arXiv subjects

Kazuhiro Yamamoto

Publications and source records attributed to Kazuhiro Yamamoto.

At least 19 recordsLinked to original sources

arXiv2026

First Observation of Multiple Very-Near-Earth Reconnection Events During a Single Storm Main Phase

For the first time, this paper presents three very-near-Earth reconnection (VNERX) events observed within the same 12-hour-long storm main phase. The THEMIS inner probes observed the hallmarks of three episodes of tailward retreating x-lines positioned between magnetic local time (MLT) 23-24 and radial distance 12-13 Earth radii (RE). The events occurred within a thin current sheet, < 1 RE thick. Simultaneously, dispersionless energetic particle injections above 10s of keV and magnetic field dipolarizations were observed near and earthward of geosynchronous altitude by the KOMPSAT and Arase satellites. Arase observed earthward flow bursts at or below geosynchronous altitude via ExB enhancements, suggesting VNERX ejecta proceed below geosynchronous orbit. These observations demonstrate that VNERX events, which predominantly occur during the storm main phase, can be frequent and essential for driving injections that can effectively power the ring current. However, they can be observed only at the pre-midnight sector, close to the neutral sheet.

physics.space-ph↗

arXiv2026

Measurement-Based Estimation of Causal Conditional Variances and Its Application to Macroscopic quantum phenomenon

We analytically investigate a quantum estimation method for a mechanical oscillator in a detuned cavity system based solely on homodyne measurement records, building on the framework developed by C.Meng et al. (Science Advances 8, 7585 (2022)). Estimation based only on measurement records is important because it enables state verification without assuming knowledge of the true system state. We construct a relative estimate operator from causal and anti-causal quantum Wiener filters and calculate its variance. The deviation from the causal conditional variance is defined as a reconstruction bias, whose magnitude is evaluated analytically. We show that, within experimentally relevant parameter regimes for typical quantum-state preparation, the reconstruction bias is sufficiently small to be neglected. As applications to state verification, we apply the method to proposals for macroscopic quantum entanglement mediated by electromagnetic interactions and for conditional momentum-squeezed states generated by homodyne detection, and clarify the conditions under which the bias remains negligible and when the reconstruction bias becomes significant.

quant-ph↗

arXiv2026

Theoretical Study of the Squeezed-Light-Enhanced Sensitivity to Gravity-Induced Entanglement via Finite-Time Analysis

We investigate the advantage of using squeezed input light for generating gravity-induced entanglement (GIE) through Fourier-domain analysis. Based on the findings of Ref.~\cite{Miki2024}, which demonstrated the feasibility of detecting GIE in optomechanical systems under quantum control, we further demonstrate that squeezed input light can reduce the optical noise in the mechanical conditional state and enhance GIE. Furthermore, we estimate the systematic and statistical errors in the measurement of GIE using the Fourier transformation over a finite measurement time. Based on the error estimations using the signal-to-noise ratio (SNR) in GIE detection, we find that a total measurement time of $10^6\,\mathrm{s}$ is required to achieve ${\rm SNR} = 1$ when using squeezed input light, whereas $10^{6.8}\,\mathrm{s}$ is needed without squeezed input light. This result highlights the effectiveness of optomechanical systems and the critical role of squeezed input light in enhancing the detectability of GIE.

quant-ph↗

arXiv2026

Momentum Squeezed State Realized via Optimal Filtering in Optomechanics: Implications for Gravity-Induced Entanglement

We analyze the conditional quantum state of a mechanical mirror in an optomechanical system subject to continuous measurement, feedback control, and quantum filtering. We identify a parameter regime in which the mirror exhibits momentum squeezing beyond the standard quantum limit, achieved through an appropriate choice of the homodyne detection angle. In this regime, we show that optimal filtering effectively realizes a free-particle-like conditional state. When this mechanism is applied to a configuration consisting of two optomechanical systems, the resulting momentum squeezing significantly enhances the signal of gravity-induced entanglement (GIE). This enhancement arises because the momentum squeezing not only amplifies the distinction between the common and differential modes, but also, in the high-purity regime, increases the position uncertainty in accordance with the uncertainty principle, thereby enlarging the spatial extent of the quantum superposition. Our results provide new insights into experimental strategies for probing the quantum nature of gravity using optomechanical platforms.

quant-ph↗

arXiv2026

Kinetic renormalization of auroral turbulence

Driven-dissipative systems often exhibit self-organization in the form of coherent dissipative structures. However, observing such critical states in natural plasmas remains elusive, leading to the traditional view that the fine structure of Earth's auroral ionosphere is shaped by local turbulent flows. Here we report the discovery of a self-organizing regime in Earth's ionosphere. We identify this by modeling the sum of saturation electric fields in the turbulent auroral electrojets as a stochastic variable that renormalizes into noise-enabled transport, via explicitly derived Bohm diffusion. This constitutes an effective field-theory for Farley-Buneman turbulence in the Martin-Siggia-Rose formalism for renormalization group theory, for which we provide strong empirical evidence. Using a composite radar-GPS power spectrum of plasma turbulence, we resolve a scale-invariant cascade that exhibits a characteristic kinetic Alfvén $k^{-8/3}$-signature across four orders of magnitude in $k$. What is more, a large statistical analysis of how the turbulence responds to magnetospheric driving reveals a clear tendency for the observed number density of turbulent waves to scale linearly with driving power, matching the predictions made by our field theory's overdamped equations of motion, which offer closed-form calculations of macroscopic transport relations that are uniquely suitable for sub-grid parameterization in space weather modeling. This establishes geospace storms as opportunities to observe non-equilibrium phase transitions imposing global constraints on collision-dominated systems.

physics.space-ph↗

arXiv2025

Space-based cm/kg-scale Laser Interferometer for Quantum Gravity

The experimental verification of the quantum nature of gravity represents a milestone in quantum gravity research. Recently, interest has grown for testing it via gravitationally induced entanglement (GIE). Here, we propose a space-based interferometer inspired by the LISA Pathfinder (LPF). Our design employs two kg-scale gold-platinum test masses which, unlike in the LPF, are surrounded by a shield below 1 K and positioned side-by-side with a centimeter-scale separation. This configuration enables the detection of GIE through simultaneous measurements of differential and common-mode motions. To estimate the integration time required for GIE detection, we simulate quantum measurements of these modes, considering noise sources such as gas damping, black-body radiation, and cosmic-ray collisions. Our results show that GIE can be demonstrated with a few modifications to the LPF setup.

gr-qc↗

arXiv2025

A Characteristic Signature of Magnetospheric Wave-Particle Interactions Found in the Turbulent E-region

Plasma waves in the magnetosphere scatter electrons, causing them to precipitate into Earth's atmosphere, imparting their temporal characteristics to diffuse auroras. In a case study of conjugate radar and satellite observations, we demonstrate a close and unprecedented association between enhanced electrostatic cyclotron harmonic wave activity in the magnetosphere and the appearance of meter-scale plasma turbulence a few seconds later in the lower ionosphere on nearby magnetic field lines. Such direct structuring of the ionosphere carries implications for our understanding of space weather.

physics.space-ph↗

arXiv2025

Performance evaluation of electron multiplier tubes as a high-intensity muon beam monitor of accelerator neutrino experiments

Upgrade work towards increasing the beam intensity of the neutrino beamline at J- PARC is underway. Monitoring tertiary muon beams is essential for stable operation of the beamline. Accordingly, we plan to replace the present muon monitor sensors with electron multiplier tubes (EMTs). We investigated the radiation tolerance and linearity response of EMTs using a 90 MeV electron beam. An EMTs was irradiated with electrons up to 470 nC. EMTs show higher radiation tolerance than the Si sensors which are presently used as one of the muon monitor detectors for the T2K long-baseline neutrino experiment at J-PARC. The integrated charge yield decrease is found to be less than 8% after a beam irradiation equivalent to 132 days of operation at the future J-PARC beam power of 1.3 MW. The EMTs show linearity better than $\pm$5% up to the future beam intensity. The observed yield decrease is likely due to dynode deterioration based on the detailed investigation. The studies described here confirm that EMTs can be used as a high-intensity muon beam monitor. From the reported results, we are proceeding with the installation in the J-PARC neutrino beamline.

physics.ins-det↗

arXiv2024

Decoherence of spin superposition state caused by a quantum electromagnetic field

In this study, we investigate the decoherence of a spatially superposed electrically neutral spin-$\frac12$ particle in the presence of a relativistic quantum electromagnetic field in Minkowski spacetime. We demonstrate that decoherence due to the spin-magnetic field coupling can be categorized into two distinct factors: local decoherence, originating from the two-point correlation functions along each branch of the superposed trajectories, and nonlocal decoherence, which arises from the correlation functions between the two superposed trajectories. These effects are linked to phase damping and amplitude damping. We also show that if the quantum field is prepared in a thermal state, decoherence monotonically increases with the field temperature.

quant-ph↗

arXiv2024

Variation of Whistler-Mode Wave Characteristics Along Magnetic Field Lines: Comparison of Near-Equatorial THEMIS and Middle-Latitude ERG Observations

The latitudinal distribution of whistler-mode wave intensity plays a crucial role in determining the efficiency and energy of electrons scattered by these waves in the outer radiation belt. Traditionally, this wave property has mostly been derived from statistical measurements of off-equatorial spacecraft, which collect intensity data at various latitudes under different geomagnetic conditions and at different times. In this study we examine a set of events captured by both the near-equatorial THEMIS spacecraft and the off-equatorial ERG spacecraft. Specifically, we compare the whistler-mode wave intensity from THEMIS and ERG measurements at the same MLT and time sectors. Similar wave spectrum characteristics confirm that THEMIS and ERG indeed observed the same wave activity. However, upon closer examination of the wave intensity variations, we identify two distinct categories of events: those that follow the statistically predicted variations in wave intensity along magnetic latitudes, and those that exhibit rapid wave intensity decay away from the equatorial plane. We analyze main characteristics of events from both categories and discuss possible implications of our analysis for radiation belt models.

physics.space-ph↗

arXiv2024

Quantumness of gravitational field: A perspective on monogamy relation

Understanding the phenomenon of quantum superposition of gravitational fields induced by massive quantum particles is an important starting point for quantum gravity. The purpose of this study is to deepen our understanding of the phenomenon of quantum superposition of gravitational fields. To this end, we consider a trade-off relation of entanglement (monogamy relation) in a tripartite system consisting of two massive particles and a gravitational field that may be entangled with each other. Consequently, if two particles cannot exchange information mutually, they are in a separable state, and the particle and gravitational field are always entangled. Furthermore, even when two particles can send information to each other, there is a trade-off between the two particles and the gravitational field. We also investigate the behavior of the quantum superposition of the gravitational field using quantum discord. We find that quantum discord increases depending on the length scale of the particle superposition. Our results may help understand the relationship between the quantization of the gravitational field and the meaning of the quantum superposition of the gravitational field.

quant-ph↗

arXiv2024

Feasible generation of gravity-induced entanglement by using optomechanical systems

We report the feasibility of detecting the gravity-induced entanglement (GIE) with optomechanical systems, which is the first investigation that clarifies the feasible experimental parameters to achieve a signal-to-noise ratio of S/N=1. Our proposal focuses on GIE generation between optomechanical mirrors, coupled via gravitational interactions, under continuous measurement, feedback control, and Kalman filtering process, which matured in connection with the field of gravitational wave observations. We solved the Riccati equation to evaluate the time evolution of the conditional covariance matrix for optomechanical mirrors that estimated the minimum variance of the motions. The results demonstrate that GIE is generated faster than a well-known time scale without optomechanical coupling. The fast generation of entanglement is associated with quantum-state squeezing by the Kalman filtering process, which is an advantage of using optomechanical systems to experimentally detect GIE.

quant-ph↗

arXiv2024

Large violation of Leggett-Garg inequalities with coherent-state projectors for a harmonic oscillator and chiral scalar field

We investigate violations of Leggett-Garg inequalities (LGIs) for a harmonic oscillator and a (1+1)-dimensional chiral scalar field with coherent-state projectors, which is equivalent to a heterodyne-type measurement scheme. For the harmonic oscillator, we found that the vacuum and thermal states violated the LGIs by evaluating the two-time quasi-probability distribution function. In particular, we demonstrate that the value of the two-time quasi-probability reaches -0.123 for a squeezed coherent-state projector, which is equivalent to 98% of the Lüders bound corresponding to the maximal violation of the LGIs. We also find a violation of the LGIs for the local mode of a quantum chiral scalar field by constructing a coherent-state projector similar to the harmonic oscillator case. In contrast to the harmonic oscillator, the periodicity in the time direction of the quasi-probability disappears, which is related to the existence of quantum entanglement between the local mode and its complementary degrees of freedom.

quant-ph↗

arXiv2023

TiO2 doping effect on reflective coating mechanical loss for gravitational wave detection at low temperature

We measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam-sputtered SiO2 and Ta2O5) with and without TiO2 on sapphire disks between 6 and 77 K. The measured loss angle exhibited a temperature dependence, and the local maximum was found at approximately 20 K. This maximum was 7.0*10^(-4) (with TiO2) and 7.7*10^(-4) (without TiO2), although the previous measurement for the coating on sapphire disks showed almost no temperature dependence (Phys. Rev. D 74 022002 (2006)). We evaluated the coating thermal noise in KAGRA and discussed future investigation strategies.

gr-qc↗

arXiv2023

Quantum uncertainty of gravitational field and entanglement in superposed massive particles

Investigating the quantum nature of gravity is an important issue in modern physics. Recently, studies pertaining to the quantum superposition of gravitational potential have garnered significant interest. Inspired by Mari \textit{et al.} [Sci. Rep. {\bf 6} 22777 (2016)] and Baym and Ozawa [Proc. Natl. Acad. Sci. U.S.A. {\bf 106}, 3035 (2009)], Belenchia \textit{et al.} [Phys. Rev. D {\bf 98}, 126009 (2018)] considered a gedanken experiment involving such a quantum superposition and mentioned that the superposition renders causality and complementarity inconsistent. They resolved this inconsistency by considering the quantized dynamical degrees of freedom of gravity. This suggests a strong relationship between the quantum superposition of the gravitational potential and the quantization of the gravitational field. In our previous study [Phys. Rev. D {\bf 106}, 125002 (2022)], we have shown that the quantum uncertainty of a field guarantees the consistency between causality and complementarity. In this study, we focus on the entanglement between two particles' states due to the electromagnetic/gravitational potential and investigate its relationship with quantum uncertainty, causality, and complementarity. Our numerical analyses show that the quantum uncertainty of the electromagnetic/gravitational field results in vacuum fluctuations and prohibits the entanglement between two particles' states when causality is satisfied. We further demonstrate that complementarity holds when the particles do not get entangled. The uncertainty relation does not cause the entanglement between two particles' states, which guarantees complementarity.

quant-ph↗

arXiv2023

Negativity volume of the generalized Wigner function in gravitating hybrid system

We evaluate the gravity-induced negativity volume of the generalized Wigner function in a hybrid system consisting of a particle in a two-localized superposition state and an oscillator. The generalized Wigner function can capture the nonclassicality of the system. The increase in the negativity volume of the generalized Wigner function can be an indicator of the entanglement generation, which is demonstrated in the hybrid system generating the gravity-induced entanglement in various initial states. Moreover, by comparing the behaviors of the negativity volume with the entanglement fidelity, we show that the nonclassical feature of entanglement is properly identified by the criterion based on the negativity volume of the generalized Wigner function when the oscillator is initially in a thermal state.

quant-ph↗

arXiv2023

Quantum state of a suspended mirror coupled to cavity light -- Wiener filter analysis of the pendulum and rotational modes

We investigated the quantum state of an optomechanical suspended mirror under continuous measurement and feedback control using Wiener filtering. We focus on the impact of the two-mode theory of suspended mirror on the quantum state, which is described by the pendulum and rotational modes. It is derived from the beam model coupled to the cavity light in the low-frequency regime, including the internal friction of the beam and the finite size effect of the mirror. We constructed a Wiener filter for the two-mode theory and predicted the quantum state by evaluating the conditional covariance matrix using Wiener filter analysis. The results demonstrate that multimode analysis may play an important role in generating the quantum squeezed state. We also point out the possibility that one-mode analysis can be a good approximation by choosing the range of the Fourier space in the Wiener filter analysis.

quant-ph↗

arXiv2023

Violation of the two-time Leggett-Garg inequalities for a coarse-grained quantum field

We investigate the violation of the Leggett-Garg inequalities for a quantum field, focusing on the two-time quasi-probability distribution function of the dichotomic variable with a coarse-grained scalar field. The Leggett-Garg inequalities are violated depending on the quantum state of the field and the size of coarse-graining. We also demonstrate that the violation of the Leggett-Garg inequalities appears even for the vacuum state and the squeezed state by properly constructing the dichotomic variable and the projection operator.

quant-ph↗