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Koji Nagata

Publications and source records attributed to Koji Nagata.

At least 19 recordsLinked to original sources
arXiv2024

Nearly homogeneous and isotropic turbulence generated by the interaction of supersonic jets

This study reports the development and characterization of a multiple-supersonic-jet wind tunnel designed to investigate the decay of nearly homogeneous and isotropic turbulence in a compressible regime. The interaction of 36 supersonic jets generates turbulence that decays in the streamwise direction. The velocity field is measured with particle image velocimetry by seeding tracer particles with ethanol condensation. Various velocity statistics are evaluated to diagnose decaying turbulence generated by the supersonic jet interaction. The flow is initially inhomogeneous and anisotropic and possesses intermittent large-scale velocity fluctuations. The flow evolves into a statistically homogeneous and isotropic state as the mean velocity profile becomes uniform. In the nearly homogeneous and isotropic region, the ratio of root-mean-squared velocity fluctuations in the streamwise and vertical directions is about 1.08, the longitudinal integral scales are also similar in these directions, and the large-scale intermittency becomes insignificant. The turbulent kinetic energy per unit mass decays according to a power law with an exponent of about 2, larger than those reported for incompressible grid turbulence. The energy spectra in the inertial subrange agree well with other turbulent flows when normalized by the dissipation rate and kinematic viscosity. The non-dimensional dissipation rate is within a range of 0.51--0.87, which is also consistent with incompressible grid turbulence. These results demonstrate that the multiple-supersonic-jet wind tunnel is helpful in the investigation of decaying homogeneous isotropic turbulence whose generation process is strongly influenced by fluid compressibility.

physics.flu-dyn↗
arXiv2020

Nonparametric Regression Quantum Neural Networks

In two pervious papers \cite{dndiep3}, \cite{dndiep4}, the first author constructed the least square quantum neural networks (LS-QNN), and ploynomial interpolation quantum neural networks ( PI-QNN), parametrico-stattistical QNN like: leanr regrassion quantum neural networks (LR-QNN), polynomial regression quantum neural networks (PR-QNN), chi-squared quantum neural netowrks ($χ^2$-QNN). We observed that the method works also in the cases by using nonparametric statistics. In this paper we analyze and implement the nonparametric tests on QNN such as: linear nonparametric regression quantum neural networks (LNR-QNN), polynomial nonparametric regression quantum neural networks (PNR-QNN). The implementation is constructed through the Gauss-Jordan Elimination quantum neural networks (GJE-QNN).The training rule is to use the high probability confidence regions or intervals.

cs.ET↗
arXiv2020

Incompleteness in the Bell Theorem Using Non-contextual Local Realistic Model

Here, we consider the Bell experiment for a system described by multipartite states in the case where n-dichotomic observables are measured per site. If n is two, we consider a two-setting Bell experiment. If n is three, we consider a three-setting Bell experiment. Twosetting model is an explicit local realistic model for the values of a correlation function, given in a two-setting Bell experiment. Three-setting model is an explicit local realistic model for the values of a correlation function, given in a three-setting Bell experiment. In the non-contextual scenario, there is not the difference between three-setting model and two-setting model. And we cannot classify local realistic theories in this case. This says that we can construct three-setting model from two-setting model. Surprisingly we can discuss incompleteness in the Bell theorem using non-contextual model. On the other hand, in the contextual scenario, there is the difference between three-setting model and two-setting model. This says that we must distinguish three-setting model from two-setting model. And we can classify local realistic theories in this case.

physics.gen-ph↗
arXiv2019

Triple decomposition of velocity gradient tensor in homogeneous isotropic turbulence

The triple decomposition of a velocity gradient tensor is studied with direct numerical simulations of homogeneous isotropic turbulence, where the velocity gradient tensor is decomposed into three components representing an irrotational straining motion. Strength of these motions can be quantified with the decomposed components. A procedure of the triple decomposition is proposed for three-dimensional flows, where the decomposition is applied in a basic reference frame identified by examining a finite number of reference frames obtained by three sequential rotational transformations of a Cartesian coordinate. Even though more than one basic reference frame may be available for the triple decomposition, the results of the decomposition depend little on the choice of basic reference frame. In homogeneous isotropic turbulence, regions with strong rigid-body rotations or straining motions are highly intermittent in space, while most flow regions exhibit moderately strong shearing motions in the absence of straining motions and rigid-body rotations. The shear tensor is also used for detecting intense shear layers.

physics.flu-dyn↗
arXiv2008

Realism violates quantum mechanics

Recently, [{arXiv:0810.3134}] is accepted and published. We show that local realistic theories violate separability of $N$-qubit quantum states by a factor of $3^N$. Thus we have a violation factor of 3 when N=1. Realistic theories violate a proposition of a single qubit (a state with two-dimensional space) system, the Bloch sphere machinery of quantum mechanics. Our method uses the ratio of each of the scalar products. The maximal possible value of the scalar product in local realistic theories grows exponentially larger than one in $N$-qubit separable states when the number of qubits increases. We find the violation factor $3^N$ when the measurement setup is entire range of settings for each of the local observers.

quant-ph↗
arXiv2008

There is no axiomatic system for the quantum theory

Recently, [arXiv:0810.3134] is accepted and published. We derive an inequality with two settings as tests for the existence of the Bloch sphere in a spin-1/2 system. The probability theory of measurement outcome within the formalism of von Neumann projective measurement violates the inequality. Namely, we have to give up the existence of the Bloch sphere. Or, we have to give up the probability theory of measurement outcome within the formalism of von Neumann projective measurement. Hence it turns out that there is a contradiction in the Hilbert space formalism of the quantum theory, viz., there is no axiomatic system for the theory.

physics.gen-ph↗
arXiv2008

Unconditional no-hidden-variables theorem

Recently, [{arXiv:0810.3134}] is accepted and published. We present ultimate version of no-hidden-variables theorem. We derive a proposition concerning the quantum theory under the existence of the Bloch sphere in a single spin-1/2 system. The existence of a single classical probability space for measurement outcome within the formalism of von Neumann's projective measurement does not coexist with the proposition concerning the quantum theory. We have to give up the existence of such a classical probability space for measurement outcome in the two-dimensional Hilbert space formalism of the quantum theory. The quantum theory does not accept a hidden-variable interpretation in the two-dimensional space.

quant-ph↗
arXiv2008

Whether quantum mechanics can be almighty even in information science

We discuss that there is a crucial contradiction within quantum mechanics. We derive a proposition concerning a quantum expectation value under the assumption of the existence of the directions in a spin-1/2 system. The quantum predictions within the formalism of von Neumann's projective measurement cannot coexist with the proposition concerning the existence of the directions. Therefore, we have to give up either the existence of the directions or the formalism of von Neumann's projective measurement. Hence there is a crucial contradiction within the Hilbert space formalism of the quantum theory. This implies that there is no axiomatic system for the quantum theory. This also reveals that we need new physical theories in order to explain the handing of raw experimental data. We discuss that this crucial contradiction makes the quantum-theoretical formulation of Deutsch's algorithm questionable.

physics.gen-ph↗
arXiv2008

Violation of time symmetry

Recently, [{arXiv:0810.3134}] is accepted and published. We would like to study the relation between a local realistic theory and commutativity of observables in a finite-dimensional space. We would like to conjecture that a realistic theory of the Bell type (a local realistic theory) for events would not imply the commutative algebraic structure into the set of all observables if all experimental events would be reproduced by a local realistic theory. We would like to suggest that a violation of Bell locality would be derived within a realistic theory of the Kochen-Specker type within quantum events.

quant-ph↗
arXiv2008

Nonlocality improves Deutsch algorithm

Recently, [{arXiv:0810.3134}] is accepted and published. We show that the Bell inequalities lead to a new type of linear-optical Deutsch algorithms. We have considered a use of entangled photon pairs to determine simultaneously and probabilistically two unknown functions. The usual Deutsch algorithm determines one unknown function and exhibits a two to one speed up in a certain computation on a quantum computer rather than on a classical computer. We found that the violation of Bell locality in the Hilbert space formalism of quantum theory predicts that the proposed {\it probabilistic} Deutsch algorithm for computing two unknown functions exhibits at least a $2\sqrt{2}(\simeq 2.83)$ to one speed up.

quant-ph↗
arXiv2008

Violation of Rotational Invariance of Local Realistic Models with Two Settings

We have considered a two-particle Bell experiment to visualize the conflict between rotational invariance of physical laws and a specific local realistic theory. The experiment is reproducible by using a local realistic theory obtained in a two-setting Bell experiment. The generalized Bell inequality {[J. Phys. A: Math. Theor. {\bf 40}, 13101 (2007)]}, which is derived under the assumption that there exists a rotationally invariant local realistic theory, turns out to disprove such a local realistic model existing with a two-setting experiment. This implies that such a model is not rotationally invariant and should, therefore, be ruled out in some situations.

quant-ph↗
arXiv2008

New Method to Reveal the Conflict Between Local Realism and Quantum Mechanics

We formulate the expectation value of the Bell-Zukowski operator acting on qubit states of a two-particle Bell experiment. By using the equivalence between a set of N copies of a two-qubit experiment and a standard two-setting Bell experiment in an entangled 2N-particle state, we obtain an inequality, which we may call the Bell-Zukowski inequality. It determines whether the measured correlation functions of two-particle states can be modeled locally and realistically. In this Bell experiment of two particles, the conflict between local realism and quantum mechanics is discussed in conjunction with the violation of the Bell-Zukowski inequality. The main point of the result is that the Bell-Zukowski operator can be represented by the Bell-Mermin operator. The threshold visibility of two-particle interference analyzed in this scheme shows good agreement with the value to cause a violation of the Bell-Zukowski inequality.

quant-ph↗
arXiv2008

Comment on ``All quantum observables in a hidden-variable model must commute simultaneously"

Malley discussed {[Phys. Rev. A {\bf 69}, 022118 (2004)]} that all quantum observables in a hidden-variable model for quantum events must commute simultaneously. In this comment, we discuss that Malley's theorem is indeed valid for the hidden-variable theoretical assumptions, which were introduced by Kochen and Specker. However, we give an example that the local hidden-variable (LHV) model for quantum events preserves noncommutativity of quantum observables. It turns out that Malley's theorem is not related with the LHV model for quantum events, in general.

quant-ph↗
arXiv2008

Multi-setting Bell inequality for qudits

We propose a generalized Bell inequality for two three-dimensional systems with three settings in each local measurement. It is shown that this inequality is maximally violated if local measurements are configured to be mutually unbiased and a composite state is maximally entangled. This feature is similar to Clauser-Horne-Shimony-Holt inequality for two qubits but is in contrast with the two types of inequalities, Collins-Gisin-Linden-Massar-Popescu and Son-Lee-Kim, for high-dimensional systems. The generalization to aribitrary prime-dimensional systems is discussed.

quant-ph↗
arXiv2008

The Conflict between Bell-Zukowski Inequality and Bell-Mermin Inequality

We consider a two-particle/two-setting Bell experiment to visualize the conflict between Bell-Żukowski inequality and Bell-Mermin inequality. The experiment is reproducible by local realistic theories which are not rotationally invariant. We found that the average value of the Bell-Żukowski operator can be evaluated only by the two-particle/two-setting Bell experiment in question. The Bell-Żukowski inequality reveals that the constructed local realistic models for the experiment are not rotationally invariant. That is, the two-particle Bell experiment in question reveals the conflict between Bell-Żukowski inequality and Bell-Mermin inequality. Our analysis has found the threshold visibility for the two-particle interference to reveal the conflict noted above. It is found that the threshold visibility agrees with the value to obtain a violation of the Bell-Żukowski inequality.

quant-ph↗
arXiv2008

Additional information decreases the estimated entanglement using the Jaynes principle

We study a particular example considered in {[Phys. Rev. A {\bf 59,} 1799 (1999)]}, concerning the statistical inference of quantum entanglement using the Jaynes principle. Assume a Clauser-Horne-Simony-Holt (CHSH) Bell operator, a sum of two operators $\sqrt{2}(X+Z)$. Given only an average of the Bell-CHSH operator, we may overestimate entanglement. However, the estimated entanglement is decreased (never increases) when we use the expectation value of the operator $X$ as additional information. A minimum entanglement state is obtained by minimizing the variance of the observable $X$.

quant-ph↗