arXiv subjects

A. Hasegawa

Publications and source records attributed to A. Hasegawa.

12 recordsLinked to original sources

arXiv2004

Theory of multiwave mixing and decoherence control in qubit array system

We develop a theory to analyze the decoherence effect in a charged qubit array system with photon echo signals in the multiwave mixing configuration. We present how the decoherence suppression effect by the {\it bang-bang} control with the $π$ pulses can be demonstrated in laboratory by using a bulk ensemble of exciton qubits and optical pulses whose pulse area is even smaller than $π$. Analysis is made on the time-integated multiwave mixing signals diffracted into certain phase matching directions from a bulk ensemble. Depending on the pulse interval conditions, the cross over from the decoherence acceleration regime to the decoherence suppression regime, which is a peculiar feature of the coherent interaction between a qubit and the reservoir bosons, may be observed in the time-integated multiwave mixing signals in the realistic case including inhomogeneous broadening effect. Our analysis will successfully be applied to precise estimation of the reservoir parameters from experimental data of the direction resolved signal intensities obtained in the multiwave mixing technique.

quant-ph↗

arXiv2003

Nonlinear Rabi Oscillations of Excitons in Dense Quantum Dot System

Nonlinear optical response and optical coherence of excitons in semiconductor dense quantum dots in GaAs single quantum wells has been studied by using photon echo techniques. At low temperatures, the optical coherence is estimated to be 2 ns from the decay curve in the photon echo signal. Rabi oscillations are also observed. Unlike the results for a single quantum dot in the literature to date, the oscillation shows nonlinear behavior affected strongly by the pre-excited carrier density. The results can be explained by the theory of the local field correction originating from exciton-exciton interaction between the inter dots.

cond-mat.mtrl-sci↗

arXiv2003

Experimental demonstration of quantum source coding

We report an experimental demonstration of Schumacher's quantum noiseless coding theorem. Our experiment employs a sequence of single photons each of which represents three qubits. We initially prepare each photon in one of a set of 8 non-orthogonal codeword states corresponding to the value of a block of three binary letters. We use quantum coding to compress this quantum data into a two-qubit quantum channel and then uncompress the two-qubit channel to restore the original data with a fidelity approaching the theoretical limit.

quant-ph↗

arXiv2002

Relativistic Band-Structure Calculations for CeTIn$_5$(T=Ir and Co)

In order to investigate electronic properties of recently discovered heavy fermion superconductors CeTIn$_5$ (T=Ir and Co), we employ the relativistic linear augmented-plane-wave (RLAPW) method to clarify the energy band structures and Fermi surfaces of those materials. The obtained energy bands mainly due to the large hybridization between Ce $4f$ and In 5$p$ states well reproduce the Fermi surfaces consistent with the de Haas-van Alphen experimental results. However, when we attempt to understand magnetism and superconductivity in CeTIn$_5$ from the microscopic viewpoint, the energy bands obtained in the RLAPW method are too complicated to analyze the system by further including electron correlations. Thus, it is necessary to prepare a more simplified model, keeping correctly the essential characters of the energy bands obtained in the band-structure calculation. For the purpose, we construct a tight-binding model for CeTIn$_5$ by including $f$-$f$ and $p$-$p$ hoppings as well as $f$-$p$ hybridization, which are expressed by the Slater-Koster integrals, determined by the direct comparison with the band-calculation result. Similarity and difference between CeIrIn$_5$ and CeCoIn$_5$ are discussed based on the obtained tight-binding model, suggesting a significant importance of the effect of crystalline electric field to understand the difference in electronic properties among CeTIn$_5$.

cond-mat.str-el↗

arXiv2002

Electronic structure and the Fermi surface of UTGa_{5} (T=Fe, Co, Rh)

The relativistic energy-band calculations have been carried out for UFeGa_{5}, UCoGa_{5} and URhGa_{5} under the assumption that 5f-electrons are itinerant. A hybridization between the U 5f state and Ga 4p state occurs in the vicinity of the Fermi level. The Fermi surface of UCoGa_{5} is quite similar to that of URhGa_{5}, which are all small in size and closed in topology. UFeGa_{5} has the quasi-two-dimensional Fermi surface which looks like a lattice structure.

cond-mat.str-el↗

arXiv1999

Effective Vector Meson Masses in a Cutoff Field Theory

Based on quantum hadrodynamics with a finite cutoff, the effective masses of vector mesons(ω, ρ) in nuclear medium are calculated. We use a low-energy effective Lagrangian which is obtained by integrating high-energy quantum fluctuations. Although we use an artificial cutoff, the cutoff-dependence can be removed order by order. It is shown that there is a strong correlation between the effective ω-meson mass and the effective nucleon mass at the normal density. It is also found that the effective ρ-meson mass m_ρ^* decreases as density increases. The rate of the decrease becomes smaller at high density. As a result, at the normal density, the m^*_ρ/m_ρis 0.85 \sim 0.95.

nucl-th↗

arXiv1999

Dispersion relations in quantum hadrodynamics with a finite cutoff

The dispersion relations in the real and imaginary parts of the meson self-energies are studied to check the consistency of the "renormalization" in cutoff field theory. It is shown that the dispersion relations are preserved by the "renormalization"even if the finite cutoff and regulator are introduced in the calculation by hand.

nucl-th↗

arXiv1998

Antiproton Production in p+d Reaction at Subthreshold Energies

An enhancement of antiprotons produced in p+d reaction in comparison with ones in p+p elementary reaction is investigated. In the neighborhood of subthreshold energy the enhancement is caused by the difference of available energies for antiproton production. The cross section in p+d reaction, on the other hand, becomes just twice of the one in elementary p+p reaction at the incident energy far from the threshold energy when non-nucleonic components in deuteron target are not considered.

nucl-th↗

arXiv1996

Properties of nuclear matter in cut-off field theory and renormalization group methods

The properties of nuclear matter are studied in the cut-off field theory. It is found that, under the Hartree approximation, the small cut-off makes the equations of state hard, especially at higher densities. The theory is modified in the framework of the renormalization group methods with arbitrary cut-off $Λ^\prime$. It is found that the expansion in terms of the $σ$ meson field is more favorable than the naive expansion of the inverse of $Λ^\prime$, when we do not use very large $Λ^\prime$.

nucl-th↗

arXiv1995

Volume, Coulomb, and volume-symmetry coefficients of nucleus incompressibility in the relativistic mean field theory with the excluded volume effects

The relation among the volume coefficient $K$(=incompressibility of the nuclear matter), the Coulomb coefficient $K_c$, and the volume-symmetry coefficient $K_{vs}$ of the nucleus incompressibility are studied in the framework of the relativistic mean field theory with the excluded volume effects of the nucleons, under the assumption of the scaling model. It is found that $K= 300\pm 50$MeV is necessary to account for the empirical values of $K$, $K_c$, and $K_{vs}$, simultaneously, as is in the case of the point-like nucleons. The result is independent on the detail descriptions of the potential of the $σ$-meson self-interaction and is almost independent on the excluded volume of the nucleons.

nucl-th↗

arXiv1995

Compressional properties of nuclear matter in the relativistic mean field theory with the excluded volume effects

Compressional properties of nuclear matter are studied by using the mean field theory with the excluded volume effects of the nucleons. It is found that the excluded volume effects make it possible to fit the empirical data of the Coulomb coefficient $K_{c}$ of nucleus incompressibility, even if the volume coefficient $K$ is small($\sim 150$MeV). However, the symmetry properties favor $K=300\pm 50$MeV as in the cases of the mean field theory of point-like nucleons.

nucl-th↗

arXiv1995

Incompressibility of nuclear matter, and Coulomb and volume-symmetry coefficients of nucleus incompressibility in the relativistic mean field theory

The volume coefficient $K$(=incompressibility of the nuclear matter), the Coulomb coefficient $K_c$, and the volume-symmetry coefficient $K_{vs}$ of the nucleus incompressibility are studied in the framework of the relativistic mean field theory, with aid of the scaling model. It is found that $K= 300\pm 50$MeV is necessary to account for the empirical values of $K_v$, $K_c$, and $K_{vs}$, simultaneously. The result is independent on the detail descriptions of the potential of the $σ$-meson self-interaction and is almost independent of the strength of the $ω$-meson self-interaction.

nucl-th↗