Resonance modes and microwave driven translational motion of skyrmion crystal under an inclined magnetic field

We theoretically investigate the microwave-active resonance modes of a skyrmion crystal on a thin-plate specimen under application of an external magnetic field that is inclined from the perpendicular direction to the skyrmion plane. In addition to the well-known breathing mode and two rotation modes, we find novel resonance modes that can be regarded as combinations of the breathing and rotation modes. Motivated by the previous theoretical work of Wang $et$ $al.$ [Phys. Rev. B {\bf 92}, 020403(R) (2015).], which demonstrated skyrmion propagation driven by breathing-mode excitation under an inclined magnetic field, we investigate the propagation of a skyrmion crystal driven by these resonance modes using micromagnetic simulations. We find that the direction and velocity of the propagation vary depending on the excited mode. In addition, it is found that a mode with a dominant counterclockwise-rotation component drives much faster propagation of the skyrmion crystal than the previously studied breathing mode. Our findings enable us to perform efficient manipulation of skyrmions in nanometer-scale devices or in magnetic materials with strong uniaxial magnetic anisotropy such as GaV$_4$S$_4$ and GaV$_4$Se$_4$, using microwave irradiation.Comment: 9+ pages, 9 figures, to be published in Physical Review

Logical Reversibility and Physical Reversibility in Quantum Measurement

A quantum measurement is logically reversible if the premeasurement density operator of the measured system can be calculated from the postmeasurement density operator and from the outcome of the measurement. This paper analyzes why many quantum measurements are logically irreversible, shows how to make them logically reversible, and discusses reversing measurement that returns the postmeasurement state to the premeasurement state by another measurement (physical reversibility). Reversing measurement and unitarily reversible quantum operation are compared from the viewpoint of error correction in quantum computation.Comment: 9pages, LaTex, Invited lecture presented at the Int. Conf. on Frontiers in Quantum Physics (Kuala Lumpur, Malaysia, 9-11 July, 1997) to be published from Springer-Verlag, e-mail: [email protected]

Security analysis of epsilon-almost dual universal2 hash functions: smoothing of min entropy vs. smoothing of R\'enyi entropy of order 2

Recently, $\varepsilon$-almost dual universal$_2$ hash functions has been proposed as a new and wider class of hash functions. Using this class of hash functions, several efficient hash functions were proposed. This paper evaluates the security performance when we apply this kind of hash functions. We evaluate the security in several kinds of setting based on the $L_1$ distinguishability criterion and the modified mutual information criterion. The obtained evaluation is based on smoothing of R\'{e}nyi entropy of order 2 and/or min entropy. We clarify the difference between these two methods.Comment: Several errors are fixed. Introduction and presentation have been improved. This paper consists of a part of the older version of arXiv:1202.0322 and several additional result. This part of the older version of arXiv:1202.0322 has been removed in the current version of arXiv:1202.0322. So, there is no overlap between this paper and the current version of arXiv:1202.032

Consideration of Covariant Quantization of Electromagnetic Field

We examine a covariant quantization of electromagnetic fields by using an operator derived from a constant scalar that can be called extended Lorentz gauge. The quantization can avoid an inconsistency between Lorentz gauge and a commutation relation, which can eliminate the need for introduction of physical state defined by a subsidiary condition and auxiliary field in Lagrangian density in Lorentz gauge. By using this quantization and indefinite metric straightforwardly, all quantum phenomena can be provided without enigmatic and paradoxical "probability interpretation".Comment: 6 pages. Related arXiv articles are arXiv:1709.04103 [physics.gen-ph] and arXiv:1711.09007 [physics.gen-ph] Some sentences and mathematical expressions have been fixe

Controlled creation of nanometric skyrmions using external magnetic fields

To exploit nanometric magnetic skyrmions as information carriers in high-density storage devices, a method is needed that creates intended number of skyrmions at specified places in the device preferably at a low energy cost. We theoretically propose that using a system with a fabricated hole or notch, the controlled creation of individual skyrmions can be achieved even when using an external magnetic field applied to the entire specimen. The fabricated defect turns out to work like a catalyst to reduce the energy barrier for the skyrmion creation.Comment: 5 pages, 4 figure

General non-asymptotic and asymptotic formulas in channel resolvability and identification capacity and their application to wire-tap channel

Several non-asymptotic formulas are established in channel resolvability and identification capacity, and they are applied to wire-tap channel. By using these formulas, the $\epsilon$ capacities of the above three problems are considered in the most general setting, where no structural assumptions such as the stationary memoryless property are made on a channel. As a result, we solve an open problem proposed in Han & Verdu and Han. Moreover, we obtain lower bounds of the exponents of error probability and the wire-tapper's information in wire-tap channel

Quantum estimation and the quantum central limit theorem

This paper gives a survey about quantum estimation. We also describes the relation between the quantum central limit theorem and the asymptotic bound of mean square error in quantum state estimation.Comment: The original Japanese version of this manuscript was published as an introductory article of quantum estimation in Bulletin of Mathematical Society of Japan, Sugaku, Vol. 55, No. 4, 368--391 (2003); Received September 2, 2002, Published October 24, 2003. It was translated to English by Michikazu Sato and Masahito Hayashi. The essential content of this manuscript is the same as that of the original version, while several descriptions and references are improved. The material in this paper was presented in English in part at Joint MaPhySto and QUANTOP Workshop on Quantum Measurements and Quantum Stochastics, Department of Mathematical Sciences, University of Aarhus, Denmark, 2003, Non-locality of Quantum Mechanics and Statistical Inference, Kyoto Sangyo Univ., Kyoto, 2003, and Special Week on Quantum Statistics, Isaac Newton Institute for Mathematical Sciences, Cambridge, UK, 200

Characterization of several kinds of quantum analogues of relative entropy

Quantum relative entropy D(\rho\|\sigma)\defeq\Tr \rho (\log \rho- \log \sigma) plays an important role in quantum information and related fields. However, there are many quantum analogues of relative entropy. In this paper, we characterize these analogues from information geometrical viewpoint. We also consider the naturalness of quantum relative entropy among these analogues

Discrete realization of group symmetric LOCC-detection of maximally entangled state

Group symmetric LOCC measurement for detecting maximally entangled state is considered. Usually, this type measurement has continuous-valued outcomes. However, any realizable measurement has finite-valued outcomes. This paper proposes discrete realizations of such a group symmetric LOCC measurement

Optimal decoy intensity for decoy quantum key distribution

In the decoy quantum key distribution, we show that a smaller decoy intensity gives a better key generation rate in the asymptotic setting when we employ only one decoy intensity and the vacuum pulse. In particular, the counting rate of single photon can be perfectly estimated when the decoy intensity is infinitesimal. The same property holds even when the intensities cannot be perfectly identified. Further, we propose a protocol to improve the key generation rate over the existing protocol under the same decoy intensity