arXiv subjects

Y. Takahashi

Publications and source records attributed to Y. Takahashi.

81 records · Page 5Linked to original sources

arXiv2005

Fast polarimetry system for the application to spin quantum non-demolition measurement

We report the development of a fast pulse polarimeter for the application to quantum non-demolition measurement of atomic spin (Spin QND). The developed system was tunable to the atomic resonance of ytterbium atom and has narrow laser linewidth suitable for spin QND. Using the developed polarimeter, we successfully demonstrated the measurement of the vacuum noise, with 10^6 to 10^7 photon number per pulse.

quant-ph↗

arXiv2005

Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6

We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized their anisotropic behaviour through low field magnetic susceptibility, field-dependent magnetization, resistivity and heat capacity measurements. The Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order anti-ferromagnetically with Neel temperatures above 450 K and the R ions' magnetism appears to behave independently. The possible origins of this unusual behaviour of the ordered Fe moments in this compound are discussed.

cond-mat.str-el↗

arXiv2004

Spin Squeezing via One-Axis Twisting with Coherent Light

We propose a new method of spin squeezing of atomic spin, based on the interactions between atoms and off-resonant light which are known as paramagnetic Faraday rotation and fictitious magnetic field of light. Since the projection process, squeezed light, or special interactions among the atoms are not required in this method, it can be widely applied to many systems. The attainable range of the squeezing parameter is S^{-2/5}, where S is the total spin, which is limited by additional fluctuations imposed by coherent light and the spherical nature of the spin distribution.

quant-ph↗

arXiv2001

Function Dynamics

We show mathematical structure of the function dynamics, i.e., the dynamics of interval maps $f_{n+1} = (1-\e)f_n + \e f_n\circ f_n$ and clarify the types of fixed points, the self-referential structure and the hierarchical structure.

nlin.AO↗

arXiv1999

Non-local bias and the problem of large-scale power in the Standard Cold Dark Matter model

We study the effect of non-radial motions, originating from the gravitational interaction of the quadrupole moment of a protogalaxy with the tidal field of the matter of the neighboring protostructures, on the angular correlation function of galaxies. We calculate the angular correlation function using a Standard Cold Dark Matter (hereafter SCDM) model (Omega=1, h=0.5, n=1) and we compare it with the angular correlation function of the APM galaxy survey (Maddox et al. 1990; Maddox et al. 1996). We find that taking account of non-radial motions in the calculation of the angular correlation function gives a better agreement of the theoretical prediction of the SCDM model to the observed estimates of large-scale power in the galaxy distribution.

astro-ph↗

arXiv1999

Surface Quantum Effects in a Fireball Model of Gamma Ray Bursts

Recent rapid advances in the observations of Gamma Ray Bursts (GRBs) lend much confidence to the cosmological fireball (FB) model: there is now compelling evidence that the radiation is emitted by a relativistic shock where a high speed upstream flow terminates. The question concerning what generates such an outflow is central to our search for the ultimate trigger mechanism. A key requirement, not well explained by current theories, is that the flow must have high entropy-to-baryon ratio. In this Letter we point out that a quantum discharge induced by the radiation of the initial FB may be the explanation. The effect is likely to be relevant, because the FB energy density inferred from GRB data is large enough that the radiation pressure leads to the formation of a surface electric field which is unstable to pair creation. Under suitable conditions, such as those of a supernova core, the discharge can convert a substantial fraction of the FB energy into surface pairs. This pair plasma is not contaminated by FB baryons because it is formed outside the FB. We demonstrate that the pairs can then develop relativistic bulk expansion, reaching a maximum speed that meets the constraints required to form a GRB.

astro-ph↗

arXiv1998

Galilei covariance and (4,1) de Sitter space

A vector space G is introduced such that the Galilei transformations are considered linear mappings in this manifold. The covariant structure of the Galilei Group (Y. Takahashi, Fortschr. Phys. 36 (1988) 63; 36 (1988) 83) is derived and the tensor analysis is developed. It is shown that the Euclidean space is embedded the (4,1) de Sitter space through in G. This is an interesting and useful aspect, in particular, for the analysis carried out for the Lie algebra of the generators of linear transformations in G.

hep-th↗

arXiv1998

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

astro-ph↗

arXiv1998

Gamma Ray Burst as Vacuum Discharge of Super-Schwinger Electric Fields

A theory is proposed to explain with simplicity the basic observed properties of a Gamma Ray Burst (GRB). It employs a well-known result of Schwinger, that static electric fields in excess of a critical value are unstable to pair creation, and catastrophically produces a thermal plasma at temperatures <= 0.5 MeV. By using observational values for the energy and volume of the source, it is shown that the radiation pressure of an expanding GRB `fireball' leads to the formation of a Schwinger critical field at the ambient medium immediately outside the `fireball'. This naturally provides a runaway solution which is inevitable, and which must involve a burst of gamma radiation in the core of the observed energy range and in an optically thin environment. The observed burst duration of 1 -- 10 seconds is also a straightforward consequence of the theory.

astro-ph↗