Searcharxiv⌕ Search

arXiv subjects

Ichiro Ohba

Publications and source records attributed to Ichiro Ohba.

10 recordsLinked to original sources
arXiv2009

Entanglement of Electrons Field-Emitted from a Superconductor

Under appropriate circumstances the electrons emitted from a superconducting tip can be entangled. We analyze these nonlocal correlations by studying the coincidences of the field-emitted electrons and show that electrons emitted in opposite directions violate Bell's inequality. We scrutinize the interplay between the bosonic nature of Cooper pairs and the fermionic nature of electrons. We further discuss the feasibility of our analysis in the light of present experimental capabilities.

quant-ph↗
arXiv2007

Decrease of entanglement by local operations in the Dür-Cirac method

One cannot always obtain information about entanglement by the Dür-Cirac (DC) method. The impracticality is attributed to the decrease of entanglement by local operations in the DC method. We show that, even in 2--qubit systems, there exist states whose entangled property the DC method never evaluates. The class of such states in 2--qubit systems is completely characterized by the value of the fully entangled fraction. Actually, a state whose fully entangled fraction is less than or equal to 1/2 is always transformed into a separable state by local operations in the DC method, even if it has negative partial transposition.

quant-ph↗
arXiv2007

Attainable entanglement of unitary transformed thermal states in liquid-state nuclear magnetic resonance with the chemical shift

Recently, Yu, Brown, and Chuang [Phys. Rev. A {\bf 71}, 032341 (2005)] investigated the entanglement attainable from unitary transformed thermal states in liquid-state nuclear magnetic resonance (NMR). Their research gave an insight into the role of the entanglement in a liquid-state NMR quantum computer. Moreover, they attempted to reveal the role of mixed-state entanglement in quantum computing. However, they assumed that the Zeeman energy of each nuclear spin which corresponds to a qubit takes a common value for all; there is no chemical shift. In this paper, we research a model with the chemical shifts and analytically derive the physical parameter region where unitary transformed thermal states are entangled, by the positive partial transposition (PPT) criterion with respect to any bipartition. We examine the effect of the chemical shifts on the boundary between the separability and the nonseparability, and find it is negligible.

quant-ph↗
arXiv2006

Potential Errors in a Scheme of Universal Quantum Gates in Kane's Model

We re--investigate a plausible proposal for universal quantum gates in Kane's model, in which the authors assumed that electron spin is always downward under a background magnetic field and the value of controlling parameters is varied instantaneously. We demonstrate that a considerable error appears, for example, in the X rotation. As result, the controlled operations don't work. Such a failure is caused by improper choice of the computational bases; actually, the electron spin is not always downward over time during quantum operations.

quant-ph↗
arXiv2003

Modified Fowler-Nordheim Field-Emission Formulae from a Nonplanar-Emitter Model

Field emission formulae, current-voltage characteristics and energy distribution of emitted electrons, are derived analytically for a nonplanar (hyperboloidal) metallic emitter model. The traditional Fowler-Nordheim formulae, which are derived from a planar emitter model, are modified, and the assumption of the planar emitter in the F-N model is reconsidered. Our analytical calculation also reveals the backgrounds of the previous numerical discussion by He et al. on the effect of the geometry of emitter on field emission. The new formulae contain a parameter which characterizes the sharpness of the hyperboloidal emitter, and experimental data of field emissions from clean tungsten emitters and nanotip emitters are analyzed by making use of this feature.

cond-mat.mtrl-sci↗
arXiv2003

Crossover from classical to quantum behavior of Duffing oscillator through "pseudo-Lyapunov exponent"

We discuss the quantum--classical correspondence in a specific dissipative chaotic system, Duffing oscillator. We quantize it on the basis of quantum state diffusion (QSD) which is a certain formulation for open quantum systems and an effective tool for analyzing complex problems numerically. We consider a sensitivity to initial conditions, `` pseudo-Lyapunov exponent '', and investigate it in detail, varying Planck constant effectively. We show that in a dissipative system there exists a certain critical stage in which the crossover from classical to quantum behavior occurs. Furthermore, we show that an effect of dissipation suppresses the occurrence of chaos in the quantum region, while it, combined with the periodic external force, plays a crucial role in the chaotic behaviors of classical system.

quant-ph↗
arXiv2002

Estimation of Buttiker-Landauer traversal time based on the visibility of transmission current

We present a proposal for the estimation of Büttiker-Landauer traversal time based on the visibility of transmission current. We analyze the tunneling phenomena with a time-dependent potential and obtain the time-dependent transmission current. We found that the visibility is directly connected to the traversal time. Furthermore, this result is valid not only for rectangular potential barrier but also for general form of potential to which the WKB approximation is applicable . We compared these results with the numerical values obtained from the simulation of Nelson's quantum mechanics. Both of them fit together and it shows our method is very effective to measure experimentally the traversal time.

quant-ph↗
arXiv1999

Quantum stochastic resonance in driven spin-boson system with stochastic limit approximation

After a brief review of stochastic limit approximation with spin-boson system from physical points of view, amplification phenomenon-stochastic resonance phenomenon-in driven spin-boson system is observed which is helped by the quantum white noise introduced through the stochastic limit approximation. The shift in frequency of the system due to the interaction with the environment-Lamb shift-has an important role in these phenomena.

quant-ph↗
arXiv1996

Effects of Inelastic Scattering on Tunneling Time in Generalized Nelson's Quantum Mechanics

We analyze the effects of inelastic scattering on the tunneling time theoretically, using generalized Nelson's quantum mechanics. This generalization enables us to describe quantum system with optical potential and channel couplings in a real time stochastic approach, which seems to give us a new insight into quantum mechanics beyond Copenhagen interpretation.

quant-ph↗