arXiv subjects

Shunichiro Kinoshita

Publications and source records attributed to Shunichiro Kinoshita.

At least 19 recordsLinked to original sources

arXiv2026

Simulating Quantum Field Theories with Boundaries in Curved Spacetimes Using Open Spin Systems

We develop a framework to simulate quantum field theories (QFTs) with boundaries in $(1+1)$-dimenmsional curved spacetimes by employing open spin systems. Building upon our previous work that established a mapping from spin systems to QFTs in periodic geometries, we extend the correspondence to systems with boundaries, where boundary conditions play a crucial role in shaping the dynamics. Focusing on Majorana fermions, we derive the allowed boundary conditions from the requirement of inner product conservation and formulate their realization in spin systems. The corresponding spin model is shown to reproduce boundary conditions of QFT accurately when a free function in the spin model is appropriately chosen. As an explicit demonstration, we analyze a flat spacetime example, comparing spectra, mode functions, and linear responses between the continuum and lattice descriptions. Our findings confirm that open spin systems can successfully replicate QFT dynamics with boundaries.

hep-th↗

arXiv2025

Spin systems as quantum simulators of quantum field theories in curved spacetimes

We demonstrate that a quantum field theory (QFT) in general two-dimensional curved spacetimes can be realized by a system of quantum spins or qubits. We consider a spin-1/2 model on a one-dimensional ring with spatially and temporally varying exchange couplings and magnetic fields. This model reduces to a QFT of Majorana fermions in the continuum limit. From this correspondence, we establish a dictionary for translating between the spacetime-dependent parameters of the spin model and the general metric on which the QFT is defined. After addressing the general case, we consider the Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) metric as a simple example. According to the dictionary, the QFT of Majorana fermions on the FLRW metric corresponds to the Ising model with a time-dependent transverse magnetic field. We demonstrate that the production of Majorana particles in the expanding universe can be simulated with the transverse-field Ising model by increasing the strength of the magnetic field. Furthermore, we examine the Unruh effect through the spin system by using our prescription and show the direct relation between the entanglement (or modular) Hamiltonian in the spin system and the Rindler Hamiltonian. This approach provides an experimentally viable system for probing various phenomena in QFT within curved spacetime, while also opening the door to uncovering nontrivial phenomena in spin systems inspired by curved spacetime physics. It offers fresh perspectives on both QFT in curved spacetimes and quantum many-body spin systems, revealing profound connections between these fields.

hep-th↗

arXiv2025

Spin systems as quantum field theories in inflationary universe: A study with Unruh-DeWitt detectors

We propose a method to probe the thermal properties of quantum field theory (QFT) in an inflationary universe simulated by spin systems. Our previous work (arXiv:2410.07587) has demonstrated that QFT of Majorana fermions in an arbitrary two-dimensional spacetime can be mapped onto a spin system. In this study, we apply this mapping to investigate the thermal properties of an inflationary universe. An interaction between a quantum field and a detector allows one to extract information about the quantum field from the excitation probability of the detector, known as the Unruh-DeWitt detector. In an inflationary universe with Hubble constant $H$, the excitation probability of an Unruh-DeWitt detector follows a thermal distribution with temperature $H/(2π)$, indicating that a static observer in the inflationary universe perceives a thermal field. We consider a spin system corresponding to QFT in an inflationary universe and introduce a single spin interacting with this system as an Unruh-DeWitt detector. We demonstrate that the detector response asymptotically approaches the result of QFT with an appropriate power of the number of spin sites. Since the dynamics of spin systems can be implemented on programmable quantum simulation platforms, our study offers a concrete route toward experimentally probing the thermal properties of an inflationary universe in controlled quantum settings. This highlights the potential of quantum technologies to emulate and investigate aspects of quantum field theory in curved spacetimes.

hep-th↗

arXiv2025

Spin current generation due to differential rotation

We study nonequilibrium spin dynamics in differentially rotating systems, deriving an effective Hamiltonian for conduction electrons in the comoving frame. In contrast to conventional spin current generation mechanisms that require vorticity, our theory describes spins and spin currents arising from differentially rotating systems regardless of vorticity. We demonstrate the generation of spin currents in differentially rotating systems, such as liquid metals with Taylor-Couette flow. Our alternative mechanism will be important in the development of nanomechanical spin devices.

cond-mat.mes-hall↗

arXiv2024

Geometrical origin of the Kodama vector

It has been known that warped-product spacetimes such as spherically symmetric ones admit the Kodama vector. This vector provides a locally conserved current made by contraction of the Einstein tensor, even though there is no Killing vector. In addition, a quasilocal mass, Birkhoff's theorem and various properties are closely related to the Kodama vector. Recently, it is shown that the notion of the Kodama vector can be extended to three-dimensional axisymmetric spacetimes even if the spacetimes are not warped product. This implies that warped product may not be a necessary condition for a spacetime to admit the Kodama vector. We show properties of the Kodama vector originate from the conformal Killing-Yano 2-form. In particular, the well-known spacetimes that admit the Kodama vector have a closed conformal Killing-Yano 2-form. Furthermore, we show the Kodama vector provides local conserved currents for each order of the Lovelock tensor as well as the Einstein tensor.

gr-qc↗

arXiv2024

Quasinormal mode spectrum of the AdS black hole with the Robin boundary condition

We study the quasinormal mode (QNM) spectrum of an asymptotically AdS black hole with the Robin boundary condition at infinity. We consider the Schwarzshild-AdS$_4$ with the flat event horizon as the background spacetime and study its scalar field perturbation. Denoting leading coefficients of slow- and fast-decay modes of the scalar field at infinity as $ϕ_1$ and $ϕ_2$, respectively, we assume a linear relation between them as $ϕ_2 = \cot(θ/2) ϕ_1$, where $θ$ is a constant called the Robin parameter and periodic under $θ\simθ+2π$. In a certain range of the Robin parameter, there is an instability driven by the boundary condition. We also find the holonomy in the QNM spectrum under the parametric cycle of the boundary condition: $θ=0\to2π$. After the one-cycle, $n$-th overtone of the QNM moves to $(n-1)$-th overtone. The fundamental tone of the QNM is swept out to the infinity in the complex plane.

gr-qc↗

arXiv2023

Stark effect and dissociation of mesons in holographic conductor

We study the meson spectrum of the ${\cal{N}}=4$ supersymmetric Yang-Mills theory with ${\cal{N}}=2$ fundamental hypermultiplets for a finite electric field by using the D3/D7 model. The spectrum for scalar and vector mesons is computed by analyzing the (quasi-)normal modes for the fluctuations of the D7-brane embedding and gauge fields. In the presence of an electric field, two different phases in the background are realized: the meson and dissociation phases. In this paper, we analyze the meson spectrum of scalar and vector mesons for all ranges of the electric field and explore the effect of the electric field on the meson spectrum, that is, the Stark effect. In the meson spectrum, we observe the avoided crossing between different levels due to the coupling of fluctuations via the electric field.

hep-th↗

arXiv2023

Note on the Time Dilation of Charged Quantum Clocks

We derive the time dilation formula for charged quantum clocks in electromagnetic fields. As a concrete example of non-inertial motion, we consider a cyclotron motion in a uniform magnetic field. Applying the time dilation formula to coherent state of the charged quantum clock, we evaluate the time dilation quantum-mechanically.

gr-qc↗

arXiv2023

Shooting null geodesics into holographic spacetimes

We find, in the AdS/CFT, a source on the boundary which generates one wave packet drawing a null geodesic inside the bulk. Once such a wave packet dives into the bulk, it comes back to the boundary after a specific time, at which the expectation value of the corresponding boundary operator finally stands up. Since this behavior strongly reflects the existence of the holographic spacetime, our technique will be helpful in identifying holographic materials.

hep-th↗

arXiv2022

Quantum Clocks, Gravitational Time Dilation, and Quantum Interference

A proper time observable for a quantum clock is introduced and it is found that the proper time read by one clock conditioned on another clock reading a different proper time obeys classical time dilation in accordance with special relativistic kinematical time dilation. Here, we extend this proposal to a weak gravitational field in order to investigate whether the weak equivalence principle holds for quantum matter. We find that for general quantum states the quantum time dilation in a weak gravitational field obeys a similar gravitational time dilation found in classical relativity. However, unlike the special relativistic case, the time dilation involves the external time (a background coordinate time at the observer on the Earth) as well as the proper times of two clocks. We also investigate a quantum time dilation effect induced by a clock in a superposition of wave packets localized in momentum space or in position space and propose the setup to observe the gravitational effect in the quantum interference effect in the time dilation.

gr-qc↗

arXiv2022

Extension of Kodama vector and quasilocal quantities in three-dimensional axisymmetric spacetimes

Spherically symmetric spacetimes admit the so-called Kodama vector, which provides a locally conserved current and a preferred time even for dynamical spacetime without any time translation symmetry. A charge associated with this conserved current leads to a quasilocal mass which agrees with the Misner-Sharp mass. In three dimensions, spherically symmetric spacetimes correspond to axisymmetric ones, while axisymmetry allows spacetimes to be rotating with angular momentum. We extend the notion of the Kodama vector to axisymmetric rotating spacetimes in three dimensions. We also define a quasilocal mass taking into account angular momentum in three-dimensional axisymmetric spacetimes.

gr-qc↗

arXiv2022

Dynamical stability and filamentary instability in holographic conductors

In this study, we analyze the dynamical stability of the D3-D7 model dual to a holographic conductor with a constant current under an external electric field. We particularly focus on the stability around the parameter region where the multivalued relation between the external electric field and the current is shown due to nonlinear conductivity. The dynamical stability of the states can be analyzed by considering linear perturbations in the background states and computing the quasinormal modes. In the multivalued region, we find that the states in one branch with a low electric current can be dynamically unstable. The turning point in the $J$-$E$ characteristic coincides with the stability switching. Further, we also find that the perturbations around the unstable states can become stable with finite wavenumber. In other words, the perturbations in the background states become static at a critical wavenumber, implying the existence of inhomogeneous steady states with current filaments.

hep-th↗

arXiv2019

Imaging black holes through AdS/CFT

Thermal states in some quantum field theories (QFTs) correspond to black holes in asymptotically AdS spacetime in the AdS/CFT correspondence. We propose a direct procedure to construct holographic images of the black hole in the bulk from a given response function of the QFT on the boundary. The response function with respect to an external source corresponds to the asymptotic data of the bulk field generated by the source on the AdS boundary. According to the wave optics, we can obtain the images from the bulk field propagating in the bulk spacetime. For a thermal state on two-dimensional sphere dual to Schwarzschild-AdS$_4$ black hole, we demonstrate that the holographic images gravitationally lensed by the black hole can be constructed from the response function. In particular, the Einstein rings on the image can be clearly observed and their radius depends on the total energy of the QFT thermal state. These results are consistent with the size of the photon sphere of the black hole calculated in geometrical optics. This implies that, if there exists a dual gravitational picture for a given quantum system, we would be able to probe existence of the dual black hole by the Einstein rings constructed from observables of the quantum system.

hep-th↗

arXiv2019

Einstein Rings in Holography

Clarifying conditions for the existence of a gravitational picture for a given quantum field theory (QFT) is one of the fundamental problems in the AdS/CFT correspondence. We propose a direct way to demonstrate the existence of the dual black holes: imaging an Einstein ring. We consider a response function of the thermal QFT on a two-dimensional sphere under a time-periodic localized source. The dual gravity picture, if it exists, is a black hole in an asymptotic global AdS$_4$ and a bulk probe field with a localized source on the AdS boundary. The response function corresponds to the asymptotic data of the bulk field propagating in the black hole spacetime. We find a formula that converts the response function to the image of the dual black hole: The view of the sky of the AdS bulk from a point on the boundary. Using the formula, we demonstrate that, for a thermal state dual to the Schwarzschild-AdS$_4$ spacetime, the Einstein ring is constructed from the response function. The evaluated Einstein radius is found to be determined by the total energy of the dual QFT. Our theoretical proposal opens a door to gravitational phenomena on strongly correlated materials.

hep-th↗

arXiv2018

The essence of the Blandford-Znajek process

From a spacetime perspective, the dynamics of magnetic field lines of force-free electromagnetic fields can be rewritten into a quite similar form for the dynamics of strings, i.e., dynamics of "field sheets". Using this formalism, we explicitly show that the field sheets of stationary and axisymmetric force-free electromagnetic fields have identical intrinsic properties to the world sheets of rigidly rotating Nambu-Goto strings. Thus, we conclude that the Blandford-Znajek process is kinematically identical to an energy-extraction mechanism by the Nambu-Goto string with an effective magnetic tension.

gr-qc↗

arXiv2017

Holographic Floquet states II: Floquet condensation of vector mesons in nonequilibrium phase diagram

With the aim to reveal universal features of hadronic matter and correlated Dirac insulators in strong AC-electric fields, we study the $\mathcal{N}=2$ supersymmetric QCD with a finite quark mass driven by a rotating electric field $\mathcal{E}_x+i\mathcal{E}_y= E e^{iΩt}$. The analysis is done in the holographically dual D3/D7 system in the co-rotating frame, effectively. The nonequilibrium phase diagram is determined from the threshold electric field at which the insulator phase breaks down to a conductive phase due to the AC version of the Schwinger mechanism. The external field induces a rotating current $\mathcal{J}_x + i \mathcal{J}_y = J e^{iΩt}$ originating from vacuum polarization and dissipative current in the insulating and conductive phases respectively. Intriguing features are observed as the frequency $Ω$ approaches resonance with the meson excitation energy $Ω_{\rm meson}$. There, the threshold minimizes and a condensate of vector mesons with oscillating current exists even in the zero driving field limit. This state, which we call Floquet condensate of vector mesons, is expected to be dynamically stable realizing a non-thermal fixed point that breaks time translational and reversal symmetries. Our finding has many similarities with exciton BEC discussed in solid state systems, where the semiconductor is to be replaced by materials hosting gapped Dirac electrons, e.g. 3D topological insulators or bismuth. Vector meson Floquet condensate may also have implications in the pre-thermalized dynamics in heavy ion collision experiments.

hep-th↗

arXiv2017

Energy extraction from Kerr black holes by rigidly rotating strings

In this paper, we show that a rigidly rotating string can extract the rotational energy from a rotating black hole. We consider Nambu-Goto strings stationary with respect to a corotating Killing vector with an uniform angular velocity $ω$ in the Kerr spacetime. We show that a necessary condition of the energy-extraction process is that an effective horizon on the string world sheet, which corresponds to the inner light surface, is inside the ergosphere of the Kerr black hole and the angular velocity $ω$ is less than that of the black hole $Ω_\mathrm{h}$. Furthermore, we discuss global configurations of such strings in both of a slow-rotation limit and the extremal Kerr case.

gr-qc↗

arXiv2016

Holographic Floquet states: (I) A strongly coupled Weyl semimetal

Floquet states can be realized in quantum systems driven by continuous time-periodic perturbations. It is known that a state known as the Floquet Weyl semimetal can be realized when free Dirac fermions are placed in a rotating electric field. What will happen if strong interaction is introduced to this system? Will the interaction wash out the characteristic features of Weyl semimetals such as the Hall response? Is there a steady state and what is its thermodynamic behavior? We answer these questions using AdS/CFT correspondence in the $\mathcal{N}=2$ supersymmetric massless QCD in a rotating electric field in the large $N_c$ limit realizing the first example of a "holographic Floquet state". In this limit, gluons not only mediate interaction, but also act as an energy reservoir and stabilize the nonequilibrium steady state (NESS). We obtain the electric current induced by a rotating electric field: In the high frequency region, the Ohm's law is satisfied, while we recover the DC nonlinear conductivity at low frequency, which was obtained holographically in a previous work. The thermodynamic properties of the NESS, e.g., fluctuation-dissipation relation, is characterized by the effective Hawking temperature that is defined from the effective horizon giving a holographic meaning to the "periodic thermodynamic" concept. In addition to the strong (pump) rotating electric field, we apply an additional weak (probe) electric field in the spirit of the pump-probe experiments done in condensed matter experiments. Weak DC and AC probe analysis in the background rotating electric field shows Hall currents as a linear response, therefore the Hall response of Floquet Weyl semimetals survives at the strong coupling limit. We also find frequency mixed response currents, i.e., a heterodyning effect, characteristic to periodically driven Floquet systems.

hep-th↗