arXiv subjects

Takafumi Fujita

Publications and source records attributed to Takafumi Fujita.

8 recordsLinked to original sources

arXiv2026

Automated Spin Readout Signal Analysis Using U-Net with Variable-Length Traces and Experimental Noise

Single-shot spin-state discrimination is essential for semiconductor spin qubits, but conventional threshold-based analysis of spin readout traces becomes unreliable under noisy conditions. Although recent neural-network-based methods improve robustness against experimental noise, they are sensitive to training conditions, restricted to fixed-length inputs, and limited to trace-level outputs without explicit temporal localization of transition events. In this work, we apply a U-Net architecture to spin readout signal analysis by formulating transition-event detection as a point-wise segmentation task in one-dimensional time-series data. The fully convolutional structure enables direct processing of variable-length traces. Point-wise and sample-wise evaluations demonstrate low readout error rates and high classification accuracy without retraining. The proposed method generalizes well to previously-unseen trace lengths and experimental non-Gaussian noise, outperforming a conventional threshold-based approach and providing a robust and practical solution for automated spin readout signal analysis.

cond-mat.mes-hall↗

arXiv2025

High fidelity flopping-mode single spin operation with tuning inter-dot orbital levels

Fast spin manipulation and long spin coherence time in quantum dots are essential features for high fidelity semiconductor spin qubits. However, generally it has not been well established how to optimize these two properties simultaneously, because these two properties are usually not independent from each other. Therefore, the scheme for high fidelity operation by simultaneous tuning Rabi frequency and coherence time, which does not rely on the material-dependent strong spin-orbit interaction and the local magnetic field gradient limiting their scalability, are strongly demanded. Here, we demonstrate an approach to achieve high-fidelity spin control by tuning inter-dot spin-orbit coupling in a GaAs triple quantum dot (TQD), where the third dot provides precise control over orbital energy levels. In an electrically stable charge state with optimized tunnel coupling, we achieve Rabi frequencies exceeding 100 MHz while maintaining coherence through proper tuning of the inter-dot orbital levels of the TQD. By implementing a machine learning-based feedback control that efficiently estimates qubit frequency using past measurement data, we characterize and mitigate the impact of low frequency noise on qubit coherence with minimal measurement overhead. Finally, we demonstrate a $π$/2 gate fidelity of 99.7\% with a gate time of 4 ns through randomized benchmarking, even in a GaAs quantum dot device where electron spin coherence is typically limited by strong hyperfine interaction with nuclear spins. Our approach provides a scalable strategy for high-fidelity spin control in semiconductor quantum dot arrays by utilizing device-specific parameters rather than relying on material properties or external field gradients.

cond-mat.mes-hall↗

arXiv2024

Accelerated adiabatic passage of a single electron spin qubit in quantum dots

Adiabatic processes can keep the quantum system in its instantaneous eigenstate, which is robust to noises and dissipation. However, it is limited by sufficiently slow evolution. Here, we experimentally demonstrate the transitionless quantum driving (TLQD) of the shortcuts to adiabaticity in gate-defined semiconductor quantum dots (QDs) to greatly accelerate the conventional adiabatic passage for the first time. For a given efficiency of quantum state transfer, the acceleration can be more than twofold. The dynamic properties also prove that the TLQD can guarantee fast and high-fidelity quantum state transfer. In order to compensate for the diabatic errors caused by dephasing noises, the modified TLQD is proposed and demonstrated in experiment by enlarging the width of the counter-diabatic drivings. The benchmarking shows that the state transfer fidelity of 97.8% can be achieved. This work will greatly promote researches and applications about quantum simulations and adiabatic quantum computation based on the gate-defined QDs.

cond-mat.mes-hall↗

arXiv2021

Noise-robust classification of single-shot electron spin readouts using a deep neural network

Single-shot readout of charge and spin states by charge sensors such as quantum point contacts and quantum dots are essential technologies for the operation of semiconductor spin qubits. The fidelity of the single-shot readout depends both on experimental conditions such as signal-to-noise ratio, system temperature and numerical parameters such as threshold values. Accurate charge sensing schemes that are robust under noisy environments are indispensable for developing a scalable fault-tolerant quantum computation architecture. In this study, we present a novel single-shot readout classification method that is robust to noises using a deep neural network (DNN). Importantly, the DNN classifier is automatically configured for spin-up and spin-down signals in any noise environment by tuning the trainable parameters using the datasets of charge transition signals experimentally obtained at a charging line. Moreover, we verify that our DNN classification is robust under noisy environment in comparison to the two conventional classification methods used for charge and spin state measurements in various quantum dot experiments.

quant-ph↗

arXiv2018

Quantum state transfer of angular momentum via single electron photo-excitation from a Zeeman-resolved light hole

Electron spins in GaAs quantum dots have been used to make qubits with high-fidelity gating and long coherence time, necessary ingredients in solid-state quantum computing. The quantum dots can also host photon qubits with energy applicable for optical communication, and can show a promising photon-to-spin conversion. The coherent interface is established through photo-excitation of a single pair of an electron and a Zeeman-resolved light-hole, not heavy-hole. However, no experiments on the single photon to spin conversion have been performed yet. Here we report on single shot readout of a single electron spin generated in a GaAs quantum dot by spin-selective excitation with linearly polarized light. A photo-electron spin generated from a Zeeman-resolved light-hole exciton is detected using an optical spin blockade method in a single quantum dot and a Pauli spin blockade method in a double quantum dot. We found that the blockade probability strongly depends on the photon polarization and the hole state, heavy- or light-hole, indicating a transfer of the angular momentum from single photons to single electron spins. Our demonstration will open a pathway to further investigation on fundamental quantum physics such as quantum entanglement between a wide variety of quantum systems and applications of quantum networking technology.

cond-mat.mes-hall↗

arXiv2017

A single eletron-photon pair generation from a single polarization-entangled photon pair

We demonstrate paired generation of a single photo-electron in a quantum dot and a single photon from a single polarization-entangled photon pair that is generated by spontaneous parametric down conversion. The electron is reated in a GaAs quantum dot via inter-band resonantexcitation by irradiating one of the paired photons on the dot, while the remaining photon is detected by a single photon counter. Because the paired photons are generated simultaneously, the photo-electron and the remaining photon should be observed as a coincident event if the traveling distance is equivalent between them. The coincidence is probabilistic due to the probabilistic generation of the paired photon. We observe the probability of finding the photon with the photo-electron detection substantially higher then that without the photo-electron detection and derive the coincident rate comparable to the emission rate of the original photon pairs.

cond-mat.mes-hall↗

arXiv2015

Single-Spin CCD

Spin-based electronics or spintronics relies on the ability to store, transport and manipulate electron spin polarization with great precision. In its ultimate limit, information is stored in the spin state of a single electron, at which point also quantum information processing becomes a possibility. Here we demonstrate the manipulation, transport and read-out of individual electron spins in a linear array of three semiconductor quantum dots. First, we demonstrate single-shot read-out of three spins with fidelities of 97% on average, using an approach analogous to the operation of a charge-coupled-device (CCD). Next, we perform site-selective control of the three spins thereby writing the content of each pixel of this "Single-Spin CCD". Finally, we show that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 $μ$m, has negligible influence on its spin projection. Extrapolating these results to the case of much larger arrays, points at a diverse range of potential applications, from quantum information to imaging and sensing.

cond-mat.mes-hall↗

arXiv2015

Single photoelectron spin detection and angular momentum transfer in a gate defined quantum dot

Recent innovations in fabricating nanoscale confined spin systems have enabled investigation of fundamental quantum correlations between single quanta of photons and matter states. Realization of quantum state transfer from photon polarization to electron spin using gate defined quantum dots (QDs) may give evidence of preserved coherence of angular momentum basis states at the photon-spin interface. The interface would enlarge the concept of quantum information technology, in which single photogenerated electron spins are manipulated with the dots, but this remains a serious challenge. Here, we report the detection of single electron spins generated by polarized single photons via a double QD (DQD) to verify the angular momentum transfer from single photons to single electrons. Pauli spin blockade (PSB) is used to project the photoelectron spin state onto the up or down spin state. Our result promises the realization of coherent quantum state transfer and development of hybrid photon and spin quantum technology.

cond-mat.mes-hall↗