arXiv subjects

Ippei Danshita

Publications and source records attributed to Ippei Danshita.

At least 19 recordsLinked to original sources

arXiv2026

Stability of current-carrying states in hard-core bosons with long-range hopping on a square lattice

We investigate the stability of current-carrying states with quasi-momentum $K$ in the Bose-condensed phase of the hard-core Bose-Hubbard model on a square lattice, where particles transfer between two sites separated by distance $r$ with hopping amplitude decaying algebraically with $r$ as $\propto r^{-α}$. Using a mean-field theory, we analyze the excitation spectrum and determine the critical quasi-momenta associated with Landau and dynamical instabilities. We find that the long-range hopping suppresses the critical quasi-momenta and makes them vanish at $α=3$. Near $α=3$, we show that the critical quasi-momentum $K_{\mathrm{c}}$ for the dynamical instability exhibits the scaling behavior $K_\mathrm{c} \propto Δ^{1+Δ}$ with $Δ=α-3$, where the scaling exponent explicitly depends on $Δ$, as a consequence of the long-range nature of the hopping.

cond-mat.quant-gas↗

arXiv2026

Entanglement-enhanced correlation propagation in the one-dimensional SU($N$) Fermi-Hubbard model

We investigate the dynamics of correlation propagation in the one-dimensional Fermi-Hubbard model with SU($N$) symmetry when the repulsive-interaction strength is quenched from a large value, at which the ground state is a Mott-insulator with $1/N$ filling, to an intermediate value. From approximate analytical insights based on a simple model that captures the essential physics of the doublon excitations, we show that entanglement in the initial state leads to collective enhancement of the propagation velocity $v_{\text{SU}(N)}$ when $N>2$, becoming equal to the velocity of the Bose-Hubbard model in the large-$N$ limit. These results are supported by numerical calculations of the density-density correlation in the quench dynamics for $N=2,3,4,$ and $6$.

cond-mat.quant-gas↗

arXiv2026

Higgs and Nambu-Goldstone modes in a spin-1 $XY$ model with long-range interactions

We theoretically study the collective excitations in a spin-1 $XY$ model with a quadratic Zeeman term and a long-range interaction that decays algebraically with the distance. Using the quantum-field theory based on the finite-temperature Green's function formalism, we analyze properties of the Nambu-Goldstone (NG) and Higgs modes in order to analytically evaluate the damping rate of the Higgs mode in the $XY$ ferromagnetic ordered phase near the quantum phase transition to the disordered phase. When the power of the algebraic decay is 3 as in the case of dipole-dipole interactions in Rydberg-atom systems, we show that at two dimensions the excitation energy of the Higgs mode exhibits a linear dispersion whereas the dispersion of the NG mode becomes proportional to the square root of the momentum. We find that the damping of the Higgs mode is significantly suppressed by the long-range interaction. We also propose how to excite and probe the Higgs mode in Rydberg-atom experiments.

cond-mat.quant-gas↗

arXiv2026

Suppressed correlation-spreading in a one-dimensional Bose-Hubbard model with strong interactions

We investigate signatures of non-ergodic behavior in the real-time evolution of a one-dimensional Bose-Hubbard model, where the initial state is a doubly occupied density-wave state. We show that the occupation dynamics at strong interactions is dominated by doublon-holon exchange which leads to a domain wall excitation and propagation. The latter manifests as a negated staggered pattern in the density-density correlations. While the single-particle and the pair correlation functions show highly localized correlations that decay rapidly away from the nearest neighbor. We show that the time scale of the domain-wall excitations depends on the inverse of the interaction strength and therefore dictates the slow relaxation dynamics. In the presence of a parabolic trap, the occupation dynamics at the edges become frozen and further suppresses the propagation of correlations. This suppression happens even for trap strengths weaker than the tunneling rate. We also show that the model can be mapped to an antiferromagnetic transverse-field Ising model in the limit of strong interactions and that the correlation-propagation velocity in the original model is well captured by the group velocity of the spin-wave excitation in the effective spin model.

cond-mat.quant-gas↗

arXiv2025

Observation of anomalous tunneling in collective excitations via a cloud experiment platform for Bose-Einstein condensates

Recent development of cloud-based experiment platforms has enabled physicists to examine theoretical concepts with unprecedented accessibility. Oqtant is a cloud-accessible platform for trapped Bose-Einstein Condensates (BECs) of neutral atomic gases, providing an invaluable experimental tool for studying the dynamics of BECs. An intriguing theoretical prediction of a characteristic phenomenon of BECs is anomalous tunneling, whereby low-energy phonon excitations of BECs easily transmit through a barrier potential. We utilize Oqtant to observe the effects of anomalous tunneling on collective excitations of BECs. For this purpose, we theoretically show that anomalous tunneling affects the frequencies of the collective excitations in the low-energy regime, and experimentally measure these frequencies using Oqtant. Our results reveal that low-energy collective modes are less affected by a potential barrier, which indicates the presence of anomalous tunneling. Our work would contribute to fundamental understandings of BECs, as well as highlight the potential of cloud-based experiments in quantum-body physics.

cond-mat.quant-gas↗

arXiv2025

Observation of slow relaxation due to Hilbert space fragmentation in strongly interacting Bose-Hubbard chains

While isolated quantum systems generally thermalize after long-time evolution, there are several exceptions defying thermalization. A notable mechanism of such nonergodicity is the Hilbert space fragmentation (HSF), where the Hamiltonian matrix splits into an exponentially large number of sectors due to the presence of nontrivial conserved quantities. Using ultracold gases, here we experimentally investigate the one-dimensional Bose-Hubbard system with neither disorder nor tilt potential, which has been predicted to exhibit HSF caused by a strong interatomic interaction. Specifically, we analyze far-from-equilibrium dynamics starting from a charge-density wave of doublons (atoms in doubly occupied sites) in a singlon and doublon-resolved manner to reveal a slowing-down of the relaxation in a strongly interacting regime. We find that the numbers of singlons and doublons are conserved during the dynamics, indicating HSF as a mechanism of the observed slow relaxation. Our results provide an experimental confirmation of the conserved quantities responsible for HSF.

cond-mat.quant-gas↗

arXiv2025

Melting of a Bosonic Mott Insulator in Kagome Optical Lattices with Sign-Inverted Hopping

Using the discrete truncated Wigner approximation (dTWA), we investigate the nonequilibrium dynamics of ultracold bosons confined in optical kagome lattices, focusing on both unfrustrated positive and frustrated negative hopping regimes. We consider a protocol in which the system is initialized in a Mott insulating state at unit filling, and the hopping amplitude is gradually increased from zero. For positive hopping, the melting of the Mott insulator is accompanied by the emergence of a sharp peak in the momentum distribution at the $Γ$ point of the lowest band, signaling the onset of superfluidity. In contrast, for negative hopping, the Mott insulator melts into a highly nontrivial state without long-range phase coherence, characterized instead by a broad momentum distribution within the flat band, consistent with recent experimental observations. These results demonstrate the applicability of dTWA to highly frustrated quantum systems and offer a new route for numerically exploring the dynamics of frustrated quantum magnets.

cond-mat.quant-gas↗

arXiv2025

Entanglement entropy dynamics of non-Gaussian states in free boson systems: Random sampling approach

We develop a random sampling method for calculating the time evolution of the Rényi entanglement entropy after a quantum quench from an insulating state in free boson systems. Because of the non-Gaussian nature of the initial state, calculating the Rényi entanglement entropy calls for the exponential cost of computing a matrix permanent. We numerically demonstrate that a simple random sampling method reduces the computational cost of a permanent; for an $N_{\mathrm{s}}\times N_{\mathrm{s}}$ matrix corresponding to $N_{\mathrm{s}}$ sites at half filling, the sampling cost becomes $\mathcal{O}(2^{αN_{\mathrm{s}}})$ with a constant $α\ll 1$, in contrast to the conventional algorithm with the $\mathcal{O}(2^{N_{\mathrm{s}}})$ number of summations requiring the exponential time cost. Although the computational cost is still exponential, this improvement allows us to obtain the entanglement entropy dynamics in free boson systems for more than $100$ sites. We present several examples of the entanglement entropy dynamics in low-dimensional free boson systems.

quant-ph↗

arXiv2024

Discrete-phase-space method for driven-dissipative dynamics of strongly interacting bosons in optical lattices

We develop a discrete truncated Wigner method to analyze the real-time evolution of dissipative SU(${\cal N}$) spin systems coupled with a Markovian environment. This semiclassical approach is not only numerically efficient but also particularly capable of accurately capturing local loss processes due to its local linearity in the dynamical equations. We apply the method to a state-of-the-art experiment involving an analog quantum simulator of a three-dimensional dissipative Bose-Hubbard model in a strongly interacting regime. Our numerical results show good agreement with experimental data, specifically capturing the continuous quantum Zeno effect in the dynamics subjected to a gradual change of the ratio between the hopping amplitude and the onsite interaction across the superfluid-Mott insulator crossover. Furthermore, we present comparative analyses with the continuous truncated Wigner method, derived as an effective Fokker-Planck equation for SU(${\cal N}$) classical spin variables, showing that the discrete method outperforms the continuous one in simulating the long-time dynamics of SU(2) and SU(3) spin models. The discrete phase space framework offers a versatile and powerful tool for exploring a wide range of open quantum many-body systems in dimensions higher than one dimension, where numerically exact methods are impractical due to the exponential growth of the Hilbert space dimension.

cond-mat.quant-gas↗

arXiv2024

Surface criticality in the mixed-field Ising model with sign-inverted next-nearest-neighbor interaction

Rydberg atoms in an optical tweezer array have been used as a quantum simulator of the spin-$1/2$ antiferromagnetic Ising model with longitudinal and transverse fields. We suggest how to implement the next-nearest-neighbor (NNN) interaction whose sign is opposite to that of the nearest neighbor one in the Rydberg atom systems. We show that this can be achieved by weakly coupling one Rydberg state with another Rydberg state. We further study the surface criticality associated with the first-order quantum phase transition between the antiferromagnetic and paramagnetic phases, which emerges due to the sign-inverted NNN interaction. From the microscopic model, we derive a Ginzburg-Landau (GL) equation, which describes static and dynamic properties of the antiferromagnetic order parameter near the transition. Using both analytical GL theory and numerical method based on a mean-field theory, we calculate the order parameter in the proximity of a boundary of the system in order to show that the healing length of the order parameter logarithmically diverges, signaling the surface criticality.

cond-mat.quant-gas↗

arXiv2024

Ground-state phase diagram of the SU($4$) Heisenberg model on a plaquette lattice

We investigate the ground state of the SU($4$) Heisenberg model on a square lattice with spatial anisotropy on each plaquette bond using the tensor-network method based on infinite projected entangled pair states. We find that the SU($4$) singlet ground state appears in the strongly anisotropic limit, whereas Néel and valence-bond crystal orders coexist in the nearly isotropic limit. By examining the intermediate parameter region, we identify a phase transition between these phases. The nature of the phase transition is likely to be of first order, and the transition point is estimated to be around $J'/J\approx 0.85(5)$, where $J$ and $J'$ are the interaction strengths of intra- and interplaquette bonds, respectively. We also calculate the anisotropy dependence of singlet correlations on a plaquette bond, which will be useful for future experiments of ultracold atoms in optical lattices.

cond-mat.quant-gas↗

arXiv2024

Quantum many-body scars in the Bose-Hubbard model with a three-body constraint

We uncover the exact athermal eigenstates in the Bose-Hubbard (BH) model with a three-body constraint, motivated by the exact construction of quantum many-body scar (QMBS) states in the $S=1$ $XY$ model. These states are generated by applying an $\rm SU(2)$ ladder operator consisting of a linear combination of two-particle annihilation operators to the fully occupied state. By using the improved Holstein-Primakoff expansion, we clarify that the QMBS states in the $S=1$ $XY$ model are equivalent to those in the constrained BH model with additional correlated hopping terms. We also find that, in the strong-coupling limit of the constrained BH model, the QMBS state exists as the lowest-energy eigenstate of the effective model in the highest-energy sector. This fact enables us to prepare the QMBS states in a certain adiabatic process and opens up the possibility of observing them in ultracold-atom experiments.

cond-mat.quant-gas↗

arXiv2024

Relation between the noise correlations and the spin structure factor for Mott-insulating states in SU$(N)$ Hubbard models

It is well established that the noise correlations measured by time-of-flight imaging in cold-atom experiments, which correspond to the density-density correlations in the momentum space of trapped atomic gases, can probe the spin structure factor deep in the Mott-insulating regime of SU(2) Hubbard models. We explicitly derive the mathematical relation between the noise correlations and the spin structure factor in the strong-interaction limit of SU$(N)$ Hubbard models at any integer filling $ρ$. By calculating the ground states of one-dimensional SU$(N)$ Fermi-Hubbard models for $2\leq N\leq 6$ with use of the density-matrix renormalization-group method, we confirm the relation numerically in the regime of strong interactions $U \gg t$, where $U$ and $t$ denote the onsite interaction and the hopping energy. We show that the deviation between the actual noise correlations and those obtained from the spin structure factor scales as approximately $(t/U)^2$ for $ρ=1$ at intermediate and large lattice sizes on the basis of numeric and semi-analytic arguments.

cond-mat.quant-gas↗

arXiv2023

Persistent-current states originating from the Hilbert space fragmentation in momentum space

Hilbert space fragmentation (HSF) is a phenomenon that the Hilbert space of an isolated quantum system splits into exponentially many disconnected subsectors. The fragmented systems do not thermalize after long-time evolution because the dynamics are restricted to a small subsector. Inspired by recent developments of the HSF, we construct the Hamiltonian that exhibits the HSF in the momentum space. We show that persistent-current (PC) states emerge due to the HSF in the momentum space. We also investigate the stability of the PC states against the random potential, which breaks the structure of the HSF, and find that the decay rate of the PC is almost independent of the current velocity.

cond-mat.quant-gas↗

arXiv2023

Higgs and Nambu-Goldstone modes in condensed matter physics

Collective dynamics of many particle systems is tightly linked to their underlying symmetry and phase transitions. Higgs and Nambu-Goldstone modes are, respectively, collective amplitude and phase modes of the order parameter that are widely observed in various physical systems at different energy scales, ranging from magnets, superfluids, superconductors to our universe. The Higgs mode is a massive excitation, which is a condensed-matter analog of Higgs particle in high-energy physics, while the Nambu-Goldstone mode is a massless excitation that appears when a continuous symmetry is spontaneously broken. They provide important information on the fundamental aspects of many particle systems, such as symmetry, phases, dynamics, response to external fields, and so on. In this article, we review the physics of Higgs and Nambu-Goldstone modes in condensed matter physics. Especially, we focus on the development on the study of collective modes in superconductors and cold-atom systems.

cond-mat.supr-con↗

arXiv2023

Dynamics of correlation spreading in low-dimensional transverse-field Ising models

We investigate the dynamical spreading of spatial correlations after a quantum quench starting from a magnetically disordered state in the transverse-field Ising model at one (1D) and two spatial dimensions (2D). We analyze specifically the longitudinal and transverse spin-spin correlation functions at equal time with use of several methods. From the comparison of the results in 1D obtained by the linear spin-wave approximation (LSWA) and those obtained by the rigorous analytical approach, we show that the LSWA can asymptotically reproduce the exact group velocity in the limit of strong transverse fields while it fails to capture the detailed time dependence of the correlation functions. By applying the LSWA to the 2D case, in which the rigorous analytical approach is unavailable, we estimate the propagation velocity to be $Ja/(2\hbar)$ at the strong-field limit, where $J$ is the Ising interaction and $a$ is the lattice spacing. We also utilize the tensor-network method based on the projected-entangled pair states for 2D and quantitatively compute the time evolution of the correlation functions for a relatively short time. Our findings provide useful benchmarks for quantum simulation experiments of correlation spreading and theoretical refinement of the Lieb-Robinson bound in the future.

cond-mat.quant-gas↗

arXiv2023

Rényi entanglement entropy after a quantum quench starting from insulating states in a free boson system

We investigate the time-dependent Rényi entanglement entropy after a quantum quench starting from the Mott-insulating and charge-density-wave states in a one-dimensional free boson system. The second Rényi entanglement entropy is found to be the negative of the logarithm of the permanent of a matrix consisting of time-dependent single-particle correlation functions. From this relation and a permanent inequality, we obtain rigorous conditions for satisfying the volume-law entanglement growth. We also succeed in calculating the time evolution of the Rényi entanglement entropy in unprecedentedly large systems by brute-force computations of the permanent. We discuss possible applications of our findings to the real-time dynamics of noninteracting bosonic systems.

quant-ph↗

arXiv2023

Evaluating thermal expectation values by almost ideal sampling with Trotter gates

We investigate the sampling efficiency for the simulations of quantum many-body systems at finite temperatures when initial sampling states are generated by applying Trotter gates to random phase product states (RPPSs). We restrict the number of applications of Trotter gates to be proportional to the system size, and thus the preparation would be easily accomplished in fault-tolerant quantum computers. When the Trotter gates are made from a nonintegrable Hamiltonian, we observe that the sampling efficiency increases with system size. This trend means that almost ideal sampling of initial states can be achieved in sufficiently large systems. We also find that the sampling efficiency is almost equal to that obtained by a typical pure quantum (TPQ) state method utilizing Haar random sampling in some cases. These findings suggest that chaotic Hamiltonian dynamics can transform RPPSs into an alternative to TPQ states for evaluating thermal expectation values.

quant-ph↗