arXiv subjects

Akira Fujiwara

Publications and source records attributed to Akira Fujiwara.

16 recordsLinked to original sources

arXiv2026

Room-temperature 1/3 suppression of diffusive shot noise

Diffusive conductors share a universal shot noise reduction of 1/3. This result can be obtained from quantum scattering, Boltzmann-Langevin kinetics, or by symmetric simple exclusion process, but experiments have been limited to cryogenic conditions and the understanding of this universality remains incomplete. Here we show that ferrocene redox cycling in a microfluidic gap is room temperature realization of this universality. An analytical model of the current and its noise is derived and confirmed experimentally. The universal 1/3 shot noise suppression is recovered in the diffusion-limited regime, providing a bridge between mesoscopic physics and electrochemistry, opening a new route for probing molecular dynamics through room-temperature noise measurements.

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arXiv2025

Scalable Parallel Single-Electron Pumps in Silicon with Split-Source Control in the Nanoampere Regime

Parallelizing single-electron pumps offers a promising route to achieving nanoampere-level currents crucial for quantum current standard applications. Achieving such current levels is essential for demonstrating the ultra-high accuracy of single-electron pumps below 0.1 ppm toward quantum metrology triangle experiments. In addition, improving the accuracy at this current range is also desirable for practical small-current measurements. However, nanoampere-level currents have not yet been achieved with parallel pumps, mainly due to challenges in optimizing operating conditions. Here, we propose a scalable and easily implementable parallelization method based on tunable-barrier single-electron pumps with split source electrodes. By tuning the source voltages, we successfully parallelize four single-electron pumps at 200 MHz and further demonstrate a current plateau exceeding 2 nA using three pumps at 2.1 GHz. The wide applicability of this parallelization technique opens a path toward advancing high-accuracy quantum current standards.

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arXiv2025

QBIOL: A quantum bioelectrochemical software based on point stochastic processes

Bioelectrochemistry is crucial for understanding biological functions and driving applications in synthetic biology, healthcare, and catalysis. However, current simulation methods fail to capture both the stochastic nature of molecular motion and electron transfer across the relevant picosecond-to-minute timescales. We present QBIOL, a web-accessible software that integrates molecular dynamics, applied mathematics, GPU programming, and quantum charge transport to address this challenge. QBIOL enables quantitative stochastic electron transfer simulations and has the potential to reproduce numerically any (bio) electrochemical experiments. We illustrate this potential by comparing our simulations with experimental data on the current generated by electrode-attached redox-labeled DNA, or by nanoconfined redox species, in response to a variety of electrical excitation waveforms, configurations of interest in biosensing and catalysis. The adaptable architecture of QBIOL extends to the development of devices for quantum and molecular technologies, positioning our software as a powerful tool for enabling new research in this rapidly evolving field.

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arXiv2024

High frequency breakdown in quantum dots

Dynamic quantum dots are known to generate very accurate currents and can be used as a source of single electron wavepackets for use in quantum metrology, sensing or information processing. To realise their full technological potential, it is desirable to maximise the frequency of operation to increase the current. However, it has been observed that the mechanism of electron transfer across the quantum dot consistently breaks down for GHz frequencies, and this remains unexplained. Here, we present a novel analysis technique, combined with detailed modelling, to present a mechanism of high frequency breakdown in quantum dots as a rapidly-imparted momentum impulse. Such understanding aids future design and operation protocols, allowing their use in high frequency and real time quantum measurements and technologies.

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arXiv2024

An ambipolar single-charge pump in silicon

The mechanism of single-charge pumping using a dynamic quantum dot needs to be precisely understood for high-accuracy and universal operation toward applications to quantum current standards and quantum information devices. The type of charge carrier (electron or hole) is an important factor for determining the pumping accuracy, but it has been so far compared just using different devices that could have different potential landscapes. Here, we report measurements of a silicon ambipolar single-charge pump. It allows a comparison between the single-electron and single-hole pumps that share the entrance tunnel barrier, which is a critical part of the pumping operation. By changing the frequency and temperature, we reveal that the entrance barrier has a better energy selectivity in the single-hole pumping, leading to a pumping error rate better than that in the single-electron pumping up to 400 MHz. This result implies that the heavy effective mass of holes is related to the superior characteristics in the single-hole pumping, which would be an important finding for stably realizing accurate single-charge pumping operation.

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arXiv2023

Advances toward high-accuracy gigahertz operation of tunable-barrier single-hole pumps in silicon

Precise and reproducible current generation is key to realize quantum current standards in metrology. A promising candidate is a tunable-barrier single-charge pump, which can accurately transfer single charges one by one with an error rate of less than ppm level. Although several high-accuracy measurements have revealed such a high performance of the pumps, it is necessary to further pursue the possibility of high-precision operation toward reproducible generation of the pumping current in many devices. Here, we investigate in detail a silicon single-hole pumps, which are potentially expected to have a superior performance to single-electron pumps because of a heavy effective mass of holes. Temperature dependence measurements of current generated by the single-hole pump revealed a high energy selectivity of the tunnel barrier, which is a critical parameter to achieve high-accuracy operation. In addition, we applied the dynamic gate compensation technique to the single-hole pump and confirm the further performance improvement. Furthermore, we demonstrate gigahertz operation of a single-hole pump with an estimated lower bound of an error rate of around 0.01 ppm. These results imply a superior capability of single-hole pumps in silicon toward high-accuracy, high-speed, and stable single-charge pumping appropriate for not only metrological applications but also quantum device applications.

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arXiv2023

Valley splitting by extended zone effective mass approximation incorporating strain in silicon

Silicon metal-oxide-semiconductor field effect transistors (MOSFETs) fabricated on a SIMOX (001) substrate, which is a kind of silicon on insulator (SOI) substrate, that is annealed at high temperature for a long time are known to exhibit large valley splitting, but the origin of this splitting has long been unknown. Extended zone effective-mass approximation (EMA) predicts that strain significantly affects valley splitting. In this study, we analyzed valley splitting based on this theory and found that the shear strain along <110> of approximately 5% near the buried oxide (BOX) interface is a promising source for large valley splitting.

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arXiv2023

Coulomb collisions of hot and cold single electrons in series-coupled silicon single-electron pumps

Control of the Coulomb interaction between single electrons is vital for realizing quantum information processing using flying electrons and, particularly, for the realization of deterministic two-qubit operations. Since the strength of the Coulomb interaction increases with decreasing distance, a collision experiment of single electrons would be an ideal way to investigate it. Moreover, it would be useful to study such a Coulomb collision in silicon system, which has been extensively studied for qubit applications but so far has not been used for making Coulomb collisions at the single-electron level. Here, we made two series-coupled tunable-barrier single-electron pumps in silicon and used one to inject a hot single electron into the other pump in each pumping cycle. The hot single electron collides with a cold single electron confined in the other single-electron pump. We observed a current flow due to ejection not only of the hot single electron but also of the confined cold single electron. The latter leads to an excess current at a current plateau at a certain voltage range. We also found that increasing the number of cold electrons from one to two increased the cold-electron current by at least twofold. These results can be explained by a charging effect due to the Coulomb interaction. This observation is an important step toward quantum manipulation of flying single electrons in silicon.

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arXiv2023

Precision measurement of an electron pump at 2 GHz

A well-characterised sample of silicon tunable-barrier electron pump has been operated at a frequency of 2 GHz using a custom drive waveform, generating a pump current of 320 pA. Precision measurements of the current were made as a function of pump control parameters, using a blind protocol, over a 7-week campaign. The combined standard uncertainty for each 10 hour measurement was 0.1 parts per million. The pump current exhibits a plateau along the exit gate voltage flat to approximately 0.1 parts per million, but offset from ef by 0.2 parts per million. This offset may be a sign of errors in the current traceability chain, indicating a limit to the accuracy of small current scaling using existing methods based on cryogenic current comparators.

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arXiv2020

Realisation of a quantum current standard at liquid helium temperature with sub-ppm reproducibility

A silicon electron pump operating at the temperature of liquid helium has demonstrated repeatable operation with sub-ppm accuracy. The pump current, approximately 168 pA, is measured by three laboratories, and the measurements agree with the expected current ef within the uncertainties which range from 0.2 ppm to 1.3 ppm. All the measurements are carried out in zero applied magnetic field, and the pump drive signal is a sine wave. The combination of simple operating conditions with high accuracy demonstrates the possibility that an electron pump can operate as a current standard in a National Measurement Institute. We also discuss other practical aspects of using the electron pump as a current standard, such as testing its robustness to changes in the control parameters, and using a rapid tuning procedure to locate the optimal operation point..

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arXiv2019

Derivation of potential profile of a dynamic quantum dot

We report a method to derive the potential barrier profile shape in a dynamic quantum dot and show the loading statistics, and hence accuracy of electron transfer, depend significantly on the shape of the barrier. This method takes a further step towards tunable barrier shapes, which would greatly increase the accuracy of single electron sources, allowing the single electron current to be useful for quantum sensing, quantum information and metrology. We apply our method to the case of a tunable-barrier single-electron pump, an exemplary device that shows promise as a source of hot single electron wavepackets.

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arXiv2019

Evidence for universality of tunable-barrier electron pumps

We review recent precision measurements on semiconductor tunable-barrier electron pumps operating in a ratchet mode. Seven studies on five different designs of pumps have reported measurements of the pump current with relative total uncertainties around $10^{-6}$ or less. Combined with theoretical models of electron capture by the pumps, these experimental data exhibits encouraging evidence that the pumps operate according to a universal mechanism, independent of the details of device design. Evidence for robustness of the pump current against changes in the control parameters is at a more preliminary stage, but also encouraging, with two studies reporting robustness of the pump current against three or more parameters in the range of $\sim\!5 \times 10^{-7}$ to $\sim\!2 \times 10^{-6}$. This review highlights the need for an agreed protocol for tuning the electron pump for optimal operation, as well as more rigorous evaluations of the robustness in a wide range of pump designs.

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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.

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arXiv2012

Simulating Capacitances to Silicon Quantum Dots: Breakdown of the Parallel Plate Capacitor Model

Many electrical applications of quantum dots rely on capacitively coupled gates; therefore, to make reliable devices we need those gate capacitances to be predictable and reproducible. We demonstrate in silicon nanowire quantum dots that gate capacitances are reproducible to within 10% for nominally identical devices. We demonstrate the experimentally that gate capacitances scale with device dimensions. We also demonstrate that a capacitance simulator can be used to predict measured gate capacitances to within 20%. A simple parallel plate capacitor model can be used to predict how the capacitances change with device dimensions; however, the parallel plate capacitor model fails for the smallest devices because the capacitances are dominated by fringing fields. We show how the capacitances due to fringing fields can be quickly estimated.

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arXiv2011

Evaluation of a gate capacitance in the sub-aF range for a chemical field-effect transistor with a silicon nanowire channel

An evaluation of the gate capacitance of a field-effect transitor (FET) whose channel length and width are several ten nanometer, is a key point for sensors applications. However, experimental and precise evaluation of capacitance in the aF range or less has been extremely difficult. Here, we report an extraction of the capacitance down to 0.55 aF for a silicon FET with a nanoscale wire channel whose width and length are 15 and 50 nm, respectively. The extraction can be achieved by using a combination of four kinds of measurements: current characteristics modulated by double gates, random-telegraph-signal noise induced by trapping and detrapping of a single electron, dielectric polarization noise, and current characteristics showing Coulomb blockade at low temperature. The extraction of such a small gate capacitance enables us to evaluate electron mobility in a nanoscale wire using a classical model of current characteristics of a FET.

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arXiv2010

Resonant escape over an oscillating barrier in single-electron ratchet transfer

Single-electron escape from a metastable state over an oscillating barrier is experimentally investigated in silicon-based ratchet transfer. When the barrier is oscillating on a time scale characteristic of the single-electron escape, synchronization occurs between the deterministic barrier modulation and the stochastic escape events. The average escape time as a function of its oscillation frequency exhibits a minimum providing a primary signature for resonant activation of single electrons.

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