Yeong Deok Han

Quantum cryptography without public announcement of bases

Abstract We give 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. Against incoherent attacks by Eve, using the bases sequence repeatedly is proven to be safe by quantum mechanical laws.

Explicit solutions for negative-probability measures for all entangled states

Abstract We obtain explicit solutions for probability measures that reproduce quantum mechanical predictions for some spinmeasurement directions for all entangled states. The necessity of negative probability in this case is shown. This constitutes another proof of Gisins theorem that all entangled states are incompatible with any local hidden-variable models (all entangled states can violate Bells inequality). A degree of freedom that remains in the solution is noted.

The detection loophole in Hardy's nonlocality theorem and minimum detection efficiency

Abstract The detection loophole is discussed in connection with Hardys nonlocality theorem. An inequality that must be satisfied in order that the Hardy nonlocality theorem avoids the detection loophole is derived. The minimum detector efficiency is calculated to be 2 3 . It is shown that the minimum value obtainable theoretically using a version of Bells inequalities is 2 3 .

Classical understanding of electron vortex beams in a uniform magnetic field

Abstract Recently, interesting observations on electron vortex beams have been made. We propose a classical model that shows vortex-like motion due to suitably-synchronized motion of each electrons cyclotron motion in a uniform magnetic field. It is shown that some basic features of electron vortex beams in a uniform magnetic field, such as azimuthal currents, the relation between energy and kinetic angular momentum, and the parallel-axis theorem are understandable by using this classical model. We also show that the time-dependence of kinetic angular momentum of electron vortex beams could be understood as an effect of a specific nonuniform distribution of classical electrons.

Quantum probability assignment limited by relativistic causality.

Quantum theory has nonlocal correlations, which bothered Einstein, but found to satisfy relativistic causality. Correlation for a shared quantum state manifests itself, in the standard quantum framework, by joint probability distributions that can be obtained by applying state reduction and probability assignment that is called Born rule. Quantum correlations, which show nonlocality when the shared state has an entanglement, can be changed if we apply different probability assignment rule. As a result, the amount of nonlocality in quantum correlation will be changed. The issue is whether the change of the rule of quantum probability assignment breaks relativistic causality. We have shown that Born rule on quantum measurement is derived by requiring relativistic causality condition. This shows how the relativistic causality limits the upper bound of quantum nonlocality through quantum probability assignment.