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Jaewan Kim

Publications and source records attributed to Jaewan Kim.

88 records · Page 5Linked to original sources
arXiv2004

Entanglement of electron spins in superconductors

We investigate entanglement of two electron spins forming Cooper pairs in an s-wave superconductor. The two-electron space-spin density matrix is obtained from the BCS ground state using a two-particle Green's function. It is demonstrated that a two spin state is not given by a spin singlet state but by a Werner state. It is found that the entanglement length, within which two spins are entangled, is not the order of the coherence length but the order of the Fermi wave length.

quant-ph↗
arXiv2004

Entanglement of electron spins of non-interacting electron gases

We study entanglement of electron spins in many-body systems based on the Green's function approach. As an application we obtain the two-particle density matrix of a non-interacting electron gas and identify its two-spin density matrix as a Werner state. We calculate entanglement measures, a classical correlation, mutual information, and a pair distribution function of two electrons at zero and finite temperatures. We find that changes of entanglement measures are proportional to $T^2$ at low temperatures.

quant-ph↗
arXiv2003

Entanglement swapping secures multiparty quantum communication

Extending the eavesdropping strategy devised by Zhang, Li and Guo [Phys. Rev. A 63, 036301 (2001)], we show that the multiparty quantum communication protocol based on entanglement swapping, which was proposed by Cabello [quant-ph/0009025], is not secure. We modify the protocol so that entanglement swapping can secure multiparty quantum communication, such as multiparty quantum key distribution and quantum secret sharing of classical information, and show that the modified protocol is secure against the Zhang-Li-Guo's strategy for eavesdropping as well as the basic intercept-resend attack.

quant-ph↗
arXiv2003

Convex-roof extended negativity as an entanglement measure for bipartite quantum systems

We extend the concept of the negativity, a good measure of entanglement for bipartite pure states, to mixed states by means of the convex-roof extension. We show that the measure does not increase under local quantum operations and classical communication, and derive explicit formulae for the entanglement measure of isotropic states and Werner states, applying the formalism presented by Vollbrecht and Werner [Phys. Rev. A {\bf 64}, 062307 (2001)].

quant-ph↗
arXiv2003

Quantum teleportation via a W state

We investigate two schemes of the quantum teleportation with a $W$ state, which belongs to a different class from a Greenberger-Horne-Zeilinger class. In the first scheme, the $W$ state is shared by three parties one of whom, called a sender, performs a Bell measurement. It is shown that quantum information of an unknown state is split between two parties and recovered with a certain probability. In the second scheme, a sender takes two particles of the $W$ state and performs positive operator valued measurements in two ways. For two schemes, we calculate the success probability and the average fidelity. We show that the average fidelity of the second scheme cannot exceed that of the first one.

quant-ph↗
arXiv2003

Multimode cat-state entanglement and network teleportation

Schemes for generation and protocols for network teleportation of multimode entangled cat-states are proposed. Explicit expressions for probability of successful teleportation are derived for both symmetric and asymmetric networks.

quant-ph↗
arXiv2003

Quantum Cryptography Using Single Particle Entanglement

A quantum cryptography scheme based on entanglement between a single particle state and a vacuum state is proposed. The scheme utilizes linear optics devices to detect the superposition of the vacuum and single particle states. Existence of an eavesdropper can be detected by using a variant of Bell's inequality.

quant-ph↗
arXiv2002

Qubit geometry and conformal mapping

Identifying the Bolch sphere with the Riemann sphere(the extended complex plane), we obtain relations between single qubit unitary operations and Möbius transformations on the extended complex plane.

quant-ph↗
arXiv2001

Quantum Shift Register

We consider a quantum circuit in which shift and rotation operations on qubits are performed by swap gates and controlled swap gates. These operations can be useful for quantum computers performing elementary arithmetic operations such as multiplication and a bit-wise comparison of qubits.

quant-ph↗
arXiv2000

Quantum Teleportation and Bell's Inequality Using Single-Particle Entanglement

A single-particle entangled state can be generated by illuminating a beam splitter with a single photon. Quantum teleportation utilizing such a single-particle entangled state can be successfully achieved with a simple setup consisting only of linear optical devices such as beam splitters and phase shifters. Application of the locality assumption to a single-particle entangled state leads to Bell's inequality, a violation of which signifies the nonlocal nature of a single particle.

quant-ph↗
arXiv1997

Light-Gravitino Production at Hadron Colliders

We consider the production of gravitinos ($\widetilde G$) in association with gluinos ($\tilde g$) or squarks ($\tilde q$) at hadron colliders, including the three main sub-processes: $q \bar q -> \tilde g\widetilde G$, $qg -> \tilde q\widetilde G$, and $gg -> \tilde g\widetilde G$. These channels become enhanced to the point of being observable for sufficiently light gravitino masses ($m_{\widetilde G} < 10^{-4} eV$), as motivated by some supersymmetric explanations of the CDF $eeγγ+ E_{T,miss}$ event. The characteristic signal of such events would be monojets, as opposed to dijets obtained in the more traditional supersymmetric process $p \bar p -> \tilde g\tilde g$. Searches for such events at the Tevatron can impose lower limits on the gravitino mass. In the Appendix, we provide a complete set of Feynman rules for gravitino interactions used in our calculation.

hep-ph↗
arXiv1994

Stretching Wiggly Strings

How does the amplitude of a wiggle on a string change when the string is stretched? We answer this question for both longitudinal and transverse wiggles and for arbitrary equation of state, {\it i.e.}, for arbitrary relation between the tension $τ$ and the energy per unit length $ε$ of the string. This completes our derivation of the renormalization of string parameters which results from averaging out small scale wiggles on a string. The program is presented here in its entirety.

hep-ph↗
arXiv1992

Wiggly Relativistic Strings

We derive the equations of motion for general strings, i.e. strings with arbitrary relation between tension $τ$ and energy per unit length $ε$. The renormalization of $τ$ and $ε$ that results from averaging out small scale wiggles on the string is obtained in the general case to lowest order in the amount of wiggliness. For Nambu-Goto strings we find deviations from the equation of state $ετ= {\rm constant}$ in higher orders. Finally we argue that wiggliness may radically modify the gauge cosmic string scenario.

hep-ph↗