arXiv subjects

Toyohiro Tsurumaru

Publications and source records attributed to Toyohiro Tsurumaru.

24 records · Page 2Linked to original sources

arXiv2006

Sequential Attack with Intensity Modulation on the Differential-Phase-Shift Quantum Key Distribution Protocol

In this paper, we discuss the security of the differential-phase-shift quantum key distribution (DPSQKD) protocol by introducing an improved version of the so-called sequential attack, which was originally discussed by Waks et al. Our attack differs from the original form of the sequential attack in that the attacker Eve modulates not only the phases but also the amplitude in the superposition of the single-photon states which she sends to the receiver. Concentrating especially on the "discretized gaussian" intensity modulation, we show that our attack is more effective than the individual attack, which had been the best attack up to present. As a result of this, the recent experiment with communication distance of 100km reported by Diamanti et al. turns out to be insecure. Moreover it can be shown that in a practical experimental setup which is commonly used today, the communication distance achievable by the DPSQKD protocol is less than 95km.

quant-ph↗

arXiv2006

Group Covariant Protocols for Quantum String Commitment

We study the security of quantum string commitment (QSC) protocols with group covariant encoding scheme. First we consider a class of QSC protocol, which is general enough to incorporate all the QSC protocols given in the preceding literatures. Then among those protocols, we consider group covariant protocols and show that the exact upperbound on the binding condition can be calculated. Next using this result, we prove that for every irreducible representation of a finite group, there always exists a corresponding nontrivial QSC protocol which reaches a level of security impossible to achieve classically.

quant-ph↗

arXiv2005

Implementable Quantum Bit-String Commitment Protocol

Quantum bit-string commitment[A.Kent, Phys.Rev.Lett., 90, 237901 (2003)] or QBSC is a variant of bit commitment (BC). In this paper, we propose a new QBSC protocol that can be implemented using currently available technology, and prove its security under the same security criteria as discussed by Kent. QBSC is a generalization of BC, but has slightly weaker requirements, and our proposed protocol is not intended to break the no-go theorem of quantum BC.

quant-ph↗

arXiv2000

Instantons, Monopoles and the Flux Quantization in the Faddeev-Niemi Decomposition

We study how instantons arise in the low energy effective theory of the SU(2) Yang-Mills theory in the context of the non-linear sigma model recently propose by Faddeev and Niemi. We find a simple relation between the instanton number $ν$ and the charge m of the monopole that appears in the effective theory. It is given by $ν= m Φ/(2π)$, where $Φ$ is the quantized flux associated with a U(1) gauge field passing through the loop formed by the singularity of the monopole.

hep-th↗

arXiv1999

On Topological Terms in the O(3) Nonlinear Sigma Model

Topological terms in the O(3) nonlinear sigma model in (1+1) and (2+1) dimensions are re-examined based on the description of the SU(2)-valued field $g$. We first show that the topological soliton term in (1+1) dimensions arises from the unitary representations of the group characterizing the global structure of the symmetry inherent in the description, in a manner analogous to the appearance of the $θ$-term in Yang-Mills theory in (3+1) dimensions. We then present a detailed argument as to why the conventional Hopf term, which is the topological counterpart in (2+1) dimensions and has been widely used to realize fractional spin and statistics, is ill-defined unless the soliton charge vanishes. We show how this restriction can be lifted by means of a procedure proposed recently, and provide its physical interpretation as well.

hep-th↗

arXiv1999

Localized and Extended States in One-Dimensional Disordered System: Random-Mass Dirac Fermions

System of Dirac fermions with random-varying mass is studied in detail. We reformulate the system by transfer-matrix formalism. Eigenvalues and wave functions are obtained numerically for various configurations of random telegraphic mass m(x). Localized and extended states are identified. For quasi-periodic m(x), low-energy wave functions are also quasi-periodic and extended, though we are not imposing the periodic boundary condition on wave function. On increasing the randomness of the varying mass, states lose periodicity and most of them tend to localize. At the band center or the low-energy limit, there exist extended states which have more than one peak spatially separate with each other comparatively large distance. Numerical calculations of the density of states and ensemble averaged Green's functions are explicitly given. They are in good agreement with analytical calculations by using the supersymmetric methods and exact form of the zero-energy wave functions.

cond-mat.dis-nn↗