arXiv subjects

Nobuki Inoue

Publications and source records attributed to Nobuki Inoue.

3 recordsLinked to original sources

arXiv2026

Correction scheme for molecular total energies from quantum phase estimation under limited qubit resources

We propose a practical method for accurately evaluating molecular total energies using a hybrid approach that integrates fault-tolerant quantum computers with classical computing. Our scheme consists of two complementary components: quantum dominant orbital selection (QDOS) and subspace dynamical correlation (SDC). QDOS extracts only the essential active orbitals from the complete active space (CAS) configuration interaction (CI) state on a quantum computer, yielding a compact active space suitable for classical CASCI calculations. SDC then evaluates dynamical-correlation corrections for the CASCI energy using this compact state, which remains tractable on classical machines. To demonstrate that the CAS energy obtained on a quantum computer can be post-corrected by SDC, we examine two frameworks: multireference perturbation theory and tailored coupled-cluster theory. Our scheme enables effective treatment of relatively large molecular systems by combining limited quantum and classical resources.

quant-ph↗

arXiv2022

Theoretical examination of QED Hamiltonian and negative-energy orbitals in relativistic molecular orbital theory

The relativistic Hartree-Fock and electron correlation methods without the negative-energy orbital problem are examined on the basis of the quantum electrodynamics (QED) Hamiltonian. First, several QED Hamiltonians previously proposed are sifted by the orbital rotation invariance, the charge conjugation and time reversal invariance, and the nonrelativistic limit. A new total energy expression is then proposed, in which a counter term corresponding to the energy of the polarized vacuum is subtracted from the total energy. This expression prevents the possibility of total energy divergence due to electron correlations, stemming from the fact that the QED Hamiltonian does not conserve the number of particles. Finally, based on the Hamiltonian and energy expression, the Dirac-Hartree-Fock (DHF) and electron correlation methods are reintroduced. The resulting QED-based DHF equation has the same form as the conventional DHF equation, but also formally describes systems specific to QED, such as the virtual positrons in the hydride ion and the positron in positronium. Three electron correlation methods are derived: the QED-based configuration interactions and single- and multireference perturbation methods. Numerical calculations show that the total energy of the QED Hamiltonian indeed diverges and that the counter term is effective in avoiding the divergence. The theoretical examinations in the present article suggest that the molecular orbital (MO) methods based on the QED Hamiltonian not only solve the problem of the negative-energy solutions of the relativistic MO method, but also provide a relativistic formalism to treat systems containing positrons.

physics.chem-ph↗

arXiv2019

Probing embedded topological modes in bulk-like GeTe-Sb$_2$Te$_3$ heterostructures

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While these topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such \textit{embedded} topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb$_2$Te$_3$ (topological insulator). We analyse their dependence on the interface and their confinement characteristics. To characterise the heterostructures, we evaluate the GeTe-Sb$_2$Te$_3$ band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. Interestingly, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen \textit{et al.}, Sci. Rep. \textbf{6}, 27716 (2016)] where ETSs were seen to couple over large distances.

cond-mat.mes-hall↗