arXiv subjects

Yonuk Chong

Publications and source records attributed to Yonuk Chong.

8 recordsLinked to original sources

arXiv2026

Optimizing Pump Conditions of Parametric Amplifiers for Fast Multiplexed Readout of Superconducting Qubits

Low-noise parametric amplifiers are widely used as the first-stage amplifier in qubit readout chains. The performance of parametric amplifiers depends sensitively on the choice of the pump condition. We propose a strategy for determining the pump condition that is tailored for fast multiplexed readout. Choosing the amplifier pump to maximize the signal-to-noise ratio (SNR) improvement at the readout frequency of the limiting qubit--the qubit that requires the longest readout time to reach a target SNR--minimizes the total multiplexed readout time. We demonstrate our pump calibration strategy experimentally on a five-qubit multiplexed readout chain with a traveling-wave parametric amplifier. Using our strategy, we reduce the multiplexed readout time by 320 ns compared to optimizing the average SNR improvement on all qubits, without degrading the target SNR for any qubit.

quant-ph↗

arXiv2026

Extended Haloscope Search and Exclusion of a Candidate Signal near 1.036 GHz

We report a follow-up axion haloscope search near 1.036 GHz that completes and extends our previous work [Phys. Rev. X 14, 031023 (2024)], in which a portion of the HEMT-based data could not be analyzed due to unrecorded experimental information. While recovering this dataset, we identified an excess near 1.036 GHz that satisfied our candidate-selection criteria, motivating dedicated validation studies, including independent cross-checks and re-examination with the original apparatus. The excess did not persist under these investigations and was not confirmed as an axion dark-matter signal. We subsequently extended the search over a 20-MHz band surrounding the candidate using a quantum-noise-limited amplifier, achieving sensitivity close to the Dine-Fischler-Srednicki-Zhitnitsky benchmark. In the absence of a confirmed signal, we set improved 90% confidence-level upper limits on the axion-photon coupling over the frequency range 1.026-1.045 GHz. This work highlights the importance of robust candidate-validation strategies as haloscope searches approach discovery-level sensitivity.

hep-ex↗

arXiv2024

Improving the accuracy of circuit quantization using the electromagnetic properties of superconductors

Recent advances in quantum information processing with superconducting qubits have fueled a growing demand for scaling and miniaturizing circuit layouts. Despite significant progress, predicting the Hamiltonian of complex circuits remains a challenging task. Here, we propose an improved method for quantizing superconducting circuits that incorporates material- and geometry-dependent kinetic inductance. Our approach models superconducting films as reactive boundary elements, seamlessly integrating into the conventional circuit quantization framework without adding computational complexity. We experimentally validate our method using superconducting devices fabricated with 35 nm-thick disordered niobium films, demonstrating significantly improved accuracy in predicting the Hamiltonian based solely on the device layout and material properties of superconducting films and Josephson junctions. Specifically, conventional methods exhibit an average error of 5.4% in mode frequencies, while our method reduces it to 1.1%. Our method enables systematic studies of superconducting devices with disordered films or compact elements, facilitating precise engineering of superconducting circuits at scale.

quant-ph↗

arXiv2024

Extensive search for axion dark matter over 1\,GHz with CAPP's Main Axion eXperiment

We report an extensive high-sensitivity search for axion dark matter above 1\,GHz at the Center for Axion and Precision Physics Research (CAPP). The cavity resonant search, exploiting the coupling between axions and photons, explored the frequency (mass) range of 1.025\,GHz (4.24\,$μ$eV) to 1.185\,GHz (4.91\,$μ$eV). We have introduced a number of innovations in this field, demonstrating the practical approach of optimizing all the relevant parameters of axion haloscopes, extending presently available technology. The CAPP 12\,T magnet with an aperture of 320\,mm made of Nb$_3$Sn and NbTi superconductors surrounding a 37-liter ultralight-weight copper cavity is expected to convert DFSZ axions into approximately $10^2$ microwave photons per second. A powerful dilution refrigerator, capable of keeping the core system below 40\,mK, combined with quantum-noise limited readout electronics, achieved a total system noise of about 200\,mK or below, which corresponds to a background of roughly $4\times 10^3$ photons per second within the axion bandwidth. The combination of all those improvements provides unprecedented search performance, imposing the most stringent exclusion limits on axion--photon coupling in this frequency range to date. These results also suggest an experimental capability suitable for highly-sensitive searches for axion dark matter above 1\,GHz.

hep-ex↗

arXiv2021

Quantum readout error mitigation via deep learning

Quantum computing devices are inevitably subject to errors. To leverage quantum technologies for computational benefits in practical applications, quantum algorithms and protocols must be implemented reliably under noise and imperfections. Since noise and imperfections limit the size of quantum circuits that can be realized on a quantum device, developing quantum error mitigation techniques that do not require extra qubits and gates is of critical importance. In this work, we present a deep learning-based protocol for reducing readout errors on quantum hardware. Our technique is based on training an artificial neural network with the measurement results obtained from experiments with simple quantum circuits consisting of singe-qubit gates only. With the neural network and deep learning, non-linear noise can be corrected, which is not possible with the existing linear inversion methods. The advantage of our method against the existing methods is demonstrated through quantum readout error mitigation experiments performed on IBM five-qubit quantum devices.

quant-ph↗

arXiv2018

Construction of controlled-NOT gate based on microwave-activated phase (MAP) gate in two transmon system

We experimentally constructed an all-microwave scheme for the controlled-NOT (cNOT) gate between two superconducting transmon qubits in a three dimensional cavity. Our cNOT gate is based on the microwave-activated phase (MAP) gate, which requires an additional procedure to compensate the accumulated phases during the operation of the MAP gate. We applied Z-axis phase gates using microwave hyperbolic secant pulse on both qubits with adequate rotation angles systematically calibrated by separate measurements.We evaluated the gate performance of the constructed cNOT gate by performing two-qubit quantum process tomography (QPT). Finally, we present the experimental implementation of Deutsch-Jozsa algorithm using the cNOT gate.

quant-ph↗

arXiv2014

Electrical Modulation of Superconducting Critical Temperature in Liquid-Gated Thin Niobium Films

We demonstrate that the superconducting critical temperature (Tc) of thin niobium films can be electrically modulated in a liquid-gated geometry device. Tc can be suppressed and enhanced by applying positive and negative gate voltage, respectively, in a reversible manner within a range of about 0.1 K. At a fixed temperature below Tc, we observed that the superconducting critical current can be modulated by gate voltage. This result suggests a possibility of an electrically-controlled switching device operating at or above liquid helium temperature, where superconductivity can be turned on or off solely by the applied gate voltage.

cond-mat.mes-hall↗

arXiv2011

Superconducting junction of a single-crystalline Au nanowire for an ideal Josephson device

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. Current-Voltage characteristic curve shows clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The sub-gap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.

cond-mat.mes-hall↗