Taichi Kosugi

To explore the electronic structure of the first aromatic superconductor, potassium-doped solid picene which has been recently discovered by Mitsuhashi et al with the transition temperatures $T_c=7 - 20$ K, we have obtained a first-principles electronic structure of solid picene as a first step toward the elucidation of the mechanism of the superconductivity. The undoped crystal is found to have four conduction bands, which are characterized in terms of the maximally localized Wannier orbitals.

We theoretically explore the crystal structures of K${}_{x}$picene for which a new aromatic superconductivity has recently been discovered for $x=3$, by systematically performing first-principles full structural optimization covering the concentration range $x=1$--4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. F

Imaginary-time evolution (ITE) on a quantum computer is a promising formalism for obtaining the ground state of a quantum system. The probabilistic ITE (PITE) exploits measurements to implement nonunitary operations, and it can avoid the restriction of dynamics to a low-dimensional subspace imposed by variational parameters unlike other types of ITE. In this paper, we propose a PITE approach that uses only one ancillary qubit. Unlike the existing PITE approaches, the one proposed here constructs

We have obtained the first-principles electronic structure of solid coronene, which has been recently reported to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provides a class of superconductors as solids of aromatic compounds, here we compare the two cases by examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit ce

We propose a scheme for the construction of the one-particle Green's function (GF) of an interacting electronic system via statistical sampling on a quantum computer. Although the nonunitarity of creation and annihilation operators for the electronic spin orbitals prevents us from preparing specific states selectively, probabilistic state preparation is demonstrated to be possible for the qubits. We provide quantum circuits equipped with at most two ancillary qubits for obtaining all the compone

The Schroedinger equation for a spinless charged particle on a curved surface under an electromagnetic field has been obtained by adopting a proper gauge which allows the separation of the on-surface and transverse dynamics. [Phys. Rev. Lett. 100 (2008) 230403] As its extension, I provide the Pauli equation for a charged spin-1/2 particle confined to a curved surface under an electromagnetic field. Energy spectra of a sphere and a corrugated surface to which a particle is confined are given as s

We study the dependence of the spin splitting on the number $N$ of atomic layers, using first-principles calculation for Au(111) surface. When the slab of the atomic layers is sufficiently thick, the lower split state has a minimum away from $\bar{\Gamma}$, which is known as the Rashba effect. As the number of layers decreases, the minimum approaches $\bar{\Gamma}$, and it is located at $\bar{\Gamma}$ for $N \leq 14$. This crossover is analyzed in detail using two models: a tight-binding model a

This work proposes a generic construction scheme for the quantum circuit implementing a nonunitary operator appearing in electronic-structure calculations. The authors demonstrate that the scheme enables one via probabilistic state preparation to calculate the linear-response functions of diatomic molecules, where simulated measurements and full configuration-interaction results are compared.

We demonstrate in the present study that self-consistent calculations based on the self-energy functional theory (SFT) are possible for the electronic structure of realistic systems in the context of quantum chemistry. We describe the procedure of a self-consistent SFT calculation in detail and perform the calculations for isolated 3d transition metal atoms from V to Cu as a preliminary study. We compare the one-particle Green's functions obtained in this way and those obtained from the coupled-

We propose a novel periodicity-free unfolding method of electronic energy spectra. Our new method solves the serious problem that a calculated electronic band structure strongly depends on the choice of the simulation cell, i.e., primitive cell or supercell. The present method projects the electronic states onto the free-electron states, giving rise to plane-wave unfolded spectra. Using the method, the energy spectra can be calculated as a quantity independent of the choice of the simulation cel

Abstract This study proposes a nonvariational scheme for geometry optimization of molecules for the first-quantized eigensolver, which is a recently proposed framework for quantum chemistry using probabilistic imaginary-time evolution (PITE). In this scheme, the nuclei in a molecule are treated as classical point charges while the electrons are treated as quantum mechanical particles. The electronic states and candidate geometries are encoded as a superposition of many-qubit states, for which a

Abstract First-quantized eigensolver (FQE) is a recently proposed quantum computation framework for obtaining the ground state of an interacting electronic system based on probabilistic imaginary-time evolution. Here, we propose a method for introducing a uniform magnetic field to the FQE calculation. Our resource estimation demonstrates that the additional circuit responsible for the magnetic field can be implemented with a linear depth in terms of the number of qubits assigned to each electron

By treating both control parameters and dynamical variables as probabilistic variables, we develop a succinct theory of perpetual extraction of work from a generic classical nonequilibrium system subject to a heat bath via repeated measurements under a Markovian feedback control. It is demonstrated that a problem for perpetual extraction of work in a nonequilibrium system is reduced to a problem of Markov chain in the higher-dimensional phase space. We derive a version of the detailed fluctuatio

For a three-electron system with finite-strength interactions confined to a one-dimensional harmonic trap, we solve the Schroedinger equation analytically to obtain the exact solutions, from which we construct explicitly the simultaneous eigenstates of the energy and total spin for the first time. The solutions for the three-electron system allow us to derive analytic expressions for the exact one-particle Green's function (GF) for the corresponding two-electron system. We calculate the GF in fr

One of the crucial generic techniques for quantum computation is amplitude encoding. Although several approaches have been proposed, each of them often requires exponential classical-computational cost or an oracle whose explicit construction is not provided. Given the growing demands for practical quantum computation, we develop moderately specialized encoding techniques that generate an arbitrary linear combination of localized complex functions. We demonstrate that ${n}_{\mathrm{loc}}$ discre