arXiv subjects

Yeong Deok Han

Publications and source records attributed to Yeong Deok Han.

13 recordsLinked to original sources

arXiv2025

Probability Density in Relativistic Quantum Mechanics

In the realm of relativistic quantum mechanics, we address a fundamental question: Which one, between the Dirac or the Foldy-Wouthuysen density, accurately provide a probability density for finding a massive particle with spin $1/2$ at a certain position and time. Recently, concerns about the Dirac density's validity have arisen due to the Zitterbewegung phenomenon, characterized by a peculiar fast-oscillating solution of the coordinate operator that disrupts the classical relation among velocity, momentum, and energy. To explore this, we applied Newton and Wigner's method to define proper position operators and their eigenstates in both representations, identifying 'localized states' orthogonal to their spatially displaced counterparts. Our analysis shows that both densities could represent the probability of locating a particle within a few Compton wavelengths. However, a critical analysis of Lorentz transformation properties reveals that only the Dirac density meets all essential physical criteria for a relativistic probability density. These criteria include covariance of the position eigenstate, adherence to a continuity equation, and Lorentz invariance of the probability of finding a particle. Our results provide a clear and consistent interpretation of the probability density for a massive spin-$1/2$ particle in relativistic quantum mechanics.

quant-ph↗

arXiv2023

Lorentz-Covariant Spin Operator for Spin 1/2 Massive Fields As a Physical Observable

We derive a relativistic-covariant spin operator for massive case directly from space-time symmetry in Minkowski space-time and investigate the physical properties of a derived spin operator. In the derivation we require only two conditions: First, a spin operator should be the generator of the SU(2) little group of the Poincare group. Second, a spin operator should covariantly transform under the Lorentz transformation. A space inversion transformation is shown to play a role to derive a unique relativistic-covariant spin operator, we call the field spin operator, whose eigenvalue labels the spin of a massive (classical) field that provides the irreducible representation space of the Poincare group. The field spin becomes the covariant spin in the covariant Dirac representation, which is shown to be the only spin that describes the Wigner rotation properly in the covariant Dirac representation. Surprisingly, the field spin also gives the non-covariant spin, which is the FW spin for the positive energy state. We also show that the field spin operator is the unique spin operator that generate the (internal) SU(2) little group transformation of the Poincare group properly.

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arXiv2019

Singularity of relativistic vortex beam and proper relativistic observables

We have studied the phase singularity of the relativistic vortex beams for the two sets of relativistic operators. One includes the new spin and orbital angular momentum (OAM) operators, which is derived from the parity-extended Poincaré group, and the other is composed of the (usual) Dirac spin and OAM operators. The first set predicts the same singular circulation as the nonrelativistic vortex beams. On the other hand, the second set anticipates that the singularity of the circulation is spin orientation-dependent and can be disappeared especially for relativistic paraxial electron beam with spin parallel to the propagating direction. These contradistinctive predictions suggest the relativistic electron beam experiment with spin-polarized electrons for the first time to answer the long-standing fundamental question, i.e., what are the proper relativistic observables, raised from the beginning of relativistic quantum mechanics since the discovery of the Dirac equation.

quant-ph↗

arXiv2016

Classical understanding of the electron vortex beams in a uniform magnetic field

Recently interesting observations on electron vortex beams, which have angular momentum about the center of the vortex beams, have been made. We have shown that the basic features of the electron vortex beams in a uniform magnetic field are understandable by using the classical motions of electrons. We have constructed a classical vortex-like motion by the collective motion of individual electrons in their cyclotron motions with a constant canonical angular momentum in the symmetric gauge, which models electron vortex beams, in a uniform magnetic field. With this model the various properties of circulating currents and the relation between energy and kinetic angular momentum in the electron vortex beams are well explained. We have also shown that the mismatch between the centers of the electron vortex beam and the classical cyclotron orbits naturally induces the parallel axis theorem and also the time-varying kinetic angular momentum of the electron vortex beam for certain distributions of classical electrons.

quant-ph↗

arXiv2015

Quantum Probability assignment limited by relativistic causality

The quantum nonlocality is limited by relativistic causality, however, the reason is not fully understood yet. The relativistic causality condition on nonlocal correlations has been usually accepted as a prohibition of faster-than-light signaling, called no-signaling condition. We propose another causality condition from the observation that space-like separate events should have no causal relationship. It is proved that the new condition is stronger than no-signaling condition for a pair of binary devices. We derive the standard probability assignment rule, so-called Born rule, on quantum measurement, which determines the degree of quantum nonlocality, by using relativistic causality constraint. This shows how the causality limits the upper bound of quantum nonlocality through quantum probability assignment.

quant-ph↗

arXiv2013

Relativistic Spin and Dirac Spin in relativistically covariant Stern-Gerlach Experiment

We have studied a relativistically covariant Stern-Gerlach (SG) experiment for a relativistic spin and a Dirac spin. We have obtained the relativistic spin in an arbitrary frame by using the classical spin dipole tensor, which gives the covariant spin dipole interactions, and the relation between a spin and a spin magnetic dipole moment. The relativistic spin is shown to have problems to become a proper spin operator for a massive relativistic particle because of two reasons. First, the relativistic spin three-vector operators cannot satisfy the spin algebra. Second, the SG experiment for the relativistic particle provides a paradox between two observers in the particle rest frame and the laboratory frame, in which the particle is moving. We have shown that the paradox in the SG experiment is resolved by the Dirac spin, which is covariantly defined by a Lorentz transformation in the Dirac spinor representation. The Dirac spin three-vector operators satisfy the spin algebra. It is shown that the SG experiment for the Dirac spin in the inertial frame, where there is only magnetic field, can determine the spin without the information of the momentum of the particle. This shows that the reduced spin density matrix for the Dirac particle can be well-defined by integrating out the momentum degrees of freedom.

quant-ph↗

arXiv2012

Limits on Quantum Probability Rule by no-Signaling Principle

We have studied the possibility of post-quantum theories more nonlocal than the (standard) quantum theory using the modification of the quantum probability rule under the no-signaling condition. For this purpose we have considered the situation that two spacelike separate parties Alice and Bob share an entangled two qubit system. We have modified the quantum probability rule as small as possible such that the first local measurements are governed by the usual Born rule and the second measurement by the modified quantum probability rule. We have shown that only the maximally entangled states can have higher nonlocality than the quantum upper bound while satisfying the no-signaling condition. This fact could be a partial explanation for why the nonlocality of the quantum theory is limited. As a by-product we have found the systematic way to obtain a variety of nonlocal boxes.

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arXiv2002

Stochastic Processes and the Dirac Equation with External Fields

The equation describing the stochastic motion of a classical particle in 1+1-dimensional space-time is connected to the Dirac equation with external gauge fields. The effects of assigning different turning probabilities to the forward and the backward moving particles in time are discussed.

hep-th↗

arXiv2001

Composite Operators and Topological Contributions in Gauge Theory

In $D$-dimensional gauge theory with a kinetic term based on the p-form tensor gauge field, we introduce a gauge invariant operator associated with the composite formed from a electric $(p-1)$-brane and a magnetic $(q-1)$-brane in $D=p+q+1$ spacetime dimensions. By evaluating the partition function for this operator, we show that the expectation value of this operator gives rise to the topological contributions identical to those in gauge theory with a topological Chern-Simons BF term.

hep-th↗

arXiv1997

Quantum Coin Tossing

We show that a secure quantum protocol for coin tossing exist. The existence of quantum coin tossing support the conjecture of D.Mayers [Phys.Rev.Lett. 78, 3414(1997)] that only asymmetrical tasks as quantum bit commitment are impossible.

quant-ph↗

arXiv1997

Quantum Cryptography without Public Announcement of Bases

This paper provides a simple variation of the basic ideas of the BB84 quantum cryptographic scheme leading to a method of key expansion. A secure random sequence (the bases sequence) determines the encoding bases in a proposed scheme. Using the bases sequence repeatedly is proven to be safe by quantum mechanical laws.

quant-ph↗

arXiv1997

The quantum cryptography and the unambiguous measurement of quantum nonorthogonal states

We discuss the unambiguous measurement of quantum nonorthogonal states in connection with the quantum cryptography. We show that checking a ratio of null one to signal is essential in detecting a certain kind of eavesdropping in the case of two nonorthogonal states quantum cryptography. We prove that it is not needed in the case of the four states quantum cryptography.

quant-ph↗

arXiv1997

A note on the supplementary variables in spin-measuring equipments in the EPR-Bell experiment

We discuss supplementary (or hidden) variables in spin-measuring equipments in EPR-Bell experiment. This theme was considered in a Bell's later work. We generalize it. First, we show why the original supplementary variable $λ$ is not to be regarded to include supplementary variables in spin-measuring equipments (why supplementary variables should be introduced additionally in spin-measuring equipments) Next, we show the followings. When the supplementary variables introduced in spin-measuring equipments have local correlations, the Bell inequality is recovered. On the other hand, when they have nonlocal correlations, the Bell inequality is not recovered. This fact is in accord with the fact that the Bell inequality is derived for local realistic models.

quant-ph↗