arXiv subjects

Hiroki Takahashi

Publications and source records attributed to Hiroki Takahashi.

At least 19 recordsLinked to original sources

arXiv2026

Stabilizing an optical cavity containing a bulk diamond crystal at millikelvin temperatures in a cryogen-free dilution refrigerator

We successfully stabilized a Fabry-Pérot optical cavity incorporating a bulk diamond crystal at millikelvin temperatures in a cryogen-free dilution refrigerator with the pulse-tube cryocooler running. In stark contrast to previous demonstrations where lasers were locked to the cavities, our setup locks the cavity to a laser. Our measurements of cavity length fluctuation suggest that the setup could stabilize a cavity up to a finesse of $1.2\times 10^4$ without the diamond and $5.8 \times10^3$ with the diamond crystal. The finesse with a diamond crystal of approximately 90 is primarily limited by the absorption loss inside the diamond.

quant-ph↗

arXiv2026

Permutation XOR cellular automata: partial-shift connections and half-primitive polynomials

This paper presents a permutation XOR cellular automaton, a simple digital dynamical system composed of an elementary cellular automaton of the XOR rule and a partial-shift permutation connection characterized by a shift-part parameter. The proposed system is suitable for precise analysis and simple hardware implementation. Depending on the partial-shift connection and system dimension, the automaton can generate long binary periodic orbits (LBPOs) with strong stability. As a principal result, we identify the partial-shift parameters and dimensions that enable the generation of LBPOs. In our analysis, these LBPOs are characterize using half-primitive polynomials over $\mathrm{GF}(2)$. Furthermore, we present a simple FPGA-based hardware implementation. The hardware transforms typical LBPOs into electrical signals for potential engineering applications.

math.DS↗

arXiv2026

A near-quantum-limited diamond maser amplifier operating at millikelvin temperatures

Current microwave quantum technologies require the amplification of weak signals with minimal added noise at millikelvin temperatures. To date, this stringent requirement has been met exclusively by superconducting technologies, such as Josephson or kinetic-inductance parametric amplifiers. A fundamentally distinct alternative approach could be offered by masers, the microwave counterpart of lasers, which were predicted as early as the 1950s to achieve quantum-limited noise performance under ideal conditions. However, their dependence on cryogenic operation historically limited further advancement. Here we demonstrate the first-ever non-superconducting, near-quantum-limited maser amplifier operating at millikelvin temperatures utilising nitrogen impurity spins (P1 centres) in diamond. Population inversion is achieved via microwave pumping, exploiting a four-spin cross-relaxation mechanism. We realise a maximum power gain exceeding 30 dB, an added noise of approximately 2.55 quanta above the standard quantum limit, and a maximum 1 dB output compression point of -63 dBm at 6.595 GHz. The ability to operate in strong static magnetic fields of arbitrary orientation may offer a complementary, non-superconducting route for applications such as semiconducting spin-qubit readout, magnetic-resonance spectroscopy, and dark-matter axion searches.

quant-ph↗

arXiv2026

All-optical control of coherent perfect absorption via frequency conversion

Coherent perfect absorption (CPA) extinguishes optical fields through interference and dissipation, but conventional implementations rely on material loss that is largely fixed after fabrication. Here we demonstrate all-optically controllable CPA based on frequency conversion in a periodically poled lithium niobate waveguide resonator. Pump-driven frequency conversion couples a resonant signal field at 1581 nm in the main system to a non-resonant environmental mode at 780 nm, creating a dynamically tunable effective loss channel. The nonlinear cavity acts as a tunable lossy beamsplitter without intrinsic material absorption. Under coherent two-sided signal injection, we observe up to 92 % absorption. We further introduce environment-assisted CPA by injecting an external field into the frequency-converted environmental mode, turning the environment from a passive loss reservoir into an addressable coherent control port. Our results establish a frequency-conversion-based platform for all-optical control of dissipation in CPA, combining pump-tunable loss with environment-assisted coherent control.

physics.optics↗

arXiv2026

Controlling the Inhomogeneous Broadening and Impedance Matching of a Spin Ensemble

We control the spectral distribution of a spin ensemble by applying a magnetic field gradient using an anti-Helmholtz coil inside a dilution refrigerator, and demonstrate impedance matching between the ensemble and a transmission line, achieving -50 dB absorption of incident radiation. This represents a first step toward a spin-ensemble-based quantum memory for itinerant microwave photons. We further model the spectral distribution under the applied gradient to predict the spin-resonator response, and use the device to systematically tune the weak-to-strong coupling transition in both continuous-wave and time-domain pulsed measurements.

quant-ph↗

arXiv2026

Toward Inclusive Avatar Design with Limb Differences Through Artificial Intelligence

As extended reality becomes more popular for social interaction and entertainment, 3D avatars must represent the full diversity of body types. Most 3D avatar systems only support normative bodies and do not accurately depict people with limb differences, amputations, or other morphological variations. This paper reviews emerging technical approaches for inclusive 3D avatar customization for this group and current guidelines that promote respectful and accurate representation. We highlight persistent challenges, including the scarcity of diverse datasets and the limitations in animation for non-normative anatomies. This paper positions artificial intelligence as a promising path to overcoming these limitations and advancing inclusive 3D avatar generation.

cs.HC↗

arXiv2026

Error-corrected phase estimation averaged over variable grids on a trapped-ion quantum computer: hyperacuity spectra of a CO molecule adsorbed onto $χ$-Fe$_5$C$_2$

Quantum phase estimation (QPE) is an underlying technology for extracting the excitation spectra of many-electron systems, yet its practical use on current hardware is hindered by low grid resolution and environmental noises. Here we propose QPE averaged over variable grids (QAVG), a vernier-type approach that combines low-resolution QPE with multiple origin shifts and physically motivated continuous parametrization to reconstruct the spectra accurately. We introduce this approach into an end-to-end workflow for the {\it ab initio}-based model system for a CO molecule adsorbed onto the $χ$-Fe$_5$C$_2$ surface. We perform experiments on Quantinuum H2-2 using both physical QPE circuits and logical QPE circuits encoded in the Steane code with offline bit-flip correction. We demonstrate that QAVG accurately reconstructs the spectra with deviations much smaller than the nominal QPE resolution, even when the noisy histograms are used. The cost landscapes averaged over the shifted grids substantially suppress the local minima arising from the spectral leakage, thereby stabilizing the optimization of trial parameters. These results indicate that QAVG provides a robust route to quantum simulations of correlated spectra toward the era of early-fault-tolerant quantum computers.

quant-ph↗

arXiv2026

Channel-agnostic finite-temperature phase estimation averaged over variable grids: reconstruction of Green's function for dynamical mean-field theory

For treating correlated electronic systems on quantum computers, we propose a quantum-classical hybrid scheme for dynamical mean-field theory (DMFT). In the quantum part of the scheme, we use modified quantum phase estimation (QPE) circuits suitable for the one-particle Green's function (GF) at a finite temperature so that we can extract spectral amplitudes and the excitation energies without knowing the excitation channel invoked at each measurement. In the classical part of the scheme, we adopt an approach that estimates reasonably the GF based on the data collected from the QPE sampling. We dub the approach the QPE averaged over variable grids (QAVG), that may help one to reconstruct the GF via optimization of trial parameters and modeling the probability distributions for various settings of the QPE circuits. We apply the QAVG-DMFT scheme to SrVO$_3$ to demonstrate its validity via numerical simulations.

quant-ph↗

arXiv2026

Pressure Effects on CeMnSi: Evolution of Ce 4f and Mn 3d Electronic States and Negative Thermal Expansion

We investigated pressure effects on the nontrivial heavy-fermion antiferromagnet CeMnSi by means of electrical resistivity and powder X-ray diffraction. With increasing pressure, the antiferromagnetic order of Mn (T_N ~ 240 K at ambient pressure) is rapidly suppressed and disappears at P_c ~ 1.3 GPa. Instead, a pressure-induced anomaly appears at T_M ~ 97 K and shifts to higher temperatures with increasing pressure. The switching of the Mn magnetic state may reflect a modification of the magnetic symmetry of the system, which could influence the stability of the heavy-fermion state. In the low-pressure region, non-Fermi-liquid-like behavior characterized by nearly T-linear resistivity is observed around 0.7 GPa. In addition, the resistivity shows a marked reduction below T_M and a qualitative change toward more metallic behavior above the structural transition pressure P_s ~ 5.7 GPa. At ambient pressure, CeMnSi exhibits negative thermal expansion below ~40 K, which is absent in LaMnSi, supporting the formation of a heavy-fermion ground state.

cond-mat.str-el↗

arXiv2026

Gamma-Ray Signatures of Thermal Misalignment Dark Matter

Thermal misalignment is a viable dark matter scenario where the misalignment of a dark matter scalar, feebly coupled to the Standard Model particles, is generated through thermal effects from the primordial plasma. In this framework, the scalar is generically metastable, and its decay can leave observable signatures. In this work, we focus on the case in which the scalar $ϕ$ is coupled to photons through $ϕF^{μν} F_{μν}$, and examine its observational signatures. We find that current gamma-ray constraints place a robust upper bound on the scalar mass of $\mathcal O(1)\,\mathrm{GeV}$. We also find that future observations can further probe the parameter region, particularly in the MeV--GeV range, an energy band expected to be explored by various gamma-ray observatories in the coming decades.

hep-ph↗

arXiv2026

The Rise of AI-Generated Anime Avatars: Trends, Challenges, and Opportunities

The rise of 3D anime-style avatars in gaming, virtual reality, and other digital media has driven significant interest in automated generation methods capable of capturing their distinctive visual characteristics. These include stylized proportions, expressive features, and non-photorealistic rendering. This paper reviews the advancements and challenges in using deep learning in 3D anime-style avatar generation. We analyze the strengths and limitations of these methods in capturing the aesthetics of anime characters and supporting customization and animation. Additionally, we identify and discuss open problems in the field, such as difficulties in resolution and detail preservation, and constraints regarding the animation of hair and loose clothing. This article aims to provide a comprehensive overview of the current state-of-the-art and identify promising research directions for advancing 3D anime-style avatar generation.

cs.GR↗

arXiv2026

Stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with Barium ions

We explore the stability of a high-finesse optical cavity at 493 nm in vacuum for cavity QED with Barium ions. A high-finesse Fabry-Perot cavity is built using mirrors with high-reflectivity (HR) coatings that are implemented by stacking multiple thin films of low-loss dielectrics on substrates. Applications of such HR mirrors in the near ultraviolet (UV) range have been hampered by degradation of coatings in vacuum. Here, we explore the degradation of mirrors with HR coatings at 493 nm in vacuum. We study both vacuum-induced and laser-induced effects on oxide-coated cavity mirrors by probing changes in cavity loss using cavity lifetime measurements. We investigate the role of circulating power in the rate of increase in cavity loss and demonstrate methods of reversal of cavity degradation. While we observe no degradation without long exposure or with short exposures at lower circulating powers, we find evidence of degradation on long exposure to high circulating powers. We discuss potential causes and conclude that laser-induced deposition is the likely cause while ruling out thermally activated processes due to laser-induced heating.

physics.optics↗

arXiv2026

Decision Support System for Technology Opportunity Discovery: An Application of the Schwartz Theory of Basic Values

Discovering technology opportunities (TOD) remains a critical challenge for innovation management, especially in early-stage development where consumer needs are often unclear. Existing methods frequently fail to systematically incorporate end-user perspectives, resulting in a misalignment between technological potentials and market relevance. This study proposes a novel decision support framework that bridges this gap by linking technological feasibility with fundamental human values. The framework integrates two distinct lenses: the engineering-based Technology Readiness Levels (TRL) and Schwartz's theory of basic human values. By combining these, the approach enables a structured exploration of how emerging technologies may satisfy diverse user motivations. To illustrate the framework's feasibility and insight potential, we conducted exploratory workshops with general consumers and internal experts at Sony Computer Science Laboratories, Inc., analyzing four real-world technologies (two commercial successes and two failures). Two consistent patterns emerged: (1) internal experts identified a wider value landscape than consumers (vision gap), and (2) successful technologies exhibited a broader range of associated human values (value breadth), suggesting strategic foresight may underpin market success. This study contributes both a practical tool for early-stage R\&D decision-making and a theoretical link between value theory and innovation outcomes. While exploratory in scope, the findings highlight the promise of value-centric evaluation as a foundation for more human-centered technology opportunity discovery.

cs.HC↗

arXiv2026

Asymmetric Dark Matter from Low-Scale Spontaneous Leptogenesis

We investigate a novel type of asymmetric dark matter (ADM) model in which the dark matter asymmetry and the baryon asymmetry in our universe (BAU) are produced simultaneously via low-scale spontaneous leptogenesis, where the mass scale of right-handed neutrino is much lower than the Davidson-Ibarra bound $M_1 \ll 10^{9}~\rm{GeV}$. In our scenario, both asymmetries are predominantly sourced by a dynamical $CP$ phase, namely the majoron. Its kinetic misalignment provides a sufficiently large, time-dependent effective $CP$ phase, allowing efficient asymmetry production even for low-scale right-handed neutrinos. In our framework, the sources of $CP$ violation responsible for the BAU and ADM are correlated with each other, leading to a predictive relation for the dark matter mass. In particular, when the dark matter asymmetry reaches its equilibrium value before freeze-out, the dark matter mass is typically predicted to lie in the range $\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_χ \lesssim \mathcal{O}(100)~\mathrm{GeV}$, which lies within the sensitivity of direct detection experiments. On the other hand, if the dark matter asymmetry does not reach its equilibrium value due to weak coupling, the allowed mass range extends over a broader interval, $\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_χ \lesssim \mathcal{O}(10)~\rm{TeV}$.

hep-ph↗

arXiv2025

Hybrid Quantum Repeater Chains with Atom-based Quantum Processing Units and Quantum Memory Multiplexers

Quantum repeaters enable the generation of reliable entanglement across long distances despite the underlying channel noise. Nevertheless, realizing quantum repeaters poses a difficult engineering challenge due to various device constraints and design tradeoffs. Herein, we propose and analyze an efficient hybrid quantum repeater design that integrates atom-based quantum processing units, spontaneous parametric down-conversion photon sources, and atomic frequency comb quantum memories. Our design leverages the strong spectro-temporal multiplexing capability of the quantum memory to enable high-rate elementary-link entanglement generation between repeater nodes. Transferring the photonic entanglement into matter-qubit entanglement, together with deterministic quantum operations, further enables reliable long-distance entanglement distribution. We analyze photon-loss channels in the hybrid architecture and propose suitable error-suppression strategies that are natively incorporated into our repeater protocol. Using numerical simulations, we demonstrate the advantages of our hybrid design for end-to-end secret key rates in a linear repeater-chain model. With continued advances in relevant hardware technologies, we envision that the proposed hybrid design is well-suited for large-scale quantum networks.

quant-ph↗

arXiv2025

Profile control of fibre-based micro-mirrors using adaptive laser shooting with $\textit{in situ}$ imaging

Fibre Fabry-Perot cavities (FFPCs) are used in various studies in cavity quantum electrodynamics (CQED) and quantum technologies due to the cavity's small mode volume and compact integration with optical fibres. We develop a novel $\text{CO}_2$ laser machining method that produces well-controlled surface profiles on the end facets of cleaved optical fibres. Using multiple shots in distinct spatial distribution patterns, our method employs a shooting algorithm that adaptively changes laser ablation parameters during the shooting to suppress deviations from the desired profile. This is made possible by $\textit{in situ}$ imaging of the machined profile, its inspection and the usage of the information in the subsequent steps. Underlying this algorithm is a newly found laser ablation parameter, the pause between shots, which controls the accumulation of heat in between successive laser shots and as a result determines the area of impact made by an individual ablation sequence. We fabricate fibre-based micro-mirrors with radii of curvature ranging from 250 $μ$m to 700 $μ$m with an effective mirror diameter of 60 $μ$m in either Gaussian or spherical profiles. Due to the self-correcting nature of our adaptive algorithm, we achieve a near 100\% success rate in the production of desired profiles with low ellipticity. After furnishing the laser machined fibre end facets with high reflectivity coating, FFPCs are formed to demonstrate a high finesse up to 150,000 at an optical wavelength of 854 nm.

physics.optics↗

arXiv2024

How to integrate a miniature optical cavity in a linear ion trap: shielding dielectrics and trap symmetry

One method of scaling up quantum systems is to adopt a modular approach. In the ion trap architecture, an efficient photonic interface between independent linear ion traps would allow for such expansion. To this end, an optical cavity with a small mode volume can be utilised to enhance the photon emission probability from the ion. Miniature fibre-based Fabry-Perot cavities have been integrated into three-dimensional Paul traps that hold a single ion, whereas an efficient interface between an optical cavity and a linear trap that can keep multiple ions has remained elusive. This presents a barrier for combining the benefits of the motional coupling in a chain of ions with optical interface between ion traps. In this paper, we show that simple electrically conductive shielding of the fibres could provide substantial advantage in mitigating the adverse effects of stray charges and motional heating by dielectrics. We also reveal that the conductive shields are not compatible with the conventional radio frequency (rf) drive in ion traps but using two rf signals with opposite phases can solve this issue. Furthermore the role played by the symmetry of the electrodes when incorporating an element that disrupts the translational symmetry of a linear trap is elucidated analytically. As a result it is realized that two-dimensional implementation of a linear ion trap such as a surface trap is inherently not suitable for integrating a shielded miniature optical cavity due to the lack of geometrical symmetry. Based on the insights obtained through the analysis, we identify essential components and a design strategy that should be incorporated in a linear ion trap for successful integration of a miniature optical cavity.

quant-ph↗

arXiv2024

Ion Trapping with a Laser-written 3D Miniaturized Monolithic Linear Paul Trap for Microcavity Integration

The miniaturization of ion trap and the precise placement of its electrodes are necessary for the integration of a microcavity to facilitate efficient ion-cavity coupling. We present a miniature monolithic ion trap made of gold-coated fused silica with high numerical aperture access. A laser writing method referred to as selective laser etching is employed to extract a trap structure from a block of fused silica. The fully monolithic structure eliminates the need for any post-fabrication alignment. Trenches are integrated into this structure such that the various electrodes on the monolithic device remain electrically isolated following their metalization via evaporative coating. We give details of the trap design and production, along with the demonstration of successful trapping of ions and characerization of the trap.

quant-ph↗