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Itaru Sasaki

Publications and source records attributed to Itaru Sasaki.

At least 19 recordsLinked to original sources
arXiv2026

Representation Theory of Canonical Commutation Relations Arising from the Quantization of the Klein-Gordon Equation with an External Potential

We investigate the Weyl representation of the canonical commutation relations for a model describing a quantized massive scalar field under the influence of an external potential. The main problem is to determine whether the Weyl representation remains equivalent to or becomes inequivalent to the original one when the mass and/or potential are changed. This problem is reduced to the study of Schrödinger operators. It turns out that the Weyl representations are inequivalent when the masses differ. Moreover, when the masses are the same, the transition between equivalence and inequivalence occurs at the decay rate $-3/2$ of the difference between the potentials. This contrasts with the decay rate $-1$ that defines the short-range condition in scattering theory.

math-ph↗
arXiv2026

Holomorphy of the ground state and energy for bosonic quadratic Hamiltonians

We study the holomorphy of the ground state and its energy with respect to the coupling constant for bosonic quadratic Hamiltonians. We prove general theorems establishing this holomorphy, thereby providing a rigorous justification for formal perturbative expansions. This theory is applied to several concrete models, including the single pair interaction model and the Pauli-Fierz model in the dipole approximation. Importantly, for the Pauli-Fierz model with a realistic ultraviolet cutoff, the perturbation expansions in the elementary charge converge at the physical charge value, even when mass renormalization is taken into account. Furthermore, we explicitly determine the radii of convergence of the ground states and their energies for both the single pair interaction model and the fibers of the translation-invariant Pauli-Fierz model.

math-ph↗
arXiv2022

Absence of Embedded Eigenvalues for Non-Local Schrödinger Operators

We consider non-local Schrödinger operators with kinetic terms given by several different types of functions of the Laplacian and potentials decaying to zero at infinity, and derive conditions ruling embedded eigenvalues out. Our goal in this paper is to advance techniques based on virial theorems, Mourre estimates, and an extended version of the Birman-Schwinger principle, previously developed for classical Schrödinger operators but thus far not used for non-local operators. We also present a number of specific cases by choosing particular classes of kinetic and potential terms, and discuss existence/non-existence of at-edge eigenvalues in a basic model case in function of the coupling parameter.

math.SP↗
arXiv2020

Explicit Diagonalization of Pair Interaction Models

We provide a general method for constructing bosonic Bogoliubov transformations that diagonalize a general class of quadratic Hamiltonians. These Hamiltonians describe the pair interaction models. Bogoliubov transformations are constructed algebraically, and the resulting Hamiltonians become the second quantizations of explicit one-particle Hamiltonians. Moreover, an explicit formula for the ground state energies is given. Our method systematically diagonalizes various models of quantum field theory, including a model of a harmonic oscillator coupled to a Bose field and the Pauli-Fierz models in the dipole approximation.

math-ph↗
arXiv2020

Time operators for continuous-time and discrete-time quantum walks

We construct concrete examples of time operators for both continuous and discrete-time homogeneous quantum walks, and we determine their deficiency indices and spectra. For a discrete-time quantum walk, the time operator can be self-adjoint if the time evolution operator has a non-zero winding number. In this case, its spectrum becomes a discrete set of real numbers.

math-ph↗
arXiv2019

Spectrum of the semi-relativistic Pauli-Fierz model II

We consider the semi-relativistic Pauli-Fierz Hamiltonian $$ H_m = |{\bf p}-{\bf A}({\bf x})| + H_{f,m} + V({\bf x}),\quad m\geq0, $$ and prove the existence of the ground state of $H_m$ for $m=0$. Here ${\bf A}({\bf x})$ denotes a quantized radiation field and $H_{f,m}$ the free field Hamiltonian with the dispersion relation $\sqrt{|{\bf k}|^2+m^2}$ with $m\geq0$. This paper is the sequel of [HH16], where the existence of the ground state $Φ_m$ of $H_m$ for $m>0$ is proven. In order to show the existence of the ground state for $m=0$ we estimate a singular and non-local pull-through formula and show the equicontinuity of set $\{a(k)Φ_m\}_{0<m<m_0}$ with some $m_0$, where $a(k)$ denotes the formal kernel of the annihilation operator. Taking a subsequence $m_j$, we can conclude that $\lim_{m_j\to0}Φ_{m_j}=Φ_0\not=0$ and $Φ_0$ is the ground state of $H_0$.

math-ph↗
arXiv2016

Embedded Eigenvalues and Neumann-Wigner Potentials for Relativistic Schrodinger Operators

The existence of potentials for relativistic Schrodinger operators allowing eigenvalues embedded in the essential spectrum is a long-standing open problem. We construct Neumann-Wigner type potentials for the massive relativistic Schrodinger operator in one and three dimensions for which an embedded eigenvalue exists. We show that in the non-relativistic limit these potentials converge to the classical Neumann-Wigner and Moses-Tuan potentials, respectively. For the massless operator in one dimension we construct two families of potentials, different by the parities of the (generalized) eigenfunctions, for which an eigenvalue equal to zero or a zero-resonance exists, dependent on the rate of decay of the corresponding eigenfunctions. We obtain explicit formulae and observe unusual decay behaviours due to the non-locality of the operator.

math-ph↗
arXiv2016

A Mathematical Analysis of Dressed Photon in Ground State of Generalized Quantum Rabi Model Using Pair Theory

We consider the generalized quantum Rabi model with the so-called $A^{2}$-term in the light of the Hepp-Lieb-Preparata quantum phase transition. We investigate the dressed photon in its ground state when the atom-light coupling strength is in the deep-strong coupling regime. We show how the dressed photon appears in the ground state. We dedicate this paper to Pavel Exner and Herbert Spohn on the occasion of their 70th birthdays, and Klaus Hepp on the occasion of his 80th birthday.

quant-ph↗
arXiv2015

Essential spectrum of the discrete Laplacian on a perturbed periodic graph

We address the Laplacian on a perturbed periodic graph which might not be a periodic graph. We present a class of perturbed graphs for which the essential spectra of the Laplacians are stable even when the graphs are perturbed by adding and removing infinitely many vertices and edges. Using this result, we demonstrate how to determine the spectra of cone-like graphs, the upper-half plane, and graphs obtained from $\mathbb{Z}^2$ by randomly adding vertices.

math-ph↗
arXiv2015

Enhanced binding of an N-particle system interacting with a scalar field II.Relativistic version

An enhanced binding of $N$-{\it relativistic} particles coupled to a massless scalar bose field is investigated. It is not assumed that the system has a ground state for the zero-coupling. It is shown, however, that there exists a ground state for sufficiently large coupling. The proof is based on checking the stability condition and showing a uniform exponential decay of infrared regularized ground states.

math-ph↗
arXiv2013

Spectral Analysis of the Dirac Polaron

A system of a Dirac particle interacting with the radiation field is considered. The Hamiltonian of the system is defined by $H = α\cdot(\hat\mathbf{p}-q\mathbf{A}(\hat\mathbf{x}))+mβ+ H_f$ where $q\in\mathbb{R}$ is a coupling constant, $\mathbf{A}(\hat\mathbf{x})$ denotes the quantized vector potential and $H_f$ denotes the free photon Hamiltonian. Since the total momentum is conserved, $H$ is decomposed with respect to the total momentum with fiber Hamiltonian $H(\mathbf{p}), (\mathbf{p}\in\mathbb{R}^3)$. Since the self-adjoint operator $H(\mathbf{p})$ is bounded from below, one can define the lowest energy $E(\mathbf{p},m):=\infσ(H(\mathbf{p}))$. We prove that $E(\mathbf{p},m)$ is an eigenvalue of $H(\mathbf{p})$ under the following conditions: (i) infrared regularization and (ii) $E(\mathbf{p},m)<E(\mathbf{p},0)$. We also discuss the polarization vectors and the angular momenta.

math-ph↗
arXiv2013

A Short Remark on the Polaron in the Semi-relativistic Pauli-Fierz Model

We consider the polaron of the spinless semi-relativistic Pauli-Fierz model. The Hamiltonian of the model is defined by $H(\mathbf{P}) = \sqrt{(\mathbf{P}-dΓ(\mathbf{k}) + e\bA)^2 + M^2} + dΓ(ω_m)$, where $\mathbf{P}\in\mathbb{R}^3$ is the momentum of the polaron, $dΓ(\cdot)$ denotes the second quantization operator and $ω_m=|\mathbf{k}|+m$ denotes the dispersion relation of the photon with virtual mass $m\geq 0$. Let $E(\mathbf{P})$ be the lowest energy of $H(\mathbf{P})$. In this paper, we prove the inequality $E(\mathbf{P} - \mathbf{k}) - E(\mathbf{P}) + ω_m(\mathbf{k}) \geq m$, for all $\mathbf{P}, \mathbf{k}\in\mathbb{R}^3$.

math-ph↗
arXiv2013

One Particle Binding of Many-Particle Semi-Relativistic Pauli-Fierz Model

It is shown that at least one particle is bound in the $N$-particle semi-relativistic Pauli-Fierz model with negative potential $V(\bx)$. It is assumed that the particles have no spin and obey the Bose or Boltzmann statistics, and the one particle Hamiltonian $\sqrt{-Δ+M^2}-M+ V(\bx)$ has a ground state with negative energy $-e_0<0$, where $M>0$ denotes the mass of the particle. We show that the ground state energy of the total system $E^V(N)$ is less than $E^0(N)-e_0$.

math-ph↗
arXiv2012

Note on the spectrum of discrete Schrödinger operators

The spectrum of discrete Schrödinger operator $L+V$ on the $d$-dimensional lattice is considered, where $L$ denotes the discrete Laplacian and $V$ a delta function with mass at a single point. Eigenvalues of $L+V$ are specified and the absence of singular continuous spectrum is proven. In particular it is shown that an embedded eigenvalue does appear for $d\geq5$ but does not for $1\leq d\leq 4$.

math-ph↗
arXiv2012

Binding condition for a general class of quantum field Hamiltonians

We consider a system of a quantum particle interacting with a quantum field and an external potential $V(\bx)$. The Hamiltonian is defined by a quadratic form $H^V = H^0 + V(\bx)$, where $H^0$ is a quadratic form which preserves the total momentum. $H^0$ and $H^V$ are assumed to be bounded from below. We give a criterion for the positivity of the binding energy $E_\mathrm{bin} = E^0-E^V$, where $E^0$ and $E^V$ are the ground state energies of $H^0$ and $H^V$. As examples of the result, the positivity of the binding energy of the semi-relativistic Pauli-Fierz model and Nelson type Hamiltonian is proved.

math-ph↗
arXiv2012

Enhanced Binding in Quantum Field Theory

This lecture note consists of three parts. Fundamental facts on Boson Fock space are introduced in Part I. Ref. 1.and 3. are reviewed in Part II and, Ref. 2. and 4. in Part III. In Part I a symplectic structure of a Boson Fock space is studied and a projective unitary representation of an infinite dimensional symplectic group through Bogoliubov transformations is constructed. In Part II the so-called Pauli-Fierz model (PF model) with the dipole approximation in non-relativistic quantum electrodynamics is investigated. This model describes a minimal interaction between a massless quantized radiation field and a quantum mechanical particle (electron) governed by Schrödinger operator. By applying the Bogoliubov transformation introduced in Part I we investigate the spectrum of the PF model. First the translation invariant case is considered and the dressed electron state with a fixed momentum is studied. Secondly the absence of ground state is proven by extending the Birman-Schwinger principle. Finally the enhanced binding of a ground state is discussed and the transition from unbinding to binding is shown. In Part III the so-called $N$-body Nelson model is studied. This model describes a linear interaction between a scalar field and $N$-body quantum mechanical particles. First the enhanced binding is shown by checking the so-called stability condition. Secondly the Nelson model with variable coefficients is discussed, which model can be derived when the Minkowskian space-time is replaced by a static Riemannian manifold, and the absence of ground state is proven, if the variable mass decays to zero sufficiently fast. The strategy is based on a path measure argument.

math-ph↗