arXiv subjects

Yasuhiro Yamada

Publications and source records attributed to Yasuhiro Yamada.

At least 19 recordsLinked to original sources

arXiv2026

Self-Limiting Mechanism of Anti-Stokes Optical Cooling in Diamond NV Centers

Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is experimentally and numerically investigated. Photoluminescence-excitation spectroscopy reveals pronounced phonon-assisted anti-Stokes emission under excitation below the zero-phonon line (ZPL). However, the below-ZPL excitation drives photoinduced charge-state conversion between negatively-charged NV- and neutral NV0, thereby suppressing the NV- mediated cooling channel. Time-resolved photoluminescence (PL) measurements reveal an increase in the effective PL lifetime with excitation density, reflecting an increasing NV0 contribution. By fitting nanosecond and millisecond PL dynamics with a minimal rate-equation model, we extract effective optical pumping and charge-conversion rates, which enables us to quantitatively simulate the cooling performance. The simulations predict a self-limiting behavior of anti-Stokes cooling and clarify the excitation conditions under which net cooling can be sustained within this effective model. The estimated cooling power per NV center is comparable, on a microscopic basis, to values discussed for semiconductor quantum dots and rare-earth optical coolers. These results identify charge-state conversion as a key bottleneck for defect-based optical refrigeration.

cond-mat.mtrl-sci↗

arXiv2025

The Cosmic Infrared Background Experiment-2: An Intensity Mapping Optimized Sounding-rocket Payload to Understand the Near-IR Extragalactic Background Light

The background light produced by emission from all sources over cosmic history is a powerful diagnostic of structure formation and evolution. At near-infrared wavelengths, this extragalactic background light (EBL) is comprised of emission from galaxies stretching all the way back to the first-light objects present during the Epoch of Reionization. The Cosmic Infrared Background Experiment 2 (CIBER-2) is a sounding-rocket experiment designed to measure both the absolute photometric brightness of the EBL over 0.5 - 2.0 microns and perform an intensity mapping measurement of EBL spatial fluctuations in six broad bands over the same wavelength range. CIBER-2 comprises a 28.5 cm, 80K telescope that images several square degrees to three separate cameras. Each camera is equipped with an HAWAII-2RG detector covered by an assembly that combines two broadband filters and a linear-variable filter, which perform the intensity mapping and absolute photometric measurements, respectively. CIBER-2 has flown three times: an engineering flight in 2021; a terminated launch in 2023; and a successful science flight in 2024. In this paper, we review the science case for the experiment; describe the factors motivating the instrument design; review the optical, mechanical, and electronic implementation of the instrument; present preflight laboratory characterization measurements; and finally assess the instrument's performance in flight.

astro-ph.IM↗

arXiv2025

Model-based framework for automated quantification of error sources in quantum state tomography

High-quality quantum state generation is essential for advanced quantum information processing, including quantum communication, quantum sensing, and quantum computing. In practice, various error sources degrade the quality of quantum states, and quantum state tomography (QST) is a standard diagnostic tool. However, in QST, multiple error sources gather in a single density matrix, making it difficult to identify individual error sources. To address this problem, we propose an automated method for quantifying error sources by combining simulation and parameter optimization to reproduce the experimental density matrix. We focus on the experimental generation of time-bin entangled photon pairs, for which we model the relevant error sources and simulate the density matrix with adjustable model parameters, thereby optimizing the parameters and minimizing the trace distance to the experimental data. Optimization of the parameters reduced the trace distance from 0.177 to 0.024, indicating that our modeled error sources explain 86% of the errors. Reducing the predicted error sources improves the state quality, consistent with our predictions and thus validating the proposed method. In addition, the modular structure of this framework makes it applicable to other quantum platforms, such as superconducting qubits, atoms, and solid-state spins.

quant-ph↗

arXiv2024

Optical Cooling of Dot-in-crystal Halide Perovskites: Challenges of Nonlinear Exciton Recombination

Highly efficient anti-Stokes (AS) photoluminescence (PL) is observed from halide perovskite quantum dots (QDs) due to their strong electron-phonon interactions. The AS PL is particularly intriguing as it suggests the potential for semiconductor optical cooling if the external quantum efficiency approaches 100%. However, the PL quantum efficiency in QDs is primarily dominated by multiparticle nonradiative Auger recombination processes under intense photoexcitation, which impose limits on the optical cooling gain. Here, we investigate the Auger recombination of dot-in-crystal perovskites. We quantitatively estimate the maximum optical cooling gain and the corresponding excitation intensity. We further conducted optical cooling experiments and demonstrate a maximum photo-cooling of approximately 9 K from room temperature. Additionally, we confirmed that increasing the excitation intensity leads to a transition from photo-cooling to photo-heating. These observations are consistent with our time-resolved measurements, offering insights into the potential and limitations of optical cooling in semiconductor QDs.

cond-mat.mtrl-sci↗

arXiv2024

Nonlocal Andreev transport through a quantum dot in a magnetic field: Interplay between Kondo, Zeeman, and Cooper-pair correlations

We study the nonlocal magnetotransport through a strongly correlated quantum dot, connected to multiple terminals consisting of two normal and one superconducting (SC) leads. Specifically, we present a comprehensive view on the interplay between the crossed Andreev reflection (CAR), the Kondo effect, and the Zeeman splitting at zero temperature in the large SC gap limit. The ground state of this network shows an interesting variety, which varies continuously with the system parameters, such as the coupling strength $\Gamma_S^{}$ between the SC lead and the quantum dot, the Coulomb repulsion $U$, the impurity level $\varepsilon_d^{}$, and the magnetic field $b$. We show, using the many-body optical theorem which is derived from the Fermi-liquid theory, that the nonlocal conductance is determined by the transmission rate of the Cooper pairs $\mathcal{T}_{\mathrm{CP}}^{} = \frac{1}{4} \sin^2 \Theta\, \sin^2 \bigl(\delta_{\uparrow}+ \delta_{\downarrow})$ and that of the Bogoliubov particles $\mathcal{T}_{\mathrm{BG}}^{}= \frac{1}{2}\sum_{\sigma} \sin^2 \delta_{\sigma}^{}$. Here, $\delta_\sigma^{}$ is the phase shift of the renormalized Bogoliubov particles, and $\Theta \equiv \cot^{-1} (\xi_d^{}/ \Gamma_S^{})$ is the Bogoliubov-rotation angle in the Nambu pseudo spin space, with $\xi_d^{} =\varepsilon_d^{}+U/2$. It is also demonstrated, using Wilson's numerical renormalization group approach, that the CAR is enhanced in the crossover region between the Kondo regime and the SC-proximity-dominated regime at zero magnetic field. The magnetic fields induce another crossover between the Zeeman-dominated regime and the SC-dominated regime. We find that the CAR is enhanced and becomes less sensitive to magnetic fields in the SC-dominated regime close to the crossover region spreading over the angular range of $\pi/4 \lesssim \Theta \lesssim 3\pi/4$.

cond-mat.mes-hall↗

arXiv2024

Kondo Screening of Local Moments in a Triangular Triple Quantum Dot Connected to Normal and Superconducting Leads

We study the interplay between the Kondo and superconducting (SC) proximity effects, taking place in a triangular triple quantum dot (TTQD) connected to one normal and two SC leads. This system shows various quantum phases. Without the SC leads, the lowest two states that belong to the different spin sectors, $S=0$ and $S=1/2$, become energetically very close to each other near half filling. The singlet one is a Kondo-screened state by conduction electrons from the normal lead, and the doublet one is a resonating valence bond state with unpaired free spin which remains unscreened. Furthermore, when one additional electron enters the TTQD, the ground state becomes a doublet in which the $S=1$ local moment due to the Nagaoka ferromagnetism is partially screened by conduction electrons.The Cooper pairs penetrating into the TTQD from the SC leads reconstruct the wavefunctions and vary the phase boundaries between these quantum states in the parameter space. We calculate ground-state phase diagrams using the numerical renormalization group, and show that the SC proximity effect induces a reentrant transition in-between the three- and four-electron fillings.

cond-mat.mes-hall↗

arXiv2023

10-GHz-clock time-multiplexed non-degenerate optical parametric oscillator network with a variable planar lightwave circuit interferometer

A coherent XY machine (CXYM) is a physical spin simulator that can simulate the XY model by mapping XY spins onto the continuous phases of non-degenerate optical parametric oscillators (NOPOs). Here, we demonstrated a large-scale CXYM with >47,000 spins by generating 10-GHz-clock time-multiplexed NOPO pulses via four-wave mixing in a highly nonlinear fiber inside a fiber ring cavity. By implementing a unidirectional coupling from the i-th pulse to the (i+1)-th pulse with a variable 1-pulse delay planar lightwave circuit interferometer, we successfully controlled the effective temperature of a one-dimensional XY spin network within two orders of magnitude.

physics.optics↗

arXiv2023

Observing a Phase Transition in a Coherent Ising Machine

A coherent Ising machine (CIM) is known to deliver the low-energy states of the Ising model. Here, we investigate how well the CIM simulates the thermodynamic properties of a two-dimensional square-lattice Ising model. Assuming that the spin sets sampled by the CIM can be regarded as a canonical ensemble, we estimate the effective temperature of the spins represented by degenerate optical parametric oscillator pulses by using maximum likelihood estimation. With the obtained temperature, we confirmed that the thermodynamic quantities obtained with the CIM exhibited a phase-transition-like behavior that better matches the analytical and numerical results than the mean field one.

cond-mat.stat-mech↗

arXiv2023

Detecting partial synchrony in a complex oscillatory network using pseudo-vortices

Partial synchronization is characteristic phase dynamics of coupled oscillators on various natural and artificial networks, which can remain undetected due to the complexity of the systems. With an analogy between pairwise asynchrony of oscillators and topological defects, i.e., vortices, in the two-dimensional XY spin model, we propose a robust and data-driven method to identify the partial synchronization on complex networks. The proposed method is based on an integer matrix whose element is pseudo-vorticity that discretely quantifies asynchronous phase dynamics in every two oscillators, which results in graphical and entropic representations of partial synchrony. As a first trial, we apply our method to 200 FitzHugh-Nagumo neurons on a complex small-world network. Partially synchronized chimera states are revealed by discriminating synchronized states even with phase lags. Such phase lags also appear in partial synchronization in chimera states. Our topological, graphical, and entropic method is implemented solely with measurable phase dynamics data, which will lead to a straightforward application to general oscillatory networks including neural networks in the brain.

cond-mat.dis-nn↗

arXiv2023

Thermodynamic quantities of two-dimensional Ising models obtained by noisy mean field annealing and coherent Ising machine

Noisy mean field annealing (NMFA) is an algorithm that mimics a coherent Ising machine (CIM), which is an optical system for solving Ising problems. The NMFA has reproduced the solver performance of the CIM for systems of limited size even though it simplifies the interaction between spins with a mean-field approximation. However, recent experiments observing various thermodynamic quantities have revealed that the CIM can capture the phase transitions of the two-dimensional Ising models that the mean field cannot capture. This situation leads to a fundamental question as to how well the NMFA can capture the features of the thermodynamic quantities around the phase transition. This paper answers that the NMFA reproduces the thermodynamic features of the mean field, but cannot reproduce the CIM results. This suggests that, in terms of sampling, the level of performance of the CIM is beyond that of the NMFA.

cond-mat.stat-mech↗

arXiv2022

Experimental observation of chimera states in spiking neural networks based on degenerate optical parametric oscillators

We experimentally demonstrate that networks of identical photonic spiking neurons based on coupled degenerate parametric oscillators can show various chimera states, in which, depending on their local synchronization and desynchronization, different kinds of spiking dynamics can develop in a self-organized manner. Even when only a static interaction is implemented, through synchronized inputs from connected neurons, the spiking mode of photonic neurons can be spontaneously and adaptively changed between the Class-I and Class-II modes classified by A. L. Hodgkin. This spontaneous spiking-mode shift induces a significant change in the spiking frequency despite the all neurons having the same natural spiking frequency, which encourages the generation of chimera states. Controllability and self-organized flexibility of the spiking modes in the present system allow us to create an experimental platform to explore the nature of chimera states in neuromorphic spiking neural networks.

physics.optics↗

arXiv2022

Anti-Stokes Photoluminescence from CsPbBr3 Nanostructures Embedded in a Cs$_4$PbBr$_6$ Crystal

Lead halide perovskites possess high photoluminescence (PL) efficiency and strong electron-phonon interactions, and therefore the optical cooling using up-conversion PL has been expected. We investigate anti-Stokes PL from green-luminescent Cs$_4$PbBr$_6$, whose origin is attributable to CsPbBr3 nanostructures embedded in a Cs$_4$PbBr$_6$ crystal. Because of the high transparency, low refractive index, and high stability of Cs$_4$PbBr$_6$, the green PL displays high external quantum efficiency without photo-degradation. Time-resolved PL spectroscopy reveals the excitonic behaviors in recombination process. The shape of the PL spectrum is almost independent of excitation photon energy, which means that the spectral width is determined by homogeneous broadening. We demonstrate that the phonon-assisted process dominates the Urbach tail of optical absorption and anti-Stokes PL at room temperature. Anti-Stokes PL is observed down to 70 K. We determine the temperature dependence of the Urbach energy and estimate the strength of the electron-phonon coupling. Our spectroscopic data show that CsPbBr3 nanostructures have potentially useful features for optical cooling.

cond-mat.mtrl-sci↗

arXiv2021

Polaron Masses in CH3NH3PbX3 Perovskites Determined by Landau Level Spectroscopy in Low Magnetic Fields

We investigate the electron-phonon coupling in CH3NH3PbX3 lead halide perovskites through the observation of Landau levels and high-order excitons at weak magnetic fields, where the cyclotron energy is significantly smaller than the longitudinal optical phonon energy. The reduced masses of the carriers and the exciton binding energies obtained from these data are clearly influenced by polaron formation. We analyze the field-dependent polaronic and excitonic properties, and show that they can be quantitatively reproduced by the Frohlich large polaron model.

cond-mat.mtrl-sci↗

arXiv2020

Quantized exciton-exciton annihilation in monolayer WS2 on SrTiO3 substrates with atomically flat terraces

Monolayer materials are strongly affected by their potential fluctuations, which are induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer WS2 on substrates with different surface topographies. We show that monolayer WS2 on the substrate with atomically flat terraces displays small effective EEA rate constant deviating from the overall tendency and multiple exciton decay components, which cannot be accounted for by a conventional EEA model. To obtain a correct description, we use a quantized EEA model. The intrinsic EEA rate constant for the flat-terrace substrates determined by this new model is comparable to that of hBN-encapsulated monolayer WS2.

cond-mat.mtrl-sci↗

arXiv2015

Exciton Lifetime Paradoxically Enhanced by Dissipation and Decoherence - Toward Efficient Energy Conversion of Solar Cell

Energy dissipation and decoherence are at first glance harmful to acquiring long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental as suggested in photosynthesis. By simulating quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of carbon nanotube, the exciton lifetime increases by nearly two orders of magnitude when the crossover triggers stable high population in the dark exciton.

cond-mat.mes-hall↗

arXiv2014

Universal departure from Johnson-Nyquist relation caused by limited resolution

Exploiting the two-point measurement statistics, we propose a quantum measurement scheme of current with limited resolution of electron counting. Our scheme is equivalent to the full counting statistics in the long-time measurement with the ideal resolution, but is theoretically extended to take into account the resolution limit of actual measurement devices. Applying our scheme to a resonant level model, we show that the limited resolution of current measurement gives rise to a positive excess noise, which leads to a deviation from the Johnson-Nyquist relation. The deviation exhibits universal single-parameter scaling with the scaling variable $Q\equiv S_{\rm{}M}/S_0$, which represents the degree of the insufficiency of the resolution. Here, $S_0$ is the intrinsic noise, and $S_{\rm{}M}$ is the positive quantity that has the same dimension as $S_0$ and is defined solely by the measurement scheme. For the lack of the ideal resolution, the deviation emerges for $Q<1$ as $2\exp[-(2π)^2/Q]$ having an essential singularity at $Q=0$, which followed by the square root dependence $\sqrt{Q/4π}$ for $Q\gg1$. Our findings offer an explanation for the anomalous enhancement of noise temperature observed in Johnson noise thermometry.

cond-mat.mes-hall↗

arXiv2011

Interplay of Kondo and superconducting correlations in the nonequilibrium Andreev transport through a quantum dot

Using the modified perturbation theory, we theoretically study the nonequilibrium Andreev transport through a quantum dot coupled to normal and superconducting leads (N-QD-S), which is strongly influenced by the Kondo and superconducting correlations. From the numerical calculation, we find that the renormalized couplings between the leads and the dot in the equilibrium states characterize the peak formation in the nonequilibrium differential conductance. In particular, in the Kondo regime, the enhancement of the Andreev transport via a Kondo resonance occurs in the differential conductance at a finite bias voltage, leading to an anomalous peak whose position is given by the renormalized parameters. In addition to the peak, we show that the energy levels of the Andreev bound states give rise to other peaks in the differential conductance in the strongly correlated N-QD-S system. All these features of the nonequilibrium transport are consistent with those in the recent experimental results [R. S. Deacon {\it et al.}, Phys. Rev. Lett. {\bf 104}, 076805 (2010); Phys. Rev. B {\bf 81}, 12308 (2010)]. We also find that the interplay of the Kondo and superconducting correlations induces an intriguing pinning effect of the Andreev resonances to the Fermi level and its counter position.

cond-mat.mes-hall↗

arXiv2010

Enhanced Andreev Tunneling via the Kondo Resonance in a Quantum Dot at Finite Bias

We study the nonequilibrium transport through a quantum dot coupled to normal and superconducting leads. We use the modified second-order perturbation theory to calculate the differential conductance and the local density of states at the quantum dot. In the strong but finite Coulomb interaction regime, the differential conductance shows an anomalous peak not at a zero bias voltage but at a finite bias voltage. We also observe an additional Kondo resonance besides the normal one in the local density of states, where the former is caused by nonequilibrium Andreev tunneling via the normal Kondo resonance. We explain that this specific Andreev tunneling gives rise to the anomalous peak in the differential conductance. Since the Andreev tunneling via the Kondo resonance is suppressed with increasing temperature, the anomalous peak in the differential conductance disappears at high temperatures.

cond-mat.mes-hall↗