Research Activities
Ongoing projects:
*Electron wavepacket in edge channels in the quantum Hall regime.
*Electron-phonon coupling controlled with surface acoustic wave
*Charge and spin measurements in semiconductor quantum dots
Two-electron double quantum dot coupled to coherent photon and phonon fields
Two-electron states of a double quantum dot (DQD) under irradiation of coherent boson (photon and phonon) fields are studied by measuring spin-flip tunneling current in the Pauli spin blockade regime. This measurement scheme allows us to investigate Rabi splitting and associated boson dressed states particularly in the deep dispersive regime where the detuning δ ≡ hbarω − E_AB between the boson energy hbarω and energy spacing E_AB of the two-level system is significantly large (δ ∼ hbarω), where the permanent dipole moment in the DQD plays a significant role in the hybridization
Y. Sato et al., .Phys. Rev. B 96, 115416 (2017).
Negative and positive cross-correlations of current noises in quantum Hall edge channels at bulk filling factor nu = 1
Cross-correlation noise in electrical currents generated from a series connection of two quantum point contacts (QPCs), the injector and the detector, is described for investigating energy relaxation in quantum Hall edge channels at bulk filling factor nu= 1. We address the importance of tuning the energy bias across the detector for this purpose. For a long channel with a macroscopic floating ohmic contact that thermalizes the electrons, the cross-correlation turns from negative values to the maximally positive value (identical noise in the two currents) by tuning the effective energy bias to zero. This can be understood by considering competition between the low-frequency charge fluctuation generated at the injector, which contributes positive correlation, and the partition noise at the detector, which gives negative correlation. Strikingly, even for a short channel without intentional thermalization, significantly large positive correlation is observed in contrast to negative values expected for coherent transport between the two QPCs.
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T. Ota et al. J. Phys. Condens. Matter 29, 225302 (2017).
Waveform measurement of charge- and spin-density wavepackets in a chiral Tomonaga–Luttinger liquid
In contrast to a free-electron system, a Tomonaga–Luttinger (TL) liquid in a one-dimensional (1D) electron system hosts charge and spin excitations as independent entities. Whenan electron is injected into a TL liquid, it transforms into chargeand spin-density wavepackets that propagate at dierent group velocities and move away from each other. This process, knownas spin–charge separation, is the hallmark of TL physics. While spin–charge separation has been probed in momentumor frequency-domain measurements in various 1D systems, waveforms of separated excitations, which are a direct manifestation of the TL behaviour, have been long awaited to be measured. Here, we present a waveform measurement for the pseudospin–charge separation process in a chiral TL liquid comprising quantum Hall edge channels. The chargeand pseudospin-density waveforms are captured by utilizing a spin-resolved sampling scope that records the spin-up or -down component of the excitations. This experimental technique provides full information for time evolution of the 1D electron system, including not only propagation of TL eigenmodes but also their decay in a practical device.
M. Hashisaka et al., Nature Physics 13, 559-562 (2017) (DOI: 10.1038/NPHYS4062)

Long-lived binary tunneling spectrum in the quantum Hall Tomonaga-Luttinger liquid
The existence of long-lived nonequilibrium states without showing thermalization, which has previously been demonstrated in time evolution of ultracold atoms, suggests the possibility of their spatial analog in transport behavior of interacting electrons in solid-state systems. Here we report long-lived nonequilibrium states in one-dimensional edge channels in the integer quantum Hall regime. An indirect heating scheme in a counterpropagating configuration is employed to generate a nontrivial binary spectrum consisting of high- and low-temperature components. This unusual spectrum is sustained even after traveling 5–10 um, much longer than the length for electronic relaxation (about 0.1um), without showing significant thermalization. This observation is consistent with the integrable model of Tomonaga-Luttinger liquid. The long-lived spectrum implies that the system is well described by noninteracting plasmons, which are attractive for carrying information for a long distance.
K. Washio et al., Phys. Rev. B 93, 075304 (2016).

Exchange-Induced Spin Blockade in a Two-Electron Double Quantum Dot
We have experimentally identified the exchange-induced spin blockade in a GaAs double quantum dot. The transport is suppressed only when the eigenstates are well-defined singlet and triplet states, and thus sensitive to dynamic nuclear-spin polarization tha causes singlet-triplet mixing. This gives rise to unusual current spectra, such as a sharp curren dip and an asymmetric current profile near the triplet resonance of a double quantum dot. Numerical simulations suggest that the current dip is a signature of identical nuclear-spin polarization in the two dots, which is attractive for coherent spin manipulations in a material with nuclear spins.
D. Imanaka et al., Phys. Rev. Lett. 115, 176802 (2015).

Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity
Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons.
J. C. H. Chen, Y. Sato, R. Kosaka, M. Hashisaka, K. Muraki, and T. Fujisawa, Sci. Rep. 5, 15176 (2015).
An edge-magnetoplasmon Mach-Zehnder interferometer
We report an edge-magnetoplasmon (EMP) Mach-Zehnder (MZ) interferometer in a quantum Hall system. The MZ interferometer, which is based on the interference of two EMP beams traveling in chiral one-dimensional edge channels, is constructed by tailoring edge channels with functional devices such as splitters and delay lines. We measured 1 GHz EMP beams transmitted through the interferometer while tuning the phase evolution along two interference paths using tunable delay lines. Clear interference patterns as a function of the phase difference ensure the MZ interference. Moreover, the MZ interferometry is applied to evaluate the EMP transport through an attenuator interposed in one of the paths. This technique will be useful for investigating the functionalities of devices in plasmonics.
N. Hiyama, M. Hashisaka, and T. Fujisawa, Appl. Phys. Lett. 107, 143101 (2015).
Shot-Noise Evidence of Fractional Quasiparticle Creation in a Local Fractional Quantum Hall State
We experimentally identify fractional quasiparticle creation in a tunneling process through a local fractional quantum Hall (FQH) state. The local FQH state is prepared in a low-density region near a quantum point contact in an integer quantum Hall (IQH) system. Shot-noise measurements reveal a clear transition from elementary-charge tunneling at low bias to fractional-charge tunneling at high bias. The fractional shot noise is proportional to T1(1-T1) over a wide range of T1, where T1 is the transmission probability of the IQH edge channel. This binomial distribution indicates that fractional quasiparticles emerge from the IQH state to be transmitted through the local FQH state. The study of this tunneling process enables us to elucidate the dynamics of Laughlin quasiparticles in FQH systems.
M. Hashisaka, T. Ota, K. Muraki and T. Fujisawa, Phys. Rev. Lett. 114, 056802 (2015).
Spin-dependent tunneling rates for electrostatically defined GaAs quantum dots
The tunneling rates for spin-up and -down electrons are investigated for a GaAs quantum dot in an in-plane magnetic field by using a real-time single-electron counting scheme with a nearby charge detector. An extremely small spin-polarized current on the order of attoamperes is analyzed with the spin and energy dependences of the tunneling rates. Fully spin-polarized current is obtained when only a spin-up Zeeman sublevel is located in the transport window. When both Zeeman sublevels are allowed to contribute to the transport, we find that the tunneling rate for spin-up electrons is considerably higher than that for spin-down electrons. This partially spin-polarized current can be explained by the exchange-enhanced spin splitting in low-density regions near the tunneling barriers.
M. Yamagishi, N. Watase, M. Hashisaka, K. Muraki, and T. Fujisawa, Phys. Rev. B. 90, 035306 (2014).
Cross-correlation measurement of quantum shot noise using homemade transimpedance amplifiers
We report a cross-correlation measurement system, based on a new approach, which can be used to measure shot noise in a mesoscopic conductor at milliKelvin temperatures. In contrast to other measurement systems in which high-speed low-noise voltage amplifiers are commonly used, our system employs homemade transimpedance amplifiers (TAs). The low input impedance of the TAs significantly reduces the crosstalk caused by unavoidable parasitic capacitance between wires. The TAs are designed to have a flat gain over a frequency band from 2 kHz to 1 MHz. Low-noise performance is attained by installing the TAs at a 4 K stage of a dilution refrigerator. Our system thus fulfills the technical requirements for cross-correlation measurements: low noise floor, high frequency band, and negligible crosstalk between two signal lines. Using our system, shot noise generated at a quantum point contact embedded in a quantum Hall system is measured. The good agreement between the obtained shot-noise data and theoretical predictions demonstrates the accuracy of the measurements.
M. Hashisaka, T. Ota, M. Yamagishi, T. Fujisawa, and K. Muraki, Rev. Sci. Instrum. 85, 054704 (2014).
Stable and unstable dynamics of Overhauser fields in a double quantum dot
Nonlinear dynamics of nuclear spin ensembles driven by a two-electron system in a double quantum dot in the Pauli spin blockade (SB) regime is studied experimentally in conjunction with numerical simulation. Dynamic nuclear spin polarization (DNP) is systematically studied by evaluating the current level and its fluctuations. We interpret large current noise in the SB regime as stable feedback noise, where identical Overhauser fields of the two dots are preferred. In contrast, stepwise increases of current in the shallow Coulomb blockade region can be understood as unstable dynamics with significant imbalance of the Overhauser fields, which cancels the external magnetic field in one of the two dots. There, an extremely small transverse Overhauser field can easily lift the SB transport, giving the highest current level, when longitudinal components cancel the applied field.
S. Sharmin et al., Phys. Rev. B 89, 115315 (2014).
Fractionalized wave packets from an artificial Tomonaga-Luttinger liquid
The model of interacting fermion systems in one dimension known as Tomonaga?Luttinger liquid (TLL) provides a simple and exactly solvable theoretical framework that predicts various intriguing physical properties. Evidence of TLL has been observed as power-law behaviour in electronic transport on various types of one-dimensional conductors. However, these measurements, which rely on d.c. transport involving electron tunneling processes, cannot identify the long-awaited hallmark of charge fractionalization, in which an injection of elementary charge e from a non-interacting lead is divided into the non-trivial effective charge e* and the remainder, e-e*. Here, we report time-resolved transport measurements on an artificial TLL composed of coupled integer quantum Hall edge channels, in which we successfully identify single charge fractionalization processes. A wave packet of charge q incident from a non-interacting region breaks up into several fractionalized charge wave packets at the edges of the artificial TLL, from which transport eigenmodes can be evaluated directly. These results are informative for elucidating the nature of TLLs and low-energy excitations in the edge channels.
Selected as an Advance Online Publication (AOP)
H. Kamata et al., Nature Nano. 9, 177-181 (2014)
DOI: 10.1038/NNANO.2013.312

Single-electron counting statistics with a finite frequency bandwidth
Single-electron counting is widely used to probe single electron dynamics and correlated electron transport through quantum dots. However, finite frequency bandwidth in amplifying and analyzing the detector current removes fast counting events and alters the statistics. We have developed a correction scheme to obtain the actual tunneling rates through a quantum dot, when the detector has a low pass filter with a cutoff frequency comparable to the rates. The accuracy of our scheme is confirmed by simulating the filtering effect on Poisson random switching events and by applying it to experimental data for self-checking.
N. Watase, M. Hashisaka, K. Muraki, and T. Fujisawa, Jpn. J. Appl. Phys. 53 04EJ01 (2014).
Distributed-element circuit model of edge magnetoplasmon transport
We report experimental and theoretical studies of edge magnetoplasmon (EMP) transport in quantum Hall (QH) devices. We develop a model that allows us to calculate the transport coefficients of EMPs in QH devices with various geometries. In our model, a QH system is described as a chiral distributed-element (CDE) circuit, where the effects of Coulomb interaction are represented by an electrochemical capacitance distributed along unidirectional transmission lines. We measure the EMP transport coefficients through single- and coupled-edge channels, a quantum point contact, and single- and double-cavity structures. These measured transmission spectra can be reproduced well by simulations using the corresponding CDE circuits. By fitting the experimental results with the simulations, we deduce the circuit parameters that characterize the electrostatic environment around the edge channels in a realistic QH system. The observed gate-voltage dependences of the EMP transport properties in gate-defined structures are explained in terms of the gate tuning of the circuit parameters in CDE circuits.
M. Hashisaka, H. Kamata, N. Kumada, K. Washio, R. Murata, K. Muraki, and T. Fujisawa, Phys. Rev. B.88, 235409 (2013).
Frequency conversion of rf edge magnetoplasmons using a quantum point contact
We report a frequency-conversion technique for edge magnetoplasmons in a quantum Hall device. The nonlinear conductance of a quantum point contact (QPC) results in the generation of high harmonics of an input microwave and the mixing of two input frequencies. We controlled the conversion efficiency by tuning the nonlinear conductance through the QPC via a gate voltage. The amplitudes of the frequency-converted waves agree well with those expected from the measured dc nonlinear conductance. We demonstrate the technique up to our instrumental limit of 12 GHz, and show that formation of charge-density waves does not change the nonlinear characteristics of the QPC.
M. Hashisaka, K. Washio, H. Kamata, K. Muraki, and T. Fujisawa, , Appl. Phys. Lett. 100, 233501 (2012)

Distributed electrochemical capacitance evidenced in high-frequency admittance measurements on a quantum Hall device
We describe high-frequency admittance measurements up to 3 GHz on a quantum point contact in a quantum Hall regime. When the excitation frequency is so high that the wavelength of the edge magnetoplasmons is comparable to the size of the system, the phase developing along the edge channels must be considered when analyzing electrochemical capacitance in admittance. The result cannot be accounted for by the local
electrochemical capacitance near the quantum point contact but instead needs to be understood by considering the distributed capacitance within the sample as a whole. This implies that all electrons within the device are interacting because the screening in one-dimensional channels is extremely weak. A distributed-element model is essential for understanding the ac response in low-dimensional structures, which is analogous to that in high-frequency electrical circuits.
M. Hashisaka, K. Washio, H. Kamata, K. Muraki, and T. Fujisawa, , Phys. Rev. B 85, 155424 (2012).

Correlation of 1/f Noise between Semiconductor Point Contacts with a Common Lead

We investigate cross spectral density between tunneling currents through closely spaced point contacts (PCs) in a semiconductor heterostructure. Analysis of 1/f noise, which originates from background charge fluctuations, is expected to reveal the characteristics of a charge detector and screened Coulomb potential in the device. However, the common resistance in the measurement circuit and the leads of the PCs causes a significant negative correlation. We find that this negative correlation is enhanced when the common electrical channel becomes so narrow that it has only a few one-dimensional conductive modes. Our finding suggests the importance of the circuit environment in integrating multiple charge detectors.
M. Yamagishi, M. Hashisaka, K. Muraki, and T. Fujisawa, Japan. J. Appl. Phys. 51, 02BJ08 (2012).

Interferometric detection of edge magnetoplasmons in AlGaAs/GaAs heterostructures

We propose and demonstrate an interferometric detection scheme for edge magnetoplasmons (EMPs) propagating in two dimensional electron gas systems. Analogous to homodyne detection of electromagnetic wave in quantum optics, we apply reference microwaves to interfere with EMPs. Tuning the delay time, hence the phase, of the reference wave and analyzing the interference signal allow us to extract the amplitude and the phase of the original EMP at the detection point. The magnetic field dependence of the amplitude and the group velocity of EMPs are investigated by the detection scheme. This detection scheme is performed with a gate electrode as well as an Ohmic contact, which will be useful in evaluating spatial distribution of EMPs.
M. Hashisaka, K. Washio, H. Kamata, K. Murak, and T. Fujisawa, Phys. Status Solidi C, (2010). (DOI:10.1002/pssc.201000477)

Multiple two-qubit operations for a coupled semiconductor charge qubit
We investigate functional two-qubit operations in coupled semiconductor charge qubits, where two spatially separated electrons are coherently excited. Their time-evolutions are investigated for an ideally closed system as well as for a realistic semiconductor device with some decoherence and dephasing processes. We find various types of quantum operations can be performed each in a single step just by adjusting the energy biases of the two qubits. These operations are promising for generating strong quantum correlation between spatially separated electrons.
T. Fujisawa, G. Shinkai, T. Hayashi, T. Ota, Physica E 43, 730 (2011). doi:10.1016/j.physe.2010.07.040

Voltage-controlled group velocity of edge magnetoplasmon in the quantum Hall regime
We investigate the group velocity of edge magnetoplasmons (EMPs) in the quantum Hall regime by means of time-of-flight measurement. The EMPs are injected from an Ohmic contact by applying a voltage pulse, and detected at a quantum point contact by applying another voltage pulse to its gate. We find that the group velocity of the EMPs traveling along the edge channel defined by a metallic gate electrode strongly depends on the voltage applied to the gate. The observed variation of the velocity can be understood to reflect the degree of screening caused by the metallic gate, which damps the in-plane electric field and, hence, reduces the velocity. The degree of screening can be controlled by changing the distance between the gate and the edge channel with the gate voltage.
H. Kamata, T. Ota, K. Muraki, and T. Fujisawa, Phys. Rev. B 81, 085329(2010).

Bidirectional current drag induced by two-electron cotunneling in coupled double quantum dots
We demonstrate a bidirectional current drag device, in which an electron tunneling through a double quantum dot (DQD) drags another electron in the other DQD in the same or opposite direction. The direction can be switched by choosing the corresponding cotunneling process of the two electrons. A reasonable drag effect is experimentally confirmed in a GaAs device. This result encourages optimizing the device for current mirror functions.
G. Shinkai, T. Hayashi, T. Ota, and T. Fujisawa, Appl. Phys. Express 2, 081101(2009).

Correlated coherent oscillations in coupled semiconductor charge qubits

We study coherent dynamics of two spatially separated electrons in a coupled semiconductor double quantum dot (DQD). Coherent oscillations in one DQD are strongly influenced by electronic states of the other DQD, or the two electrons simultaneously tunnel in a correlated manner. The observed coherent oscillations are interpreted as various two-qubit operations. The results encourage searching quantum entanglement in electronic devices.
G. Shinkai, T. Hayashi, T. Ota, and T. Fujisawa, Phys. Rev. Lett. 103, 056802 (2009)

Correlation measurement of time-dependent potentials in a semiconductor quantum point contact
A novel time-correlation measurement technique is proposed and demonstrated to investigate time-dependent potentials in nanostructures. We focus on a semiconductor quantum point contact, in which the tunneling barrier potential and the source-drain bias potential can be independently controlled by external voltage pulses. Time-correlation of the two potentials is obtained by measuring the dc current at various time differences of the two pulses. The observed correlation function is consistent with the model, and can be used to evaluate the waveform of the time-dependent potential.
H. Kamata, T. Ota, and T. Fujisawa, Japan. J. Appl. Phys. 48, 04C149 (2009).

Zeeman splitting in single-electron transport through a few-electron quantum dot
Single-electron transport through a few-electron quantum dot is investigated under in-plane and perpendicular magnetic fields. Zeeman splitting always appears as two conductance peaks, whose conditions depend on whether the total spin is raised or lowered by single-electron tunneling. The total spin of the ground state can be identified by consecutively investigating the Zeeman splitting from a known spin state. Zeeman splitting for some excited states is also discussed.
T. Fujisawa, G. Shinkai, and T. Hayashi, "Zeeman splitting in single-electron transport through a few-electron quantum dot", Phys. Rev. B 72, 041302(R), (2007).

Controlled resonant tunneling in a coupled double-quantum-dot system
The authors investigate electrostatic coupling between two double quantum dots (DQDs) defined in an AlGaAs/GaAs heterostructure by measuring the correlation between resonant tunneling currents through the DQDs. Resonant tunneling in one DQD can be controlled by the charge state of the other DQD. This controlled resonant tunneling is consistent with the capacitance model for the geometry and can be used to investigate the statistics of single-electron charge states in the DQD. The observed electrostatic coupling is strong enough to perform two-qubit quantum gates both for charge- and electron-spin-based qubit schemes.
G. Shinkai, T. Hayashi, Y. Hirayama and T. Fujisawa, "Controlled resonant tunneling in a coupled double-quantum-dot system", Appl. Phys. Lett. 90, 103116 (2007).

Bidirectional single electron counting device
A bidirectional single-electron counting device is demonstrated. Individual electrons flowing in forward and reverse directions through a double quantum dot are detected with a quantum point contact acting as a charge sensor. A comprehensive statistical analysis in the frequency and time domains and of higher order moments of noise reveals antibunching correlation in single-electron transport through the device itself. The device can also be used to investigate current flow in the attoampere range, which cannot be measured by existing current meters.
T. Fujisawa, R. Tomita, T. Hayashi, and Y. Hirayama, "Bidirectional counting of single electrons", Science 312, 1634 (2006).