arXiv subjects

Hoon Ryu

Publications and source records attributed to Hoon Ryu.

At least 19 recordsLinked to original sources

arXiv2025

An approach using geometric diagrams to generic Bell inequalities with multiple observables

We extend the generic Bell inequalities suggested by Son, Lee, and Kim [Phys. Rev. Lett. 96, 060406 (2006)] to incorporate multiple observables for tripartite systems and introduce a geometric methodology for calculating classical upper bounds of the inequalities. Our method transforms the problem of finding the classical upper bounds into identifying constraints in linear congruence relations. Using this approach, we derive the upper bounds for scenarios with three and four observables per party. In order to demonstrate quantum violations, we employ Greenberger-Horne-Zeilinger entangled states that can achieve values exceeding the classical upper bounds, with the violation becoming more pronounced as the number of observables increases.

quant-ph↗

arXiv2025

Operational Quasiprobability in Quantum Thermodynamics: Work Extraction by Coherence and Non-joint Measurability

We employ the operational quasiprobability (OQ) as a work distribution, which reproduces the Jarzynski equality and yields the average work consistent with the classical definition. The OQ distribution can be experimentally implemented through the end-point measurement and the two-point measurement scheme. Using this framework, we demonstrate the explicit contribution of coherence to the fluctuation, the average, and the second moment of work. In a two-level system, we show that non-joint measurability, a generalized notion of measurement incompatibility, can increase the amount of extractable work beyond the classical bound imposed by jointly measurable measurements. We further prove that the real part of Kirkwood-Dirac quasiprobability (KDQ) and the OQ are equivalent in two-level systems, and they are nonnegative for binary unbiased measurements if and only if the measurements are jointly measurable. In a three-level Nitrogen-vacancy center system, the OQ and the KDQ exhibit different amounts of negativities while enabling the same work extraction, implying that the magnitude of negativity is not a faithful indicator of nonclassical work. These results highlight that coherence and non-joint measurability play fundamental roles in the enhancement of work.

quant-ph↗

arXiv2025

Subspace Variational Quantum Simulation: Fidelity Lower Bounds as Measures of Training Success

We propose an iterative variational quantum algorithm to simulate the time evolution of arbitrary initial states within a given subspace. The algorithm compresses the Trotter circuit into a shorter-depth parameterized circuit, which is optimized simultaneously over multiple initial states in a single training process using fidelity-based cost functions. After the whole training procedure, we provide an efficiently computable lower bound on the fidelities for arbitrary states within the subspace, which guarantees the performance of the algorithm in the worst-case training scenario. We also show our cost function exhibits a barren-plateau-free region near the initial parameters at each iteration in the training landscape. The experimental demonstration of the algorithm is presented through the simulation of a 2-qubit Ising model on an IBMQ processor. As a demonstration for a larger system, a simulation of a 10-qubit Ising model is also provided.

quant-ph↗

arXiv2025

PennyLane-Lightning MPI: A massively scalable quantum circuit simulator based on distributed computing in CPU clusters

Quantum circuit simulations play a critical role in bridging the gap between theoretical quantum algorithms and their practical realization on physical quantum hardware, yet they face computational challenges due to the exponential growth of quantum state spaces with increasing qubit size. This work presents PennyLane-Lightning MPI, an MPI-based extension of the PennyLane-Lightning suite, developed to enable scalable quantum circuit simulations through parallelization of quantum state vectors and gate operations across distributed-memory systems. The core of this implementation is an index-dependent, gate-specific parallelization strategy, which fully exploits the characteristic of individual gates as well as the locality of computation associated with qubit indices in partitioned state vectors. Benchmarking tests with single gates and well-designed quantum circuits show that the present method offers advantages in performance over general methods based on unitary matrix operations and exhibits excellent scalability, supporting simulations of up to 41-qubit with hundreds of thousands of parallel processes. Being equipped with a Python plug-in for seamless integration to the PennyLane framework, this work contributes to extending the PennyLane ecosystem by enabling high-performance quantum simulations in standard multi-core CPU clusters with no library-specific requirements, providing a back-end resource for the cloud-based service framework of quantum computing that is under development in the Republic of Korea.

quant-ph↗

arXiv2025

A variational quantum eigensolver tailored to multi-band tight-binding simulations of electronic structures

We propose a cost-efficient measurement scheme of the variational quantum eigensolver (VQE) for atomistic simulations of electronic structures based on a tight-binding (TB) theory. Leveraging the lattice geometry of a material domain, the sparse TB Hamiltonian is constructed in a bottom-up manner and is represented as a linear combination of the standard-basis (SB) operators. The cost function is evaluated with an extended version of the Bell measurement circuit that can simultaneously measure multiple SB operators and therefore reduces the number of circuits required bythe evaluation process. The proposed VQE scheme is applied to find band-gap energies of metal-halide-perovskite supercells that have finite dimensions with closed boundaries and are described with a sp3 TB model. Experimental results confirm that the proposed scheme gives solutions that follow well the accurate ones, but, more importantly, has the computing efficiency that is obviously superior to the commutativity-based Pauli grouping methods. Extending the application scope of VQE to three-dimensional confined atomic structures, this work can serve as a practical guideline for handling TB simulations in the noise-intermediate-scale quantum devices.

quant-ph↗

arXiv2024

A variational quantum algorithm for tackling multi-dimensional Poisson equations with inhomogeneous boundary conditions

We design a variational quantum algorithm to solve multi-dimensional Poisson equations with mixed boundary conditions that are typically required in various fields of computational science. Employing an objective function that is formulated with the concept of the minimal potential energy, we not only present in-depth discussion on the cost-efficient & noise-robust design of quantum circuits that are essential for evaluation of the objective function, but, more remarkably, employ the proposed algorithm to calculate bias-dependent spatial distributions of electric fields in semiconductor systems that are described with a two-dimensional domain and up to 10-qubit circuits. Extending the application scope to multi-dimensional problems with mixed boundary conditions for the first time, fairly solid computational results of this work clearly demonstrate the potential of variational quantum algorithms to tackle Poisson equations derived from physically meaningful problems.

quant-ph↗

arXiv2024

On the feasibility of quantum teleportation protocols implemented with Silicon devices

With recent experimental advancements demonstrating high-fidelity universal logic gates and basic programmability, Silicon-based spin quantum bit (qubit) have emerged as promising candidates for scalable quantum computing. However, implementation of more complex quantum information protocols with many qubits still remains a critical challenge for realization of practical programmability in Silicon devices. In this study, we present a computational investigation of entanglement-based quantum information applications implemented on an electrically defined quantum dot structure in Silicon. Using in-house multi-scale simulations based on tight-binding calculations augmented with bulk physics, we model a five quantum dot system that can create up to five electron spin qubits, and discuss details of control engineering needed to implement single-qubit rotations and two-qubit logic operations in a programmable manner. Using these elementary operations, then, we design a five-qubit quantum teleportation protocol and computationally verify its end-to-end operation including a simple but clear analysis on how the designed circuit can be affected by charge noise. With engineering details that are not well uncovered by experiments, our results demonstrate the advanced programmability of Silicon quantum dot systems, delivering the practical guidelines for potential designs of quantum information processes based on electrically defined Silicon quantum dot structures.

quant-ph↗

arXiv2023

Quantum-centric Supercomputing for Materials Science: A Perspective on Challenges and Future Directions

Computational models are an essential tool for the design, characterization, and discovery of novel materials. Hard computational tasks in materials science stretch the limits of existing high-performance supercomputing centers, consuming much of their simulation, analysis, and data resources. Quantum computing, on the other hand, is an emerging technology with the potential to accelerate many of the computational tasks needed for materials science. In order to do that, the quantum technology must interact with conventional high-performance computing in several ways: approximate results validation, identification of hard problems, and synergies in quantum-centric supercomputing. In this paper, we provide a perspective on how quantum-centric supercomputing can help address critical computational problems in materials science, the challenges to face in order to solve representative use cases, and new suggested directions.

quant-ph↗

arXiv2022

Exploring entanglement resource in Si quantum dot systems with operational quasiprobability approach

We characterize the quantum entanglement of the realistic two-qubit signals that are sensitive to charge noises. Our working example is the time response generated from a silicon double quantum dot (DQD) platform, where a single-qubit rotation and a two-qubit controlled-NOT operation are conducted sequentially in time to generate arbitrary entangled states. In order to characterize the entanglement of two-qubit states, we employ the marginal operational quasiprobability (OQ) approach that allows negative values of the probability function if a given state is entangled. While the charge noise, which is omnipresent in semiconductor devices, severely affects logic operations implemented in the DQD platform, causing huge degradation in fidelity of unitary operations as well as resulting two-qubit states, the pattern in the OQ-driven entanglement strength turns out to be quite invariant, indicating that the resource of quantum entanglement is not significantly broken though the physical system is exposed to noise-driven fluctuations in exchange interaction between quantum dots.

quant-ph↗

arXiv2022

Devitalizing noise-driven instability of entangling logic in silicon devices with bias controls

The quality of quantum bits (qubits) in silicon is highly vulnerable to charge noise that is omni-present in semiconductor devices and is in principle hard to be suppressed. For a realistically sized quantum dot system based on a silicon-germanium heterostructure whose confinement is manipulated with electrical biases imposed on top electrodes, we computationally explore the noise-robustness of 2-qubit entangling operations with a focus on the controlled-X (CNOT) logic that is essential for designs of gate-based universal quantum logic circuits. With device simulations based on the physics of bulk semiconductors augmented with electronic structure calculations, we not only quantify the degradation in fidelity of single-step CNOT operations with respect to the strength of charge noise, but also discuss a strategy of device engineering that can significantly enhance noise-robustness of CNOT operations with almost no sacrifice of speed compared to the single-step case. Details of device designs and controls that this work presents can establish a rare but practical guideline for potential efforts to secure silicon-based quantum processors using an electrode-driven quantum dot platform.

quant-ph↗

arXiv2014

Atomistic and continuum modeling of a zincblende quantum dot heterostructure

A multiscale approach was adopted for the calculation of confined states in self-assembled semiconductor quantum dots (QDs). While results close to experimental data have been obtained with a combination of atomistic strain and tight-binding (TB) electronic structure description for the confined quantum states in the QD, the TB calculation requires substantial computational resources. To alleviate this problem an integrated approach was adopted to compute the energy states from a continuum 8-band k.p Hamiltonian under the influence of an atomistic strain field. Such multi-scale simulations yield a roughly six-fold faster simulation. Atomic-resolution strain is added to the k.p Hamiltonian through interpolation onto a coarser continuum grid. Sufficient numerical accuracy is obtained by the multi-scale approach. Optical transition wavelengths are within 7% of the corresponding TB results with a proper splitting of p-type sub-bands. The systematically lower emission wavelengths in k.p are attributable to an underestimation of the coupling between the conduction and valence bands.

cond-mat.mes-hall↗

arXiv2014

Numerical guidelines for setting up a general purpose k.p simulator with applications to quantum dot heterostructures and topological insulators

The k.p perturbation method for determination of electronic structure first pioneered by Kohn and Luttinger continues to provide valuable insight to several band structure features. This method has been adopted to heterostructures confined up to three directions. In this paper, numerical details of setting up a k.p Hamiltonian using the finite difference approximation for such confined nanostructures is explicitly demonstrated. Nanostructures belonging to two symmetry classes namely cubic zincblende and rhombohedral crystals are considered. Rhombohedral crystals, of late, have gained prominence as candidates for the recently discovered topological insulator (TI) class of materials. Lastly the incorporation of strain field to the k.p Hamiltonian and matrix equations for computing the intrinsic and externally applied strain in heterostructures within a continuum approximation is shown. Two applications are considered 1)Computation of the eigen states of a multi-million zincblende InAs quantum dot with a stress-reducing InGaAs layer of varying Indium composition embedded in a GaAs matrix and 2)Dispersion of a rhombohedral topological insulator Bi$_{2}$Se$_{3}$ film.

cond-mat.mes-hall↗

arXiv2011

Feasibility, Accuracy and Performance of Contact Block Reduction method for multi-band simulations of ballistic quantum transport

Numerical utilities of the Contact Block Reduction (CBR) method in evaluating the retarded Green's function, are discussed for 3-D multi-band open systems that are represented by the atomic tight-binding (TB) and continuum k\cdotp (KP) band model. It is shown that the methodology to approximate solutions of open systems which has been already reported for the single-band effective mass model, cannot be directly used for atomic TB systems, since the use of a set of zincblende crystal grids makes the inter-coupling matrix be non-invertible. We derive and test an alternative with which the CBR method can be still practical in solving TB systems. This multi-band CBR method is validated by a proof of principles on small systems, and also shown to work excellent with the KP approach. Further detailed analysis on the accuracy, speed, and scalability on high performance computing clusters, is performed with respect to the reference results obtained by the state-of- the-art Recursive Green's Function and Wavefunction algorithm. This work shows that the CBR method could be particularly useful in calculating resonant tunneling features, but show a limited practicality in simulating field effect transistors (FETs) when the system is described with the atomic TB model. Coupled to the KP model, however, the utility of the CBR method can be extended to simulations of nanowire FETs.

cond-mat.mes-hall↗

arXiv2011

Quantitative Excited State Spectroscopy of a Single InGaAs Quantum Dot Molecule through Multi-million Atom Electronic Structure Calculations

Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum [12] is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15 million atom structures. An excited state spectroscopy technique is presented in which the externally applied electric field is swept to probe the ladder of the electronic energy levels (electron or hole) of one quantum dot through anti-crossings with the energy levels of the other quantum dot in a two quantum dot molecule. This technique can be applied to estimate the spatial electron-hole spacing inside the quantum dot molecule as well as to reverse engineer quantum dot geometry parameters such as the quantum dot separation. Crystal deformation induced piezoelectric effects have been discussed in the literature as minor perturbations lifting degeneracies of the electron excited (P and D) states, thus affecting polarization alignment of wave function lobes for III-V Heterostructures such as single InAs/GaAs quantum dots. In contrast this work demonstrates the crucial importance of piezoelectricity to resolve the symmetries and energies of the excited states through matching the experimentally measured spectrum in an InGaAs quantum dot molecule under the influence of an electric field. Both linear and quadratic piezoelectric effects are studied for the first time for a quantum dot molecule and demonstrated to be indeed important. The net piezoelectric contribution is found to be critical in determining the correct energy spectrum, which is in contrast to recent studies reporting vanishing net piezoelectric contributions.

cond-mat.mes-hall↗

arXiv2010

Experimental and Theoretical Study of Polarization-dependent Optical Transitions from InAs Quantum Dots at Telecommunication-Wavelengths (1.3-1.5μm)

The design of some optical devices such as semiconductor optical amplifiers for telecommunication applications requires polarization-insensitive optical emission at the long wavelengths (1300-1550 nm). Self-assembled InAs/GaAs quantum dots (QDs) typically exhibit ground state optical emission at wavelengths shorter than 1300 nm with highly polarization-sensitive characteristics, although this can be modified by using low growth rates, the incorporation of strain-reducing capping layers or growth of closely-stacked QD layers. Exploiting the strain interactions between closely stacked QD layers also allows greater freedom in the choice of growth conditions for the upper layers, so that both a significant extension in their emission wavelength and an improved polarization response can be achieved due to modification of the QD size, strain and composition. In this paper we investigate the polarization behavior of single and stacked QD layers using room temperature sub-lasing-threshold electroluminescence and photovoltage measurements as well as atomistic modeling with the NEMO 3-D simulator. A reduction is observed in the ratio of the transverse electric (TE) to transverse magnetic (TM) optical mode response for a GaAs-capped QD stack compared to a single QD layer, but when the second layer of the two-layer stack is InGaAs-capped an increase in the TE/TM ratio is observed, in contrast to recent reports for single QD layers.

cond-mat.mtrl-sci↗

arXiv2010

A Study of Temperature-dependent Properties of N-type delta-doped Si Band-structures in Equilibrium

A highly phosphrous delta-doped Si device is modeled with a quantum well with periodic boundary conditions and the semi-empirical spds* tight-binding band model. Its temperature-dependent electronic properties are studied. To account for high doping density with many electrons, a highly parallelized self-consistent Schroedinger-Poisson solver is used with atomistic representations of multiple impurity ions. The band-structure in equilibrium and the corresponding Fermi-level position are computed for a selective set of temperatures. The result at room temperature is compared with previous studies and the temperature-dependent electronic properties are discussed further in detail with the calculated 3-D self-consistent potential profile.

cond-mat.mtrl-sci↗

arXiv2010

Million Atom Electronic Structure and Device Calculations on Peta-Scale Computers

Semiconductor devices are scaled down to the level which constituent materials are no longer considered continuous. To account for atomistic randomness, surface effects and quantum mechanical effects, an atomistic modeling approach needs to be pursued. The Nanoelectronic Modeling Tool (NEMO 3-D) has satisfied the requirement by including emprical $sp^{3}s^{*}$ and $sp^{3}d^{5}s^{*}$ tight binding models and considering strain to successfully simulate various semiconductor material systems. Computationally, however, NEMO 3-D needs significant improvements to utilize increasing supply of processors. This paper introduces the new modeling tool, OMEN 3-D, and discusses the major computational improvements, the 3-D domain decomposition and the multi-level parallelism. As a featured application, a full 3-D parallelized Schrödinger-Poisson solver and its application to calculate the bandstructure of $δ$ doped phosphorus(P) layer in silicon is demonstrated. Impurity bands due to the donor ion potentials are computed.

physics.comp-ph↗

arXiv2009

Multimillion Atom Simulations with NEMO 3-D

The rapid progress in nanofabrication technologies has led to the emergence of new classes of nanodevices and structures. At the atomic scale of novel nanostructured semiconductors the distinction between new device and new material is blurred and device physics and material science meet. The quantum mechanical effects in the electronic states of the device and the granular, atomistic representation of the underlying material become important. The variety of geometries, materials, and doping configurations in semiconductor devices at the nanoscale suggests that a general nanoelectronic modeling tool is needed. The Nanoelectronic Modeling tool (NEMO 3-D) has been developed to address these needs. Based on the atomistic valence-force field (VFF) method and a variety of nearest-neighbor tight-binding models, NEMO 3-D enables the computation of strain for over 64 million atoms and of electronic structure for over 52 million atoms, corresponding to volumes of (110nmx110nmx110nm) and (101nmx101nmx101nm), respectively. This article discusses the theoretical models, essential algorithmic and computational components, and optimization methods that have been used in the development and the deployment of NEMO 3-D. Also, successful applications of NEMO 3-D are demonstrated in the atomistic calculation of single-particle electronic states of (1) self-assembled quantum dots including long-range strain and piezoelectricity; (2) stacked quantum dots ; (3) Phosphorus impurities in Silicon used in quantum computation; (4) Si on SiGe quantum wells (QWs); and (5) SiGe nanowires.

physics.comp-ph↗