arXiv subjects

Woojun Lee

Publications and source records attributed to Woojun Lee.

12 recordsLinked to original sources

arXiv2026

A Single-Tuning-Element Loaded Pixelated Tunable Band-Pass Filters with Low Loss via Inverse Synthesis on Massive-Scale Dataset

This paper presents a novel three-phase inverse-design flow for synthesizing tunable pixelated band-pass filters. The workflow consists of: 1) acquisition of a massive dataset ~300000 5-port S-parameter samples using the group's custom electromagnetic solvers accelerated by method-of-moments pre-computation; 2) identification of an initial seed through brute-force search with dataset augmentation by dynamically reassigning the input, output, and tuning ports and randomly applying open or short terminations to the remaining ports; and 3) fine-tuning of the selected seed using a direct-binary-search algorithm. Two design targets are considered: 4G mid-/high-band tunable band-pass filters and 5G n79/UNIII-band tunable band-pass filters. One designed filter achieves a 28% tuning range, with passband peaks reconfigured from 4.64 to 6.16 GHz and insertion loss from 1.3 to 1.8 dB in simulation. All inversely designed filters use only a single varactor, representing the first reported demonstration of continuously reconfigurable inverse design in the microwave regime.

physics.app-ph↗

arXiv2025

Radio-Frequency Pseudo-Null Induced by Light in an Ion Trap

In a linear radio-frequency (rf) ion trap, the rf null is the point of zero electric field in the dynamic trapping potential where the ion motion is approximately harmonic. When displaced from the rf null, the ion is superimposed by fast oscillations known as micromotion, which can be probed through motion-sensitive light-atom interactions. In this work, we report on the emergence of the rf pseudo-null, a locus of points where the ion responds to light as if it were at the true rf null, despite being displaced from it. The phenomenon is fully explained by accounting for the general two-dimensional structure of micromotion and is experimentally verified under various potential configurations, with observations in great agreement with numerical simulations. The rf pseudo-null manifests as a line in a two-dimensional parameter space, determined by the geometry of the incident light and its overlap with the motional structure of the ion. The true rf null occurs uniquely at the concurrent point of the pseudo-null lines induced by different light sources.

physics.atom-ph↗

arXiv2024

Inverse Design of Multi-Layered Manufacturable Pixelated Diplexers Through Optimized Geometrical Configuration and Meshing Strategy in MoM

This paper presents a fast inverse design framework for complex multilayered, multiport pixelated surfaces - a class of structures largely unexplored in current research. Leveraging a method-of-moments (MoM) electromagnetic (EM) solver, the framework enables the rapid synthesis of pixelated device designs. A novel matrix reconstruction technique, based on pre-labeling matrix entries as "inter-pixel" or "inner-pixel," accelerates simulations for each variation of the pixelated structure. To mitigate the cubic increase in computation time associated with additional layers, GPU acceleration is employed. Further enhancing convergence speed, a stochastic multi-pixel flipping search algorithm is integrated into the framework. The effectiveness of this approach is demonstrated through the design of a diplexer achieving a -3-dB bandwidth for one channel spanning 5.23-5.94 GHz and another covering 6.17-7.15 GHz, validated by a full-wave solver.

physics.app-ph↗

arXiv2024

A silicon-based ion trap chip protected from semiconductor charging

Silicon-based ion trap chips can benefit from existing advanced fabrication technologies, such as multi-metal layer techniques for two-dimensional architectures and silicon photonics for the integration of on-chip optical components. However, the scalability of these technologies may be compromised by semiconductor charging, where photogenerated charge carriers produce electric potentials that disrupt ion motion. Inspired by recent studies on charge distribution mechanisms in semiconductors, we developed a silicon-based chip with gold coated on all exposed silicon surfaces. This modification significantly stabilized ion motion compared to a chip without such metallic shielding, a result that underscores the detrimental effects of exposed silicon. With the mitigation of background silicon-induced fields to negligible levels, quantum operations such as sideband cooling and two-ion entangling gates, which were previously infeasible with the unshielded chip, can now be implemented.

quant-ph↗

arXiv2024

Technical Report of NICE Challenge at CVPR 2024: Caption Re-ranking Evaluation Using Ensembled CLIP and Consensus Scores

This report presents the ECO (Ensembled Clip score and cOnsensus score) pipeline from team DSBA LAB, which is a new framework used to evaluate and rank captions for a given image. ECO selects the most accurate caption describing image. It is made possible by combining an Ensembled CLIP score, which considers the semantic alignment between the image and captions, with a Consensus score that accounts for the essentialness of the captions. Using this framework, we achieved notable success in the CVPR 2024 Workshop Challenge on Caption Re-ranking Evaluation at the New Frontiers for Zero-Shot Image Captioning Evaluation (NICE). Specifically, we secured third place based on the CIDEr metric, second in both the SPICE and METEOR metrics, and first in the ROUGE-L and all BLEU Score metrics. The code and configuration for the ECO framework are available at https://github.com/DSBA-Lab/ECO .

cs.CV↗

arXiv2023

Photo-induced charge carrier dynamics in a semiconductor-based ion trap investigated via motion-sensitive qubit transitions

Ion trap systems built upon microfabricated chips have emerged as a promising platform for quantum computing to achieve reproducible and scalable structures. However, photo-induced charging of materials in such chips can generate undesired stray electric fields that disrupt the quantum state of the ion, limiting high-fidelity quantum control essential for practical quantum computing. While crude understanding of the phenomena has been gained heuristically over the past years, explanations for the microscopic mechanism of photo-generated charge carrier dynamics remains largely elusive. Here, we present a photo-induced charging model for semiconductors, whose verification is enabled by a systematic interaction between trapped ions and photo-induced stray fields from exposed silicon surfaces in our chip. We use motion-sensitive qubit transitions to directly characterize the stray field and analyze its effect on the quantum dynamics of the trapped ion. In contrast to incoherent errors arising from the thermal motion of the ion, coherent errors are induced by the stray field, whose effect is significantly imprinted during the quantum control of the ion. These errors are investigated in depth and methods to mitigate them are discussed. Finally, we extend the implications of our study to other photo-induced charging mechanisms prevalent in ion traps.

quant-ph↗

arXiv2023

Micromotion compensation of trapped ions by qubit transition and direct scanning of dc voltages

Excess micromotion is detrimental to accurate qubit control of trapped ions, thus measuring and minimizing it is crucial. In this paper, we present a simple approach for measuring and suppressing excess micromotion of trapped ions by leveraging the existing laser-driven qubit transition scheme combined with direct scanning of dc voltages. The compensation voltage is deduced by analyzing the Bessel expansion of a scanned qubit transition rate. The method provides a fair level of sensitivity for practical quantum computing applications, while demanding minimal deviation of trap condition. By accomplishing compensation of excess micromotion in the qubit momentum-excitation direction, the scheme offers an additional avenue for excess micromotion compensation, complementing existing compensation schemes.

quant-ph↗

arXiv2020

Quantum annealing of Cayley-tree Ising spins at small scales

Significant efforts are being directed towards developing a quantum annealer capable of solving combinatorial optimization problems. The challenges are Hamiltonian programming and large-scale implementations. Here we report quantum annealing demonstration of Ising Hamiltonians programmed with up to $N=22$ spins mapped on various Cayley tree graphs. Experiments are performed with a Rydberg-atom quantum simulator, in which rubidium single atoms are arranged in three dimensional space in such a way that their Rydberg atoms and blockaded strong couplings respectively represent the nodes and edges of each graph. Three different Cayley-tree graphs of $Z=3$ neighbors and of up to $S=4$ shells are constructed, and their ground-state phases and N\'{e}el's order formations are probed. In good agreement with model calculations, the anti-ferromagnetic phase in regular Cayley trees and frustrated competing ground-states in a dual-center Cayley tree are directly observed. This demonstrates the possibilities of high-dimensional qubit connection programming in quantum simulators.

quant-ph↗

arXiv2019

Coherent and dissipative dynamics of entangled few-body systems of Rydberg atoms

Experimentally observed quantum few-body dynamics of neutral atoms excited to a Rydberg state are numerically analyzed with Lindblad master equation formalism. For this, up to five rubidium atoms are trapped with optical tweezers, arranged in various two-dimensional configurations, and excited to Rydberg 67S state in the nearest-neighbor blockade regime. Their coherent evolutions are measured with time-varying ground-state projections. The experimental results are analyzed with a model Lindblad equation with the homogeneous and inhomogeneous dampings determined by systematic and statistical error analysis. The coherent evolutions of the entangled systems are successfully reproduced by the resulting model analysis for the experimental results with optimal parameters in consistent with external calibrations.

physics.atom-ph↗

arXiv2017

Defect-free atomic array formation using Hungarian matching algorithm

Deterministic loading of single atoms onto arbitrary two-dimensional lattice points has recently been demonstrated, where by dynamically controlling the optical-dipole potential, atoms from a probabilistically loaded lattice were relocated to target lattice points to form a zero-entropy atomic lattice. In this atom rearrangement, how to pair atoms with the target sites is a combinatorial optimization problem: brute-force methods search all possible combinations so the process is slow, while heuristic methods are time-efficient but optimal solutions are not guaranteed. Here, we use the Hungarian matching algorithm as a fast and rigorous alternative to this problem of defect-free atomic lattice formation. Our approach utilizes an optimization cost function that restricts collision-free guiding paths so that atom loss due to collision is minimized during rearrangement. Experiments were performed with cold rubidium atoms that were trapped and guided with holographically controlled optical-dipole traps. The result of atom relocation from a partially filled 7-by-7 lattice to a 3-by-3 target lattice strongly agrees with the theoretical analysis: using the Hungarian algorithm minimizes the collisional and trespassing paths and results in improved performance, with over 50\% higher success probability than the heuristic shortest-move method.

physics.atom-ph↗

arXiv2016

In situ single-atom array synthesis by dynamic holographic optical tweezers

Cooling and trapping of atoms by light has enabled one to build and manipulate quantum systems at the single atom level. Such a bottom-up approach becomes one of the fascinating challenges toward scalable and highly controllable quantum systems, e.g., a large-scale quantum information machine. Their implementation requires crucial pre-requisites: scalablity, site distinguishability, and reliable single-atom loading into sites. The widely adopted methods satisfies the two former conditions relatively well, but the last condition, filling single atoms onto individual sites, relies mostly on the probabilistic loading, implying that loading a pre-defined set of atoms in given positions will be hampered exponentially. Two approaches are readily thinkable to overcome this issue: increasing the single-atom loading efficiency and relocating abundant atoms into unfilled positions. Realizing the relocation is directly related to how many atoms can be transportable in a designer way. Here, we demonstrate a dynamic holographic single-atom tweezers with unprecedented degrees of freedom of 2N. In a proof-of-principle experiment conducted with cold rubidium atoms, simultaneous rearrangements of N=9 single atoms are successfully performed. This method may be further applicable to deterministic N single-atom loading, coherent transport, and controlled collisions.

physics.atom-ph↗

arXiv2015

Coherent Control of Resonant Two-Photon Transitions by Counter-Propagating Ultrashort Pulse Pairs

We describe optimized coherent control methods for two-photon transitions in atoms of a ladder-type three-state energy configuration. Our approach is based on the spatial coherent control scheme which utilizes counter-propagating ultrashort laser pulses to produce complex excitation patterns in an extended space. Since coherent control requires constructive interference of constituent transition pathways, applying it to an atomic transition with a specific energy configuration requires specially designed laser pulses. Here, we show, in an experimental demonstration, that the two-photon transition with an intermediate resonant energy state can be coherently controlled and retrieved out from the resonance-induced background, when phase-flipping of the laser spectrum near the resonant intermediate transition is used. A simple reason for this behavior is the fact that the transition amplitude function (to be added to give an overall two-photon transition) changes its sign at the intermediate resonant frequency, thus, by a proper spectral-phase programming, the excitation patterns (or the position-dependent interference of the transition given as a consequence of the spatial coherent control) are well isolated in space along the focal region of the counter-propagating pulses.

physics.atom-ph↗