arXiv subjects

Toshimasa Fujisawa

Publications and source records attributed to Toshimasa Fujisawa.

At least 19 recordsLinked to original sources

arXiv2026

Dispersive detection of a charge qubit with a broadband high-impedance quantum-Hall plasmon resonator

Cavity quantum electrodynamics (cQED) provides strong light-matter interactions that can be used for manipulating and detecting quantum states. The interaction can be enhanced by increasing the resonator's impedance, while approaching the quantum impedance ($h/e^2$) remains challenging. Edge plasmons emergent as chiral bosonic modes in the quantum Hall channels provide high quantized impedance of $h/ νe^2$ that can exceed 10 k$Ω$ for the Landau-level filling factor $ν\leq 2$, well beyond the impedance of free space. Here, we apply such a high-impedance plasmon mode in a quantum-Hall plasmon resonator to demonstrate dispersive detection of a nearby charge qubit formed in a double quantum dot. The phase shift in microwave transmission through the plasmon resonator follows the dispersive shift associated with the qubit state in agreement with the cQED theory. The high impedance allows us to perform dispersive detection of qubit spectroscopy with a plasmon resonator having a broad bandwidth. Leveraging these topological edge modes, our results establish two-dimensional topological insulators as a new platform of cQED.

cond-mat.mes-hall↗

arXiv2025

Efficient heat-energy conversion from a non-thermal Tomonaga-Luttinger liquid

Energy harvesting is a technique that generates useful work from waste heat. Conventional energy harvesters acting on local thermal equilibrium states are constrained by thermodynamic limits, such as the Carnot efficiency. Quantum heat engines with non-thermal reservoirs are expected to exceed such limits. Here, we demonstrate energy harvesting from a nonthermal Tomonaga-Luttinger (TL) liquid in quantum Hall edge channels, where the non-thermal state is naturally formed due to the absence of thermalization. The scheme is tested with a quantum-dot energy harvester working on a non-thermal TL liquid supplied with waste heat from a quantum-point-contact transistor. Compared to the quasi-thermalized TL liquid, the non-thermal state prepared under the same heat is capable of a larger electromotive force and higher conversion efficiency. These characteristics can be understood by considering a binary Fermi distribution function of the non-thermal state induced by entropy-conserving equilibration. TL liquids are attractive non-thermal carriers for excellent energy harvesting.

cond-mat.mes-hall↗

arXiv2024

Coulomb oscillations of a quantum antidot formed by an airbridged pillar gate in the integer and fractional quantum Hall regime

Quantum antidots (QAD) are attractive for manipulating quasiparticles in quantum Hall (QH) systems. Here, we form a QAD in the integer and fractional QH regimes at nominal Landau-level filling factor $ν$ = 2, 1, and 2/3 using a submicron pillar gate with an airbridge connection. After confirming the required conditions for a fully depleted QAD, we analyze the observed Coulomb oscillations in terms of the area of the QAD and the effective charge for the oscillation period in an identical gate voltage range. The area at $ν$ = 2/3 is significantly smaller than that at $ν$ = 2 and 1, in qualitative agreement with the previous report. By assuming a constant gate capacitance, the effective charge at $ν$ = 2/3 is about 2/3 of that at $ν$ = 2 and 1. The QAD device can be used to capture and emit charges in the unit of 2e/3.

cond-mat.mes-hall↗

arXiv2024

Static quantum dot on a potential hilltop for generating and analyzing hot electrons in the quantum Hall regime

We propose and demonstrate a static quantum dot on a potential hilltop to generate and analyze ballistic hot electrons along a quantum Hall edge channel well above the chemical potential. High energy resolution associated with discrete energy levels is attractive for studying hot-electron dynamics. Particularly, the energy distribution function of hot electrons weakly coupled to cold electrons is investigated to reveal spectral diffusion with energy relaxation. The analysis allows us to estimate the maximum energy exchange per scattering, which is an important parameter to describe interacting electrons in the edge channel.

cond-mat.mes-hall↗

arXiv2024

Resonant plasmon-assisted tunneling in a double quantum dot coupled to a quantum-Hall plasmon resonator

Edge magnetoplasmon is an emergent chiral bosonic mode promising for studying electronic quantum optics. While the plasmon transport has been investigated with various techniques for decades,its coupling to a mesoscopic device remained unexplored. Here, we demonstrate the coupling between a single plasmon mode in a quantum Hall plasmon resonator and a double quantum dot (DQD). Resonant plasmon-assisted tunneling is observed in the DQD through absorbing or emitting plasmons stored in the resonator. By using the DQD as a spectrometer, the plasmon energy and the coupling strength are evaluated, which can be controlled by changing the electrostatic environment of the quantum Hall edge. The observed plasmon-electron coupling encourages us for studying strong coupling regimes of plasmonic cavity quantum electrodynamics.

cond-mat.mes-hall↗

arXiv2023

Tunable tunnel coupling in a double quantum antidot with cotunneling via localized state

Controlling tunnel coupling between quantum antidots (QADs) in the quantum Hall (QH) regime is problematic. We propose and demonstrate a scheme for tunable tunnel coupling between two QADs by utilizing a cotunneling process via a localized state as a third QAD. The effective tunnel coupling can be tuned by changing the localized level even with constant nearest-neighbor tunnel couplings. We systematically study the variation of transport characteristics in the effectively triple QAD system at the Landau level filling factor $ν=2$. The tunable tunnel coupling is clarified by analyzing the anti-crossing of Coulomb blockade peaks in the charge stability diagram, in agreement with numerical simulations based on the master equation. The scheme is attractive for studying coherence and interaction in QH systems.

cond-mat.mes-hall↗

arXiv2023

Non-thermal Tomonaga-Luttinger liquid eventually emerging from hot electrons in the quantum Hall regime

Dynamics of integrable systems, such as Tomonaga-Luttinger (TL) liquids, is deterministic, and the absence of stochastic thermalization processes provides unique characteristics, such as long-lived non-thermal metastable states with many conserved quantities. Here, we show such non-thermal states can emerge even when the TL liquid is excited with extremely high-energy hot electrons in chiral quantum-Hall edge channels. This demonstrates the robustness of the integrable model against the excitation energy. Crossover from the single-particle hot electrons to the many-body TL liquid is investigated by using on-chip detectors with a quantum point contact and a quantum dot. The charge dynamics can be understood with a single-particle picture only for hot electrons. The resulting electron-hole plasma in the TL liquid shows a non-thermal metastable state, in which warm and cold electrons coexist without further thermalization. The multi-temperature constituents are attractive for transporting information with conserved quantities along the channels.

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arXiv2022

Non-uniform heat redistribution among multiple channels in the integer quantum Hall regime

Heat transport in multiple quantum-Hall edge channels at Landau-level filling factor nu = 2, 4, and 8 is investigated with a quantum point contact as a heat generator and a quantum dot as a local thermometer. Heat distribution among the channels remains highly non-uniform, which can be understood with the plasmon eigenmodes of the multiple channels. The heat transport can be controlled with another quantum point contact as a quantized heat valve, as manifested by stepwise increases of heat current at the thermometer. This encourages developing integrated heat circuits with quantum-Hall edge channels.

cond-mat.mes-hall↗

arXiv2021

Time-resolved investigation of plasmon mode along interface channels in integer and fractional quantum Hall regimes

Quantum Hall (QH) edge channels appear not only along the edge of the electron gas but also along an interface between two QH regions with different filling factors. However, the fundamental transport characteristics of such interface channels are not well understood, particularly in the high-frequency regime. In this study, we investigate the interface plasmon mode along the edge of a metal gate electrode with ungated and gated QH regions in both integer and fractional QH regimes using a time-resolved measurement scheme. The observed plasmon waveform was delayed and broadened due to the influence of the charge puddles formed around the channel. The charge velocity and diffusion constant of the plasmon mode were evaluated by analyzing the waveform using a distributed circuit model. We found that the conductive puddles in the gated region induce significant dissipation in plasmon transport. For instance, a fractional interface channel with a reasonably fast velocity was obtained by preparing a fractional state in the ungated region and an integer state in the gated region, whereas a channel in the swapped configuration was quite dissipative. This reveals a high-quality interface channel that provides a clean path to transport fractional charges for studying various fractional QH phenomena.

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arXiv2021

Plasmon modes of coupled quantum Hall edge channels in the presence of disorder-induced tunneling

Coupled quantum Hall edge channels show intriguing non-trivial modes, for example, charge and neutral modes at Landau level filling factors 2 and 2/3. We propose an appropriate and effective model with Coulomb interaction and disorder-induced tunneling characterized by coupling capacitances and tunneling conductances, respectively. This model explains how the transport eigenmodes, within the interaction- and disorder-dominated regimes, change with the coupling capacitance, tunneling conductance, and measurement frequency. We propose frequency- and time-domain transport experiments, from which eigenmodes can be determined using this model.

cond-mat.mes-hall↗

arXiv2020

Quantized charge fractionalization at quantum Hall Y junctions in the disorder-dominated regime

Fractionalization is a phenomenon where an elementary excitation partitions into several pieces. This picture explains non-trivial transport through a junction of one-dimensional edge channels defined by topologically distinct quantum Hall states, for example, a hole-conjugate state at Landau-level filling factor $ν$ = 2/3. Here we employ a time-resolved scheme to identify an elementary fractionalization process; injection of charge q from a non-interaction region into an interacting and scattering region of one-dimensional channels results in the formation of a collective excitation with charge $(1-\textit{r})\textit{q}$ by reflecting fractionalized charge $\textit{rq}$. The fractionalization factors, $\textit{r}$ = 0.34$\pm$0.03 for $ν$ = 2/3 and $\textit{r}$ = 0.49$\pm$0.03 for $ν$ = 2, are consistent with the quantized values of 1/3 and 1/2, respectively, which are expected in the disorder dominated regime. The scheme can be used for generating and transporting fractionalized charges with a well-defined time course along a well-defined path.

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arXiv2019

Ballistic hot-electron transport in a quantum Hall edge channel defined by a double gate

Ballistic transport of hot electrons in a quantum Hall edge channel is attractive for studying electronic analog of quantum optics, where the edge potential profile is an important parameter that governs the charge velocity and scattering by longitudinal-optical (LO) phonons. Here we use a parallel double gate to control the electric field of the edge potential, and investigate the ballistic length of the channel by using hot-electron spectroscopy. The ballistic length is significantly enhanced by reducing the LO phonon scattering rate in the tailored potential.

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arXiv2019

Two-step breakdown of a local v = 1 quantum Hall state

We report quantum Hall effect breakdown of a local filling factor v_local = 1 state formed in a bulk v_bulk = 2 system in an AlGaAs/GaAs heterostructure. When a finite source-drain bias is applied across the local system, the breakdown occurs in two steps. At low bias, quantized conductance through the v_local = 1 system breaks down due to inter-edge electron tunneling. At high bias, the incompressibility of the v_local = 1 system breaks down because the spin gap closes. The two steps are resolved by combining measurements of resistively detected nuclear magnetic resonance and shot noise, which allows one to evaluate electron spin polarization in the local system and spin-dependent charge transport through the system, respectively.

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arXiv2019

Charge equilibration in integer and fractional quantum Hall edge channels in a generalized Hall-bar device

Charge equilibration between quantum-Hall edge states can be studied to reveal geometric structure of edge channels not only in the integer quantum Hall (IQH) regime but also in the fractional quantum Hall (FQH) regime particularly for hole-conjugate states. Here we report on a systematic study of charge equilibration in both IQH and FQH regimes by using a generalized Hall bar, in which a quantum Hall state is nested in another quantum Hall state with different Landau filling factors. This provides a feasible way to evaluate equilibration in various conditions even in the presence of scattering in the bulk region. The validity of the analysis is tested in the IQH regime by confirming consistency with previous works. In the FQH regime, we find that the equilibration length for counter-propagating $δν$ = 1 and $δν$ = -1/3 channels along a hole-conjugate state at Landau filling factor $ν$ = 2/3 is much shorter than that for co-propagating $δν$ = 1 and $δν$ = 1/3 channels along a particle state at $ν$ = 4/3. The difference can be associated to the distinct geometric structures of the edge channels. Our analysis with generalized Hall bar devices would be useful in studying edge equilibration and edge structures.

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arXiv2019

Spectroscopic study on hot-electron transport in a quantum Hall edge channel

Hot electron transport in a quantum Hall edge channel of an AlGaAs/GaAs heterostructure is studied by investigating the energy distribution function in the channel. Ballistic hot-electron transport, its optical-phonon replicas, weak electron-electron scattering, and electron-hole excitation in the Fermi sea are clearly identified in the energy spectra. The optical-phonon scattering is analyzed to evaluate the edge potential profile. We find that the electron-electron scattering is significantly suppressed with increasing the hot-electron's energy well above the Fermi energy. This can be understood with suppressed Coulomb potential with longer distance for higher energy. The results suggest that the relaxation can be suppressed further by softening the edge potential. This is essential for studying non-interacting chiral transport over a long distance.

cond-mat.mes-hall↗

arXiv2018

Coupling between Quantum Hall Edge Channels on Opposite Sides of a Hall Bar

We investigate the coupling between quantum Hall (QH) edge channels (ECs) located at opposite sides of a 50-um-wide Hall bar by exciting a charged wavepacket in one EC and detecting time-dependent current in the other EC. In a QH state, the current shows a peak followed by a dip, demonstrating the existence of capacitive coupling across the incompressible two-dimensional electron system (2DES). The observed magnetic field dependence of the amplitude and time delay of the current suggests that the capacitance is affected by the presence of localized states. We also show that the dominant manner of the coupling changes gradually as the system changes between the QH and non-QH states.

cond-mat.str-el↗

arXiv2018

Signatures of a non-thermal metastable state in copropagating quantum Hall edge channels

A Tomonaga-Luttinger (TL) liquid is known as an integrable system, in which a non-equilibrium many-body state survives without relaxing to a thermalized state. This intriguing characteristic is tested experimentally in copropagating quantum Hall edge channels at bulk filling factor nu = 2. The unidirectional transport allows us to investigate the time evolution by measuring the spatial evolution of the electronic states. The initial state is prepared with a biased quantum point contact, and its spatial evolution is measured with a quantum-dot energy spectrometer. We find strong evidences for a non-thermal metastable state in agreement with the TL theory before the system relaxes to thermal equilibrium with coupling to the environment.

cond-mat.mes-hall↗

arXiv2017

Charge Fractionalization in Artificial Tomonaga-Luttinger Liquids with Controlled Interaction Strength

We investigate charge fractionalizations in artificial Tomonaga-Luttinger liquids (TLLs) composed of two capacitively coupled quantum Hall edge channels (ECs) in graphene. The interaction strength of the artificial TLLs can be controlled through distance W between the ECs. We show that the fractionalization ratio r and the TLL mode velocity v vary with W. The experimentally obtained relation between v and r follows a unique function predicted by the TLL theory. We also show that charged wavepackets are reflected back and forth multiple times at both ends of the TLL region.

cond-mat.str-el↗