19,739 research outputs found
Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis
We consider the cloning of sequences of qubits prepared in the states used in
the BB84 or 6-state quantum cryptography protocol, and show that the
single-qubit fidelity is unaffected even if entire sequences of qubits are
prepared in the same basis. This result is of great importance for practical
quantum cryptosystems because it reduces the need for high-speed random number
generation without impairing on the security against finite-size attacks.Comment: 8 pages, submitted to PR
Undetermined states: how to find them and their applications
We investigate the undetermined sets consisting of two-level, multi-partite
pure quantum states, whose reduced density matrices give absolutely no
information of their original states. Two approached of finding these quantum
states are proposed. One is to establish the relation between codewords of the
stabilizer quantum error correction codes (SQECCs) and the undetermined states.
The other is to study the local complementation rules of the graph states. As
an application, the undetermined states can be exploited in the quantum secret
sharing scheme. The security is guaranteed by their undetermineness.Comment: 6 pages, no figur
Relativistic quantum coin tossing
A relativistic quantum information exchange protocol is proposed allowing two
distant users to realize ``coin tossing'' procedure. The protocol is based on
the point that in relativistic quantum theory reliable distinguishing between
the two orthogonal states generally requires a finite time depending on the
structure of these states.Comment: 6 pages, no figure
Separately contacted electron-hole double layer in a GaAs/AlxGa1−xAs heterostructure
We describe a method for creating closely spaced parallel two-dimensional electron and hole gases confined in 200 Å GaAs wells separated by a 200 Å wide AlxGa1−xAs barrier. Low-temperature ohmic contacts are made to both the electrons and holes, whose densities are individually adjustable between 10^(10)/cm^2 to greater than 10^(11)/cm^2
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
Tight Finite-Key Analysis for Quantum Cryptography
Despite enormous progress both in theoretical and experimental quantum
cryptography, the security of most current implementations of quantum key
distribution is still not established rigorously. One of the main problems is
that the security of the final key is highly dependent on the number, M, of
signals exchanged between the legitimate parties. While, in any practical
implementation, M is limited by the available resources, existing security
proofs are often only valid asymptotically for unrealistically large values of
M. Here, we demonstrate that this gap between theory and practice can be
overcome using a recently developed proof technique based on the uncertainty
relation for smooth entropies. Specifically, we consider a family of
Bennett-Brassard 1984 quantum key distribution protocols and show that security
against general attacks can be guaranteed already for moderate values of M.Comment: 11 pages, 2 figure
Star polymers: A study of the structural arrest in presence of attractive interactions
Simulations and Mode-Coupling Theory calculations, for a large range of the
arm number and packing fraction have shown that the structural
arrest and the dynamics of star polymers in a good solvent are extremely rich:
the systems show a reentrant melting of the disordered glass nested between two
stable fluid phases that strongly resemble the equilibrium phase diagram.
Starting from a simple model potential we investigate the effect of the
interplay between attractive interactions of different range and ultrasoft core
repulsion, on the dynamics and on the occurrence of the ideal glass transition
line. In the two cases considered so far, we observed some significant
differences with respect to the purely repulsive pair interaction. We also
discuss the interplay between equilibrium and non equilibrium phase behavior.
The accuracy of the theoretical tools we utilized in our investigation has been
checked by comparing the results with molecular dynamics simulations.Comment: 24 pages, 14 figures, accepted for publication in Physical Review
Deterministic secure direct communication using GHZ states and swapping quantum entanglement
We present a deterministic secure direct communication scheme via
entanglement swapping, where a set of ordered maximally entangled
three-particle states (GHZ states), initially shared by three spatially
separated parties, Alice, Bob and Charlie, functions as a quantum information
channel. After ensuring the safety of the quantum channel, Alice and Bob apply
a series local operations on their respective particles according to the
tripartite stipulation and the secret message they both want to send to
Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is
able to infer the secret messages directly. The secret messages are faithfully
transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ
states without revealing any information to a potential eavesdropper. Since
there is not a transmission of the qubits carrying the secret message between
any two of them in the public channel, it is completely secure for direct
secret communication if perfect quantum channel is used.Comment: 9 pages, no figur
- …