arXiv subjects

Sungguen Ryu

Publications and source records attributed to Sungguen Ryu.

14 recordsLinked to original sources

arXiv2026

Transport in magnetic-topological-insulator nanoribbons containing multiple superconductor-proximitized sectors

Transport in devices with multiple proximitized sectors depends heavily on the complex phases of the pairing gaps of those sectors. We investigate magnetic topological insulator nanoribbons in two- and three-terminal setups with proximitized sectors and asymptotic normal leads. Our focus is on the regime of single chiral Majoranas. The characteristic electric and thermal interferometries of chiral Majoranas can be controlled by the complex phases of the pairing. We predict an AC Majorana effect, in which phase dynamics induced by a voltage bias generate measurable time-dependent conductance oscillations. A three-terminal junction with superconducting islands can be used as a Majorana router when the relative complex phases are configured.

cond-mat.mes-hall↗

arXiv2025

Nonlinear Transport in Carbon Quantum Dot Electronic Devices: Experiment and Theory

Carbon quantum dots (CQDs) are a promising material for electronic applications due to their easy fabrication and interesting semiconductor properties. Further, CQDs exhibit quantum confinement and charging effects, which may lead not only to improved performances but also to devices with novel functionalities. Here, we investigate the electronic transport of CQDs embedded on epoxy polymer. Our samples are coupled to interdigitated electrodes with individually addressable microelectrodes. Remarkably, the current-voltage characteristics show strongly nonlinear regimes at room temperature, ranging from Schottky diode to Coulomb blockade and even negative differential conductance behavior. We propose a master equation theoretical framework which allows us to compute current curves that agree well with the observations. This model emphasizes the importance of interacting dots and electron traps in generating a cohesive picture that encompasses all transport regimes. Overall, our results suggest that CQDs constitute a versatile materials platform for 3D integrated electronic purposes.

cond-mat.mes-hall↗

arXiv2025

Entanglement Generation and Stabilization by Coherent Collisions

Collision is a useful tool for revealing quantum effects and realizing quantum informational tasks. We demonstrate that repeated collisions by itinerant electrons can dissipatively drive two remote spin qubits into an entangled state in a generic collisional framework. A coherent spin exchange with either qubit facilitates entanglement generation. When combined with proper local driving, these collisions induce an entangled steady state in most collision configurations. Particularly, the collision which is symmetric for the two qubits results in a unique steady state close to a maximally entangled state. Due to the dissipative nature of the process, the entanglement persists in the presence of decoherence, provided the collision frequency exceeds the decoherence rate. Our model can be experimentally implemented using single-electron sources.

cond-mat.mes-hall↗

arXiv2024

Characterizing and Mitigating Timing Noise-Induced Decoherence in Single Electron Sources

Identifying and controlling decoherence in single electron sources (SES) is important for their applications in quantum information processing. The recent experiments with ultrashort electron pulses [J. D. Fletcher et al., Nat. Commun. 10, 5298 (2019)] demonstrate strong decoherence that cannot be caused by traditional mechanisms such as electron-electron or electron-phonon interactions. Here we propose timing noise as a universal model, consistent with existing experimental data, to explain strong decoherence of ultrafast SES pulses, without resorting to any specific microscopic mechanism for such decoherence. We also propose a protocol to filter out timing noise which works even in the presence of other decoherence effects, such as those present in, e.g., low-energy SESs.

cond-mat.mes-hall↗

arXiv2023

Measuring the arrival time of an electron wave packet using a dynamical potential barrier

A time-dependent potential barrier has been used to probe the arrival-time distribution of the wave packet of a hot electron by raising the barrier to block the packet upon arrival of the packet at the barrier. To see whether the barrier precisely detects the distribution, it is necessary to study an error caused by a finite rising speed of the barrier. For this purpose, we study transmission of an electron wave packet through the dynamical barrier, and identify two regimes, the semiclassical regime and the quasistatic regime. In each regime, we calculate the arrival-time distribution reconstructed by using the barrier and quantify the error in the detection, the difference of the temporal uncertainty between the wave-packet distribution and the reconstructed distribution. Our finding suggests that for precise detection, the time scale, in which the barrier height rises over the energy distribution of the wave packet and the tunneling energy window of the barrier, has to be much shorter than the temporal uncertainty of the wave packet. The analytical results are confirmed with numerical calculations.

cond-mat.mes-hall↗

arXiv2022

Partition of Two Interacting Electrons by a Potential Barrier

Scattering or tunneling of an electron at a potential barrier is a fundamental quantum effect. Electron-electron interactions often affect the scattering, and understanding of the interaction effect is crucial in detection of various phenomena of electron transport and their application to electron quantum optics. We theoretically study the partition and collision of two interacting hot electrons at a potential barrier in the quantum Hall regime. We predict their kinetic energy change by their Coulomb interaction during the scattering delay time inside the barrier. The energy change results in characteristic deviation of the partition probabilities from the noninteracting case. The derivation includes nonmonotonic dependence of the probabilities on the barrier height, which agrees with recent experiments, and reduction of the fermionic antibunching.

cond-mat.mes-hall↗

arXiv2022

Conductance of electrostatic wire junctions in bilayer graphene

The conductance of electrostatic wire junctions in bilayer graphene, classified as trivial-trivial or trivial-topological regarding the confinement character on each junction side, is calculated. The topological side always corresponds to a kink-antikink system, as required for a proper connection with a trivial side. We report a conductance quench of the trivial-topological junction, with a conductance {\it near} quantization to $4e^2/h$, which is only half of the maximum value allowed by the Chern number of a kink-antikink system. The analysis allowed us to uncover the existence of a chiral edge mode in the trivial wire under quite general conditions. A double junction, trivial-topological-trivial, displays periodic Fano-like conductance resonances (dips or peaks) induced by the created topological loop.

cond-mat.mes-hall↗

arXiv2022

Beating Carnot efficiency with periodically driven chiral conductors

Classically, the power generated by an ideal thermal machine cannot be larger than the Carnot limit. This profound result is rooted in the second law of thermodynamics. A hot question is whether this bound is still valid for microengines operating far from equilibrium. Here, we demonstrate that a quantum chiral conductor driven by AC voltage can indeed work with efficiencies much larger than the Carnot bound. The system also extracts work from common temperature baths, violating Kelvin-Planck statement. Nonetheless, with the proper definition, entropy production is always positive and the second law is preserved. The crucial ingredients to obtain efficiencies beyond the Carnot limit are: i) irreversible entropy production by the photoassisted excitation processes due to the AC field and ii) absence of power injection thanks to chirality. Our results are relevant in view of recent developments that use small conductors to test the fundamental limits of thermodynamic engines.

cond-mat.mes-hall↗

arXiv2021

Quantum Consensus Dynamics by Entangling Maxwell Demon

We introduce a Maxwell demon which generates many-body entanglement robustly against bit-flip noises, which allows us to obtain quantum advantage. Adopting the protocol of the voter model used for opinion dynamics approaching consensus, the demon randomly selects a qubit pair and performs a quantum feedback control, in continuous repetitions. We derive upper bounds of the entropy reduction and the work extraction rates by demon's operation, which are determined by a competition between the quantum-classical mutual information acquired by the demon and the absolute irreversibility of the feedback control. Our finding of the upper bounds corresponds to a reformulation of the second law of thermodynamics under a class of Maxwell demon which generates many-body entanglement in a working substance.

cond-mat.stat-mech↗

arXiv2021

Asymmetric arms maximise visibility in hot-electron interferometers

We consider theoretically an electronic Mach-Zehnder interferometer constructed from quantum Hall edge channels and quantum point contacts, fed with single electrons from a dynamic quantum dot source. By considering the energy dependence of the edge-channel guide centres, we give an account of the phase averaging in this set up that is particularly relevant for the short, high-energy wavepackets injected by this type of electron source. We present both analytic and numerical results for the energy-dependent arrival time distributions of the electrons and also give an analysis of the delay times associated with the quantum point contacts and their effects on the interference patterns. A key finding is that, contrary to expectation, maximum visibility requires the interferometer arms to be different in length, with an offset of up to a micron for typical parameters. By designing interferometers that incorporate this asymmetry in their geometry, phase-averaging effects can be overcome such that visibility is only limited by other incoherent mechanisms.

cond-mat.mes-hall↗

arXiv2019

Picosecond coherent electron motion in a silicon single-electron source

Understanding ultrafast coherent electron dynamics is necessary for application of a single-electron source to metrological standards, quantum information processing, including electron quantum optics, and quantum sensing. While the dynamics of an electron emitted from the source has been extensively studied, there is as yet no study of the dynamics inside the source. This is because the speed of the internal dynamics is typically higher than 100 GHz, beyond state-of-the-art experimental bandwidth. Here, we theoretically and experimentally demonstrate that the internal dynamics in a silicon singleelectron source comprising a dynamic quantum dot can be detected, utilising a resonant level with which the dynamics is read out as gate-dependent current oscillations. Our experimental observation and simulation with realistic parameters show that an electron wave packet spatially oscillates quantum-coherently at $\sim$ 200 GHz inside the source. Our results will lead to a protocol for detecting such fast dynamics in a cavity and offer a means of engineering electron wave packets. This could allow high-accuracy current sources, high-resolution and high-speed electromagnetic-field sensing, and high-fidelity initialisation of flying qubits.

cond-mat.mes-hall↗

arXiv2016

Ultrafast Emission and Detection of a Single-Electron Gaussian Wave Packet: A Theoretical Study

Generating and detecting a prescribed single-electron state is an important step towards solid-state fermion optics. We propose how to generate an electron in a Gaussian state, using a quantum-dot pump with gigahertz operation and realistic parameters. With the help of a strong magnetic field, the electron occupies a coherent state in the pump, insensitive to the details of nonadiabatic evolution. The state changes during the emission from the pump, governed by competition between the Landauer-Buttiker traversal time and the passage time. When the former is much shorter than the latter, the emitted state is a Gaussian wave packet. The Gaussian packet can be identified by using a dynamical potential barrier, with a resolution reaching the Heisenberg minimal uncertainty $\hbar/2$.

cond-mat.mes-hall↗

arXiv2015

Phonon emission and arrival times of electrons from a single-electron source

In recent charge-pump experiments, single electrons are injected into quantum Hall edge channels at energies significantly above the Fermi level. We consider here the relaxation of these hot edge-channel electrons through longitudinal-optical phonon emission. Our results show that the probability for an electron in the outermost edge channel to emit one or more phonons en route to a detector some microns distant along the edge channel suffers a double-exponential suppression with increasing magnetic field. This explains recent experimental observations. We also describe how the shape of the arrival-time distribution of electrons at the detector reflects the velocities of the electronic states post phonon emission. We show how this can give rise to pronounced oscillations in the arrival-time-distribution width as a function of magnetic field or electron energy.

cond-mat.mes-hall↗

arXiv2012

Minimax optimization of entanglement witness operator for the quantification of three-qubit mixed-state entanglement

We develop a numerical approach for quantifying entanglement in mixed quantum states by convex-roof entanglement measures, based on the optimal entanglement witness operator and the minimax optimization method. Our approach is applicable to general entanglement measures and states and is an efficient alternative to the conventional approach based on the optimal pure-state decomposition. Compared with the conventional one, it has two important merits: (i) that the global optimality of the solution is quantitatively verifiable, and (ii) that the optimization is considerably simplified by exploiting the common symmetry of the target state and measure. To demonstrate the merits, we quantify Greenberger-Horne-Zeilinger (GHZ) entanglement in a class of three-qubit full-rank mixed states composed of the GHZ state, the W state, and the white noise, the simplest mixtures of states with different genuine multipartite entanglement, which have not been quantified before this work. We discuss some general properties of the form of the optimal witness operator and of the convex structure of mixed states, which are related to the symmetry and the rank of states.

quant-ph↗