共查询到20条相似文献,搜索用时 15 毫秒
1.
Guo-Ping Guo Xiao-Jie Hao Tao Tu Zhi-Cheng Zhu Guang-Can Guo 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):141-146
We propose a scheme to eliminate the effect of non-nearest-neighbor
qubits in preparing cluster state with double-dot molecules. As the
interaction Hamiltonians between qubits are Ising-model and mutually
commute, we can get positive and negative effective interactions
between qubits to cancel the effect of non-nearest-neighbor qubits
by properly changing the electron charge states of each quantum dot
molecule. The total time for the present multi-step cluster state
preparation scheme is only doubled for one-dimensional qubit chain
and tripled for two-dimensional qubit array comparing with the time
of previous protocol leaving out the non-nearest-neighbor
interactions. 相似文献
2.
G. Lagmago Kamta A. Y. Istomin A. F. Starace 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,44(2):389-400
We study systematically the entanglement of a two-qubit
Heisenberg XY model in thermal equilibrium in the presence of an
external arbitrarily-directed static magnetic field, thereby
generalizing our prior work [G. Lagmago Kamta, A.F. Starace, Phys.
Rev. Lett. 88, 107901 (2002)]. We show that a magnetic field
having a component in the xy-plane containing the spin-spin
interaction components produces different entanglement for
ferromagnetic (FM) and antiferromagnetic (AFM) couplings. In
particular, quantum phase transitions induced by the magnetic
field-driven level crossings always occur for the AFM-coupled
qubits, but only occur in FM-coupled qubits when the coupling is of
Ising type or when the magnetic field has a component perpendicular
to the xy-plane. When the magnetic field has a component in the
xy-plane, the cut-off temperature above which the entanglement of
both the FM- and AFM-coupled qubits vanishes can always be
controlled using the magnetic field for any value of the XY coupling
anisotropy parameter. Thus, by adjusting the magnetic field, an
entangled state of two spins can be produced at any finite
temperature. Finally, we find that a higher level of entanglement is
achieved when the in-plane component of the magnetic field is
parallel to the direction in which the XY exchange coupling is
smaller. 相似文献
3.
A. Fubini T. Roscilde V. Tognetti M. Tusa P. Verrucchi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,38(3):563-570
We consider a quantum many-body system made of N interacting
S=1/2 spins on a lattice, and develop a formalism which allows to
extract, out of conventional magnetic observables, the quantum
probabilities for any selected spin pair to be in maximally entangled
or factorized two-spin states. This result is used in order to
capture the meaning of entanglement properties in terms of magnetic
behavior. In particular, we consider the concurrence between two
spins and show how its expression extracts information on the presence
of bipartite entanglement out of the probability distributions
relative to specific sets of two-spin quantum states. We apply the
above findings to the antiferromagnetic Heisenberg model in a uniform
magnetic field, both on a chain and on a two-leg ladder. Using Quantum
Monte Carlo simulations, we obtain the above probability distributions
and the associated entanglement, discussing their evolution under
application of the field. 相似文献
4.
A. Ramšak J. Mravlje 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):419-425
We investigate entanglement between electrons in
serially coupled double quantum dots attached to noninteracting
leads. In addition to local repulsion we consider the influence of
capacitive inter-dot interaction. We show how the competition
between extended Kondo and local singlet phases determines the
ground state and thereby the entanglement. The results are
additionally discussed in connection with the linear conductance
through the system. 相似文献
5.
A. Rycerz 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):291-296
We study quantum entanglement in a single-level quantum dot in the
linear-response regime.
The results show, that the maximal quantum value of the conductance
2e2/h not always match the maximal entanglement.
The pairwise entanglement between the quantum dot and the nearest atom of the
lead is also analyzed by utilizing the Wootters formula
for charge and spin degrees of freedom separately.
The coexistence of zero concurrence and the maximal conductance is observed
for low values of the dot-lead hybridization.
Moreover, the pairwise concurrence vanish simultaneously for charge and
spin degrees of freedom, when the Kondo resonance is present in the system.
The values of a Kondo temperature, corresponding to the zero-concurrence
boundary, are also provided. 相似文献
6.
I. M. Herbauts S. Bettelli H. Hübel M. Peev 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,46(2):395-406
Some MIT researchers [Phys. Rev. A
75, 042327 (2007)] have recently claimed that
their implementation of the Slutsky-Brandt attack [Phys. Rev. A
57, 2383 (1998); Phys. Rev. A 71, 042312 (2005)] to
the BB84 quantum-key-distribution (QKD) protocol puts the security of this
protocol “to the test” by simulating “the most powerful
individual-photon attack” [Phys. Rev. A 73, 012315
(2006)].
A related unfortunate news feature by a
scientific journal [G. Brumfiel, Quantum cryptography is hacked, News @ Nature (april 2007); Nature 447, 372 (2007)]
has spurred some concern in the QKD community and among the general public
by misinterpreting the implications of this work. The present article
proves the existence of a stronger individual attack on QKD protocols with
encrypted error correction, for which tight bounds are shown, and clarifies
why the claims of the news feature incorrectly suggest a contradiction with
the established “old-style” theory of BB84 individual attacks.
The full implementation of a quantum cryptographic protocol includes a
reconciliation and a privacy-amplification stage, whose choice alters in
general both the maximum extractable secret and the optimal eavesdropping
attack. The authors of [Phys. Rev. A
75, 042327 (2007)] are concerned only with the error-free
part of the so-called sifted string, and do not consider faulty bits,
which, in the version of their protocol, are discarded. When using the
provably superior reconciliation approach of encrypted error correction
(instead of error discard), the Slutsky-Brandt attack is no more optimal
and does not “threaten” the security bound derived by Lütkenhaus [Phys. Rev. A 59, 3301 (1999)].
It is shown that the method of Slutsky and collaborators [Phys. Rev. A
57, 2383 (1998)] can
be adapted to reconciliation with error correction, and that the optimal
entangling probe can be explicitly found. Moreover, this attack fills
Lütkenhaus bound, proving that it is tight (a fact which was not
previously known). 相似文献
7.
C.-S. Yu H.-S. Song 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,42(1):147-151
In this paper, an intuitive approach is employed to
generalize the full separability criterion of tripartite quantum
states of qubits to the higher-dimensional systems [Phys. Rev. A
72, 022333 (2005)]. A distinct characteristic of the
present generalization is that less restrictive conditions are
needed to characterize the properties of full separability.
Furthermore, the formulation for pure states can be conveniently
extended to the case of mixed states by utilizing the Kronecker
product approximate technique. As applications, we give the analytic
approximation of the criterion for weakly mixed tripartite quantum
states and investigate the full separability of some weakly mixed
states. 相似文献
8.
A. Khalique G. M. Nikolopoulos G. Alber 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,40(3):453-464
The influence of imperfections on
achievable secret-key generation rates of quantum key distribution protocols
is investigated. As examples of relevant imperfections, we consider
tagging of Alice's qubits and dark counts at Bob's detectors, while
we focus on a powerful eavesdropping strategy which takes full advantage of
tagged signals.
It is demonstrated that error correction and privacy amplification based on
a combination of a two-way classical communication protocol and
asymmetric Calderbank-Shor-Steane codes may significantly postpone the
disastrous influence of dark counts. As a result, the
distances are increased considerably
over which a secret key can be distributed in optical fibres reliably.
Results are presented for the four-state, the six-state, and
the decoy-state protocols. 相似文献
9.
The entanglement-assisted capacity of a generalized amplitude damping channel is investigated by using the properties of partial symmetry and concavity of mutual information. The numerical and analytical results of the entanglement-assisted capacity are obtained under certain conditions. It is shown that the entanglement-assisted capacity depends on the channel parameters representing the ambient temperature and dissipation, and the prior entanglement between sender and receiver can approximately double the classical capacity of the generalized amplitude damping channel. 相似文献
10.
We address quantum state reconstruction for d-dimensional
systems based on measuring, on the system of interest and a
probe, of a single entangled observable defined on the bipartite
system/probe Hilbert space. We show that the statistics of the
measurement and the knowledge of the probe preparation suffice to
reliably reconstruct the density matrix of the system, as well
as the expectation value of any desired operator, including those
not corresponding to observable quantities. The statistical
robustness of the reconstruction is examined and a method is
developed to minimize statistical errors by tuning the probe
preparation. Numerical simulations of the whole reconstruction
procedure are also presented for qubit systems. 相似文献
11.
J. Bouda M. Koniorczyk A. Varga 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,53(3):365-372
Channels encrypting quantum bits by the application of randomly
chosen unitary operators are studied. Quantities based on averages of
linear entropies which characterize certain aspects
of the encoding quality and the non-malleability of the
channels are introduced. The relation between the entropy of the classical key and
the choice of the encryption operators with the behaviour of these
properties is discussed. The extension of exact private quantum
channels in order to improve non-malleability via additional
encryption operators is considered. 相似文献
12.
F. Casagrande A. Lulli 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,46(1):165-172
We investigate the entanglement of an open tripartite
system where a cavity field mode in thermal equilibrium is
off-resonantly coupled with two atoms that are simultaneously driven
by a resonant coherent field. For moderately detuned atom-field
coupling and strong atomic driving we show the generation, at given
interaction times and for low enough cavity decay rates, of atomic
Bell states and of Bell state superpositions relevant for quantum
gates implementation. The system can oscillate between bi-separable
and fully separable states. Also we describe the distribution of
quantum correlations between the atom-atom and the two atom-field
subsystems. In the dispersive coupling regime with strongly driven
atoms we show the generation of nearly stationary Bell states which
remain protected from cavity dissipation. 相似文献
13.
An experimental scheme is proposed for faithful teleportation of a unknown optical cat-state via attenuated quantum channel due to energy loss or photon absorption during the process of entanglement sharing. The scheme is probabilistic, yet conclusive, and the effective classical communication costs just Log23 bits, instead of five bits which are necessary for full record of the measurement outcome. The scheme uses only threshold (i.e., yes/no) detectors so that exact photon counting is not needed. However, it requires application of a nonlinear element called cross-phase modulator. Feasibility of the scheme is also discussed with respect to EIT-based modern techniques. 相似文献
14.
Generation of field cluster states through collective operation of cavity QED disentanglement eraser
R. Ul-Islam A. H. Khosa H.-W. Lee F. Saif 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,48(2):271-277
We investigate effects of a collective disentanglement eraser performed over
states of two pairs of pre-entangled cavities tagged independently with two
identical three-level atoms. It is shown that the collective disentanglement
operation ensures not only the recovery of initial coherence but also its
extension from the initial two to four qubits, generating four-qubit field
cluster states. We also propose a cavity QED scheme to generate an arbitrary
field graph state by means of a collective operation of disentanglement erasers. 相似文献
15.
We propose a protocol Dn for faithfully teleporting an arbitrary n-qudit state with the tensor product state (TPS) of n generalized Bell states (GBSs) as the quantum channel. We also put forward explicit protocols and for faithfully teleporting an arbitrary n-qudit state with two classes of 2n-qudit GESs as the quantum channel, where the GESs we constructed are a kind of genuine entangled states which cannot be reducible to the TPS of n GBSs. 相似文献
16.
A. R.R. Carvalho F. Mintert S. Palzer A. Buchleitner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(2):425-432
We investigate the time evolution of entanglement for bipartite systems of
arbitrary dimensions under the influence of decoherence. For qubits, we
determine the precise entanglement decay rates under different
system-environment couplings, including finite temperature effects. For qudits, we show how
to obtain upper bounds for the decay rates and also present exact solutions
for various classes of states. 相似文献
17.
Quantum teleportation is one of the most important subjects in quantum information science. This is because quantum teleportation can be regarded as not only quantum information transfer but also a building block for universal quantum information processing. Furthermore, deterministic quantum information processing is very important for efficient processing and it can be realized with continuous-variable quantum information processing. In this review, quantum teleportation for continuous variables and related quantum information processing are reviewed from these points of view. 相似文献
18.
Q. Xie W. Hai 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,39(2):277-282
We investigate the relation between quantum states and classical
fixed-point bifurcations in a coupled two-component Bose-Einstein
condensate (BEC). It is shown that the classical bifurcations are
closely related to a topological change of the corresponding
quantum levels, and such a structure change can be manifested in
the entanglement properties of the corresponding quantum states.
That is, the entanglement of the quantum states displays some
peaks near the classical bifurcation points. 相似文献
19.
Peng Xue 《Physics letters. A》2008,372(46):6859-6866
We show how to realize long-distance quantum communication using a long-lived quantum memory, which is embedded in a decoherence-free subspace (DFS). Neutral atoms were used in the present scheme, whose interactions are catalyzed by single photons or weak coherent light. The generation, purification and swapping of logical entangled states are performed with help of cavity-assisted photon scattering which is robust to random variation in the atom-photon coupling rate. 相似文献
20.
P. Chang B. Shao J. Zou 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(4):371-377
The dynamics of the entanglement for a solid polariton
system is investigated. The polariton system is a photon-phonon complex and
its time-dependent characteristic function in the Wigner representation for
the system is obtained analytically. It is found that when the photon field
is initially prepared in the squeezed vacuum state, and the phonon in the
thermal state, the polariton system can evolve into a two-mode Gaussian
mixed state. The entanglement between photon and phonon turns out to be
apparently dependent on the squeezing parameter and exhibits a critical
behavior with respect to the temperature. 相似文献