arXiv subjects

Takeru Utsugi

Publications and source records attributed to Takeru Utsugi.

11 recordsLinked to original sources

arXiv2026

Valley enhanced Rabi frequency in n-type planar Silicon-MOS quantum dot

Electron spin resonance spectroscopy (ESR) of a single electron in planar Si-MOS quantum dot is reported in the vicinity of a valley level anti-crossing. A number of one and two-photon resonances are observed due to mixing of magnetic spin-flip and electric valley-flip transitions. This allows the reconstruction of the energy-level diagram of a four state system with two valley and two spin states. Near the anti-crossing, an enhancement of the Rabi frequency is observed. This is attributed to an electric-dipole transition activated by admixing of the upper energy level due to inter-valley spin coupling. The electric-dipole transition may be driven via capacitive coupling between the ESR antenna, and the confinement gate. To characterize spin-valley coupling responsible for the enhancement, we measure the anisotropy of the g-factor difference between the two valley states, the mean g-factor and the inter-valley spin coupling for both in and out-of-plane magnetic fields. The inter-valley spin coupling is strongly modulated by the direction of the B-field, and is strongest for out-of-plane B-field, consistent with an in-plane spin-valley field. In principle, this strong Electric dipole spin resonance (EDSR) effect could be utilized for fast all-electrical spin control in small-scale devices.

cond-mat.mes-hall↗

arXiv2026

Concatenated continuous driving of silicon qubit by amplitude and phase modulation

The rate of coherence loss is lower for a qubit under the Rabi drive than a freely evolving qubit $T_{2}^{\rm{Rabi}}>T_{2}^*$. Building on this principle, concatenated continuous driving (CCD) keeps the qubit under continuous drive to suppress noise and manipulate dressed states by either phase or amplitude modulation. In this work, we propose a variant of CCD which simultaneously modulates both the amplitude and phase of the driving field to generate a circularly polarized field in the rotating frame of the carrier frequency. This circular-modulated CCD(CMCCD) cancels the counter-rotating term in the second rotating frame, eliminating a systematic pulse-area error that arises from an imperfect rotating wave approximation for fast gates. Numerical simulations demonstrate that the proposed CMCCD achieves higher gate fidelity than conventional CCD schemes. We further implement and compare different CCD protocols using an electron spin-qubit in an isotopically purified $^{28}$Si-MOS quantum dot and evaluate its robustness by applying static detuning and Rabi frequency errors. The robustness is significantly improved compared with the standard Rabi drive, showing the effectiveness of this scheme for qubit arrays with variation in qubit frequency, coupling to the Rabi drive, and low-frequency noise. The proposed scheme can be applied to various physical systems, including trapped atoms, cold atoms, superconducting qubits, and NV centers.

quant-ph↗

arXiv2025

High Fidelity Qubit Control in a Natural Si-MOS Quantum Dot using a 300 mm Silicon on Insulator Wafer

We demonstrate high-fidelity single qubit control in a natural Si-MOS quantum dot fabricated in an industrial 300 mm wafer process on a silicon on insulator (SOI) wafer using electron spin resonance. A relatively high optimal Rabi frequency of 5 MHz is achieved, dynamically decoupling the electron spin from its 29-Si environment. Tracking the qubit frequency reduces the impact of low frequency noise in the qubit frequency and improves the $T^{Rabi}$ from 7 to 11 $\mu$s at a Rabi frequency of 5 MHz, resulting in Q-factors exceeding 50. Randomized benchmarking returns an average single gate control fidelity of 99.5 $\pm$ 0.3%. As a result of pulse-area calibration, this fidelity is limited by the Rabi Q-factor. These results show that a fast Rabi frequency, low charge noise, and a feedback protocol enable high fidelity in these Si-MOS devices, despite the low-frequency magnetic noise.

cond-mat.mes-hall↗

arXiv2025

Robust spin-qubit control in a natural Si-MOS quantum dot using phase modulation

Silicon quantum dots are one of the most promising candidates for practical quantum computers because of their scalability and compatibility with the well-established complementary metal-oxide-semiconductor technology. However, the coherence time is limited in industry-standard natural silicon because of the $^{29}$Si isotopes, which have non-zero nuclear spin. Here, we protect an isotopically natural silicon metal-oxide-semiconductor (Si-MOS) quantum dot spin qubit from environmental noise via electron spin resonance with a phase-modulated microwave (MW) drive. This concatenated continuous drive (CCD) method extends the decay time of Rabi oscillations from 1.2 $\mathrm{\mu s}$ to over 200 $\mathrm{\mu s}$. Furthermore, we define a protected qubit basis and propose robust gate operations. We find the coherence time measured by Ramsey sequence is improved from 143 ns to 40.7 $\mu$s compared to that of the bare spin qubit. The single qubit gate fidelity measured with randomized benchmarking is improved from 95% to 99%, underscoring the effectiveness of the CCD method. The method shows promise for improving control fidelity of noisy qubits, overcoming the qubit variability for global control, and maintaining qubit coherence while idling.

cond-mat.mes-hall↗

arXiv2025

Optimal cavity design for minimizing errors in cavity-QED-based atom-photon entangling gates with finite temporal duration

We investigate atom-photon entangling gates based on cavity quantum electrodynamics (QED) for a finite photon-pulse duration, where not only the photon loss but also the temporal mode-mismatch of the photon pulse becomes a severe source of error. We analytically derive relations between cavity parameters, including transmittance, length, and effective cross-sectional area of the cavity, that minimize both the photon loss probability and the error rate due to temporal mode-mismatch by taking it into account as state-dependent pulse delay. We also investigate the effects of pulse distortion using numerical simulations for the case of short pulse duration. We believe that these analyses are the first to suggest that a cavity has an optimal length for the atom-photon gate, providing a fundamental guideline for implementing quantum information processing.

quant-ph↗

arXiv2025

Gaussian-wavepacket-model for single-photon generation based on cavity QED in the adiabatic and nonadiabatic conditions

For single-photon generation based on cavity quantum electrodynamics, we investigate a practical model assuming a Gaussian wavepacket. This model makes it possible to comprehensively analyze the temporal dynamics of an atom-cavity system with both adiabatic and nonadiabatic conditions using analytical expressions. These results enable us to clarify the relationship between pulse width and maximum success probability, over the full range of coupling regimes. We demonstrate how to achieve a high success probability while keeping a short pulse width by optimizing the cavity transmittance parameter and the time-controlled exexternal field. Our formulations provide a practical tool for efficient single-photon generation in a wide variety of experimental platforms.

quant-ph↗

arXiv2025

Single electron routing in a silicon quantum-dot array

The ability to transport single electrons on a quantum dot array dramatically increases the freedom in designing quantum computation schemes that can be implemented on solid-state devices. So far, however, routing schemes to precisely control the transport paths of single electrons have yet to be established. Here, we propose a silicon single-electron router that transports pumped electrons along the desired route on the branches of a T-shaped quantum dot array by inputting a synchronous phase-controlled signal to multiple gates. Notably, we show that it is possible to achieve a routing accuracy above 99\% by boosting the accuracy of the electron-transport timing with an assist gate in front of the branching paths. We also evaluated the minimum error rate of routing by the model of electron transport based on the Wigner representation in an energy-time space. The results suggest new possibilities for fast and accurate transport of single electrons on the two-dimensional quantum dot arrays.

cond-mat.mes-hall↗

arXiv2024

Thermal circuit model for silicon quantum-dot array structures

Temperature rise of qubits due to heating is a critical issue in large-scale quantum computers based on quantum-dot (QD) arrays. This leads to shorter coherence times, induced readout errors, and increased charge noise. Here, we propose a simple thermal circuit model to describe the heating effect on silicon QD array structures. Noting that the QD array is a periodic structure, we represent it as a thermal distributed-element circuit, forming a thermal transmission line. We validate this model by measuring the electron temperature in a QD array device using Coulomb blockade thermometry, finding that the model effectively reproduces experimental results. This simple and scalable model can be used to develop the thermal design of large-scale silicon-based quantum computers.

cond-mat.mes-hall↗

arXiv2024

Generating Shuttling Procedures for Constrained Silicon Quantum Dot Array

In silicon quantum computers, a single electron is trapped in a microstructure called a quantum dot, and its spin is used as a qubit. For large-scale integration of qubits, we previously proposed an approach of arranging the quantum dots in a two-dimensional array and sharing a control gate in a row or column of the array. In our array, the shuttling of electrons is a useful technique to operate the target qubit independently and avoid crosstalk. However, since the shuttling is also conducted using shared control gates, the movement of qubits is complexly constrained. We therefore propose a formal model on the basis of state transition systems to describe those constraints and operation procedures on the array. We also present an approach to generate operation procedures under the constraints. Utilizing this approach, we present a concrete method for our 16 $\times$ 8 quantum dot array. By implementing the proposed method as a quantum compiler, we confirmed that it is possible to generate operation procedures in a practical amount of time for arbitrary quantum circuits. We also demonstrated that crosstalk can be avoided by shuttling and that the fidelity in that case is higher than when crosstalk is not avoided.

quant-ph↗

arXiv2023

Speed Limit of Efficient Cavity-Mediated Adiabatic Transfer

Cavity-mediated adiabatic transfer (CMAT) is a robust way to perform a two-qubit gate between trapped atoms inside an optical cavity. In the previous study by Goto and Ichimura [H. Goto and K. Ichimura, Phys. Rev. A 77, 013816 (2008).], the upper bound of success probability of CMAT was shown where the operation is adiabatically slow. For practical applications, however, it is crucial to operate CMAT as fast as possible without sacrificing the success probability. In this paper, we investigate the operational speed limit of CMAT conditioned on the success probability being close to the upper bound. In CMAT both the adiabatic condition and the decay of atoms and cavity modes limit the operational speed. We show which of these two conditions more severely limits the operational speed in each cavity-QED parameter region, and find that the maximal operational speed is achieved when the influence of cavity decay is dominant compared to spontaneous emission.

quant-ph↗

arXiv2023

Electron Charge Sensor with Hole Current Operating at Cryogenic Temperature

When SOI-PMOS functions like a capacitor-less 1T-DRAM cell, it is possible for the number of electrons to be sensed at cryogenic temperatures (5K). We developed a structure that combines SOI-NMOS and SOI-PMOS with multiple gates to form a silicon quantum-dot array. In this structure, a variable number of electrons is injected into the SOI-PMOS body transporting them by means of the bucket-brigade operation of SOI-NMOS connected in series. The channel-hole current was changed by the injected electrons due to the body bias effect in SOI-PMOS, and the change appeared to be step-like, suggesting a dependence on the elementary charge.

cond-mat.mes-hall↗