-body phenomena in quantum magnets. This includes the potential diagnosis of fractionalized excitations, for which linear response often struggles due to the indistinguishability of a continuum of fractional excitations from that caused by disorders or impurities. However, the interpretation of the complex results produced by 2DCS remains a challenge, and a general prediction of the spectral characteristics of different types of excitations has not yet been established. In this paper, we develop a numerical approach based on exact diagonalization (ED) to push our understanding of 2DCS towards different scenarios. We first validate our approach by comparing numerical ED and exact analytical results for the spectroscopic signatures of spinons in one-dimensional transverse field Ising model and develop how to deal with the inherently small system sizes in ED calculations. Augmenting the model by a longitudinal field, we demonstrate significant changes to the 2DCS spectrum upon the field-induced spinon pair confinement, which can be rationalized in our ED calculations and from a "two-kink" model (in the absence of integrability). One advantage of our ED approach is its possible extension to finite temperatures, which we explore using thermally pure quantum states and demonstrate to change the intensity and spectroscopical patterns of 2DCS when going beyond the integrable model. Our numerically exact results provide a benchmark for future experiments and theoretical studies relying on approximation methods, and pave the way for the exploration of fractionalized excitations in quantum magnets.

cond-mat.str-el↗

arXiv2023

Rhombic skyrmion lattice coupled with orthorhombic structural distortion in EuAl$_{4}$

The centrosymmetric tetragonal itinerant magnet EuAl$_{4}$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl$_{4}$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim$0.10% and $\sim$0.03% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in EuAl$_{4}$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

cond-mat.str-el↗

arXiv2022

Frustrated Ferromagnetism of Honeycomb Cobaltates: Incommensurate Spirals, Quantum Disordered Phases, and Out-of-Plane Ising Order

Recent experiments on the Co-based $d^7$ honeycomb materials BaCo$_2$(AsO$_4$)$_2$ and BaCo$_2$(PO$_4$)$_2$ have drawn renewed interest to XXZ models with competing interactions beyond the nearest neighbor exchange. While the case of dominant antiferromagnetic exchange has been extensively studied, the actual materials exhibit a ferromagnetic nearest neighbor exchange. Here we show that such a sign change of the dominant nearest neighbor coupling has substantial consequences on the phase diagram. In particular, the nature of the quantum disordered phase of interest changes from a plaquette valence bond crystal to a long-range entangled spin liquid phase. By means of complementary numerical simulations, based on exact diagonalization and the pseudo-fermion functional renormalization group, we demonstrate the formation of a gapless spin liquid state at the heart of the ferromagnetic phase diagram in the isotropic Heisenberg limit, which gives way to out-of-plane Ising magnetic order upon inclusion of XXZ anisotropy. The magnetically ordered phases surrounding this quantum disordered region are much less sensitive to the sign change of the dominant interaction and can be understood from their classical analogs. We briefly comment on the relevance of our results for Co-based $d^7$ honeycomb materials.

cond-mat.str-el↗