arXiv subjects

Yusuke Hama

Publications and source records attributed to Yusuke Hama.

12 recordsLinked to original sources

arXiv2025

Subsampling Factorization Machine Annealing

Quantum computing and machine learning are state-of-the-art technologies that have been investigated intensively in both academia and industry. The hybrid technology of these two ingredients is expected to be a powerful tool to solve complex problems in many branches of science and engineering such as combinatorial optimization problems and accelerate the creation of next-generation technologies. In this work, we develop an algorithm to solve a black-box optimization problem by improving Factorization Machine Annealing (FMA) such that the training of a machine learning model called Factorization Machine is performed not by a full dataset but by a subdataset that is sampled from a full dataset: Subsampling Factorization Machine Annealing (SFMA). According to such a probabilistic training process, the performance of FMA on exploring a solution space gets enhanced. As a result, SFMA exhibits balanced performance of exploration and exploitation, which we call exploitation-exploration functionality. We conduct numerical benchmarking tests to compare the performance of SFMA with that of FMA. Consequently, SFMA certainly exhibits the exploration-exploitation functionality and outperforms FMA in speed and accuracy. In addition, the performance of SFMA can be further improved by sequentially using two subsampling datasets with different sizes such that the size of the latter dataset is substantially smaller than the former. Such a substantial reduction not only enhances the exploration performance of SFMA but also enables us to run it with correspondingly low computational cost even for a large-scale problem. These results indicate the effectiveness of SFMA in a certain class of black-box optimization problems of significant size: the potential scalability of SFMA in solving large-scale problems with correspondingly low computational cost.

quant-ph↗

arXiv2023

Quantum-Error-Mitigation Circuit Groups for Noisy Quantum Metrology

Quantum technologies work by utilizing properties inherent in quantum systems such as quantum coherence and quantum entanglement and are expected to be superior to classical counterparts for solving certain problems in science and engineering. The quantum technologies are, however, fragile against an interaction with an environment (decoherence) and in order to utilize them with high accuracy we need to develop error mitigation techniques which reduce decoherence effects. In this work, we analyze quantum error mitigation (QEM) protocol for quantum metrology in the presence of quantum noise. We demonstrate the effectiveness of our QEM protocol by analyzing three types of quantum Fisher information (QFI), ideal (error-free) QFI, noisy (erroneous) QFI, and quantum-error-mitigated QFI, and show both analytically and numerically that the scaling behaviors of quantum-error-mitigated QFI with respect to the number of probes become restored to the those exhibited in the ideal quantum metrology. Our QEM protocol is constructed by an ensemble of quantum circuits, namely QEM circuit groups, and has advantages such that it can be applied to noisy quantum metrology for any type of initial state as well as any type of the probe-system Hamiltonian, and it can be physically implemented in any type of quantum device. Furthermore, the quantum-error-mitigated QFI become approximately equal to the ideal QFI for almost any values of physical quantities to be sensed. Our protocol enables us to use quantum entanglement as a resource to perform high-sensitive quantum metrology even under the influence of quantum noise.

quant-ph↗

arXiv2022

Quantum Error Mitigation via Quantum-Noise-Effect Circuit Groups

Near-term quantum computers have been built as intermediate-scale quantum devices and are fragile against quantum noise effects, namely, NISQ devices. Traditional quantum-error-correcting codes are not implemented on such devices and to perform quantum computation in good accuracy with these machines we need to develop alternative approaches for mitigating quantum computational errors. In this work, we propose quantum error mitigation (QEM) scheme for quantum computational errors which occur due to couplings with environments during gate operations, i.e., decoherence. To establish our QEM scheme, first we estimate the quantum noise effects on single-qubit states and represent them as groups of quantum circuits, namely, quantum-noise-effect circuit groups. Then our QEM scheme is conducted by subtracting expectation values generated by the quantum-noise-effect circuit groups from that obtained by the quantum circuits for the quantum algorithms under consideration. As a result, the quantum noise effects are reduced, and we obtain approximately the ideal expectation values via the quantum-noise-effect circuit groups and the numbers of elementary quantum circuits composing them scale polynomial with respect to the products of the depths of quantum algorithms and the numbers of register bits. To numerically demonstrate the validity of our QEM scheme, we run noisy quantum simulations of qubits under amplitude damping effects for four types of quantum algorithms. Furthermore, we implement our QEM scheme on IBM Q Experience processors and examine its efficacy. Consequently, the validity of our scheme is verified via both the quantum simulations and the quantum computations on the real quantum devices.

quant-ph↗

arXiv2020

Quantum Circuits for Collective Amplitude Damping in Two-Qubit Systems

Quantum computers have now appeared in our society and are utilized for the investigation of science and engineering. At present, they have been built as intermediate-size computers containing about fifty qubits and are weak against noise effects. Hence, they are called noisy-intermediate scale quantum devices. In order to accomplish efficient quantum computation with using these machines, a key issue is going to be the coherent control of individual and collective quantum noises. In this work, we focus on a latter type and investigate formulations of the collective quantum noises represented as quantum circuits. To simplify our discussions and make them concrete, we analyze collective amplitude damping processes in two-qubit systems. As verifications of our formalisms and the quantum circuits, we demonstrate digital quantum simulations of the collective amplitude damping by examining six different initial conditions with varying the number of execution of an overall operation for our quantum simulations. We observe that our results show good numerical matching with the solution of quantum master equation for the two-qubit systems as we increase such a number. In addition, we explain the essence of the way to extend our formalisms to analyze the collective amplitude damping in larger qubit systems. These results pave the way for establishing systematic approaches to control the quantum noises and designing large-scale quantum computers.

quant-ph↗

arXiv2020

Spin-Pumping-Induced Non-Linear Electric Current on the Surface of a Ferromagnetic Topological Insulator

We investigate the spin-pumping-induced electric current on the surface of a three-dimensional topological insulator hybridized with a ferromagnet, namely, ferromagnetic topological insulator. In order to do this, we establish the microscopic formalism and construct the perturbation theory using a Keldysh Green's function approach. We analyze how this electric current is generated by an exchange interaction and an external ac magnetic field, which is the driving force of ferromagnetic resonance as well as the spin pumping. The mechanism is as follows. First, the ferromagnetic resonance is driven and a zero-momentum magnon emerges. It is the fluctuation from the saturation magnetization pointing parallel to the precession axis of the ferromagnetic resonance. After then, the spin pumping is generated with the zero-momentum magnon being the carrier of spin. The zero-momentum magnon and the topological insulator surface state couples through the exchange interaction and the spin carried by the magnon is transferred to it. Owing to the spin-momentum locking, the transferred spin is converted into the momentum of topological insulator surface state leading to the generation of electric current flowing perpendicular to the precession axis of the ferromagnetic resonance. It is quadratic in the amplitude of external ac magnetic field whereas it is linear to the strength of the exchange interaction. The associated electric voltage is described by the spectrum of zero-momentum magnon. The non-linearity of spin-pumping-induced electric current in the ac magnetic field as well as the linearity in the exchange-interaction strength reflects that the surface of ferromagnetic topological insulator has a high-performing functionality of generating the electric charge current by magnetic controlling.

cond-mat.mes-hall↗

arXiv2018

Negative-Temperature State Relaxation and Reservoir-Assisted Quantum Entanglement in Double Spin Domain Systems

Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two novel collective behaviors: the negative-temperature state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.

quant-ph↗

arXiv2017

Electromagnon on the Surface of Magnetic Topological Insulator

We investigate theoretically the electromagnon on the surfaces of the magnetic topological insulator thin films. It is found that when the magnetic asymmetry between the top and bottom surfaces is there, the ferromagnetic resonance is driven by the electric field which is two orders of magnitude more efficient compared with that by the magnetic field. The resonant frequency of the electromagnon is also estimated.

cond-mat.mes-hall↗

arXiv2016

Relaxation to Negative Temperatures in Double Domain Systems

We investigate the relaxation of two collective spins in double domain system, which are individually coupled to a single bosonic reservoir, by varying the total number of spins in each domain and their initial spin configurations. A particularly interesting situation occurs when the spin domains are set in an antiparallel configuration. Further for an unbalanced configuration where the number of spins are in the excited state initially is much greater than that in the ground state, the spin ensemble prepared in the ground state relaxes towards a negative-temperature state.

quant-ph↗

arXiv2016

Green's Function of Magnetic Topological Insulator in Gradient Expansion Approach

We study the Keldysh Green's function of the Weyl-fermion surface state of the three-dimensional topological insulator coupled with a space-time dependent magnetization in the gradient expansion. Based on it we analyze the electric charge and current densities as well as the energy density and current induced by spatially and temporally slowly-varying magnetization fields. We show that all the above quantities except the energy current are generated by the emergent electromagnetic fields. The energy current emerges as the circular current reflecting the spatial modulation of an induced gap of the Weyl fermion.

cond-mat.mes-hall↗

arXiv2013

Nambu-Goldstone modes and the Josephson supercurrent in the bilayer quantum Hall system

An interlayer phase coherence develops spontaneously in the bilayer quantum Hall system at the filling factor $ν=1$. On the other hand, the spin and pseudospin degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor $ν=2$. There emerges a complex Nambu-Goldstone mode with a linear dispersion in the zero tunneling-interaction limit for both cases. Then its phase field provokes a Josephson supercurrent in each layer, which is dissipationless as in a superconductor. We study what kind of phase coherence the Nambu-Goldstone mode develops in association with the Josephson supercurrent and its effect on the Hall resistance in the bilayer quantum Hall system at $ν=1,2$, by employing the Grassmannian formalism.

cond-mat.mes-hall↗

arXiv2013

Spin Supercurrent in the Canted Antiferromagnetic Phase

The spin and layer (pseudospin) degrees of freedom are entangled coherently in the canted antiferromagnetic phase of the bilayer quantum Hall system at the filling factor $ν=2$. There emerges a complex Goldstone mode describing such a combined degree of freedom. In the zero tunneling-interaction limit ($Δ_{\text{SAS}}\rightarrow 0$), its phase field provokes a supercurrent carrying both spin and charge within each layer. The Hall resistance is predicted to become anomalous precisely as in the $ν=1$ bilayer system in the counterflow and drag experiments. Furthermore, it is shown that the total current flowing in the bilayer system is a supercurrent carrying solely spins in the counterflow geometry. It is intriguing that all these phenomena occur only in imbalanced bilayer systems.

cond-mat.mes-hall↗

arXiv2011

Higgs Mechanism with Type-II Nambu-Goldstone Bosons at Finite Chemical Potential

When the spontaneous symmetry breaking occurs for systems without Lorentz covariance, there arises possible mismatch, $N_{\rm NG} < N_{\rm BG}$, between numbers of Nambu-Goldstone (NG) bosons ($N_{\rm NG}$) and the numbers of broken generators ($N_{\rm BG}$. In such a situation, so-called type-II NG bosons emerge. We study how the gauge bosons acquire masses through the Higgs mechanism under this mismatch by employing gauge theories with complex scalar field at finite chemical potential and by enforcing "charge" neutrality. To separate the physical spectra from unphysical ones, the $R_ξ$ gauge is adopted. Not only massless NG bosons but also massive scalar bosons generated by the chemical potential are absorbed into spatial components of the gauge bosons. Although the chemical potential induces a non-trivial mixings among the scalar bosons and temporal components of the gauge bosons, it does not affect the structure of the physical spectra, so that the total number of physical modes is not modified even for $N_{\rm NG} < N_{\rm BG}$.

hep-ph↗