8,204 research outputs found
Quantum correlations in the collective spin systems
Quantum and classical pairwise correlations in two typical collective spin
systems (i.e., the Dicke model and the Lipkin-Meshkov-Glick model) are
discussed. These correlations in the thermodynamical limit are obtained
analytically and in a finite-size system are calculated numerically. Large-size
scaling behavior for the quantum discord itself is observed, which has never
been reported in another critical system. A logarithmic diverging behavior for
the first derivative of the quantum discord is also found in both models, which
might be universal in the second-order quantum phase transition. It is
suggested that the pronounced maximum or minimum of first derivative of quantum
discord signifies the critical point. Comparisons between the quantum discord
and the scaled concurrence are performed. It is shown that the quantum discord
is very small in one phase and robust in the other phase, while the scaled
concurrence shows maximum at the critical point and decays rapidly when away
from the the critical point.Comment: 8 pages, 4 figure
Quantum correlations in a cluster-like system
We discuss a cluster-like 1D system with triplet interaction. We study the
topological properties of this system. We find that the degeneracy depends on
the topology of the system, and well protected against external local
perturbations. All these facts show that the system is topologically ordered.
We also find a string order parameter to characterize the quantum phase
transition. Besides, we investigate two-site correlations including
entanglement, quantum discord and mutual information. We study the different
divergency behaviour of the correlations. The quantum correlation decays
exponentially in both topological and magnetic phases, and diverges in reversed
power law at the critical point. And we find that in TQPT systems, the global
difference of topology induced by dimension can be reflected in local quantum
correlations.Comment: 7 pages, 6 figure
Quantum correlations in topological quantum phase transitions
We study the quantum correlations in a 2D system that possesses a topological
quantum phase transition. The quantumness of two-body correlations is measured
by quantum discord. We calculate both the correlation of two local spins and
that of an arbitrary spin with the rest of the lattice. It is notable that
local spins are classically correlated, while the quantum correlation is hidden
in the global lattice. This is different from other systems which are not
topologically orderd. Moreover, the mutual information and global quantum
discord show critical behavior in the topological quantum phase transition.Comment: 6 pages, 3 figure
Topological order in 1D Cluster state protected by symmetry
We demonstrate how to construct the Z2*Z2 global symmetry which protects the
ground state degeneracy of cluster states for open boundary conditions. Such a
degeneracy ultimately arises because the set of stabilizers do not span a
complete set of integrals of motion of the cluster state Hamiltonian for open
boundary conditions. By applying control phase transformations, our
construction makes the stabilizers into the Pauli operators spanning the qubit
Hilbert space from which the degeneracy comes.Comment: 1 figure, To be published in Quantum Information Processin
Entanglement in spin-1/2 dimerized Heisenberg systems
We study entanglement in dimerized Heisenberg systems. In particular, we give
exact results of ground-state pairwise entanglement for the four-qubit model by
identifying a Z_2 symmetry. Although the entanglements cannot identify the
critical point of the system, the mean entanglement of nearest-neighbor qubits
really does, namely, it reaches a maximum at the critical point.Comment: Four pages, three figures, accepted in Communications in Theoretical
Physic
Classification of Gapped Symmetric Phases in 1D Spin Systems
Quantum many-body systems divide into a variety of phases with very different
physical properties. The question of what kind of phases exist and how to
identify them seems hard especially for strongly interacting systems. Here we
make an attempt to answer this question for gapped interacting quantum spin
systems whose ground states are short-range correlated. Based on the local
unitary equivalence relation between short-range correlated states in the same
phase, we classify possible quantum phases for 1D matrix product states, which
represent well the class of 1D gapped ground states. We find that in the
absence of any symmetry all states are equivalent to trivial product states,
which means that there is no topological order in 1D. However, if certain
symmetry is required, many phases exist with different symmetry protected
topological orders. The symmetric local unitary equivalence relation also
allows us to obtain some simple results for quantum phases in higher dimensions
when some symmetries are present.Comment: 21 pages, 7 figures. Version 2, classification for parity and
translation symmetry update
Entanglement and Quantum Phase Transition in Low Dimensional Spin Systems
Entanglement of the ground states in and dimerized Heisenberg spin
chains as well as in a two-leg spin ladder is analyzed by using the spin-spin
concurrence and the entanglement entropy between a selected sublattice of spins
and the rest of the system. In particular, we reveal that quantum phase
transition points/boundaries may be identified based on the analysis on the
local extreme of this entanglement entropy, which is illustrated to be superior
over the concurrence scenario and may enable us to explore quantum phase
transitions in many other systems including higher dimensional ones.Comment: 4 pages, 4 figure
Peierls distorted chain as a quantum data bus for quantum state transfer
We systematically study the transfer of quantum state of electron spin as the
flying qubit along a half-filled Peierls distorted tight-binding chain
described by the Su-Schrieffer-Heeger (SSH) model, which behaves as a quantum
data bus. This enables a novel physical mechanism for quantum communication
with always-on interaction: the effective hopping of the spin carrier between
sites and connected to two sites in this SSH chain can be induced by
the quasi-excitations of the SSH model. As we prove, it is the Peierls energy
gap of the SSH quasi-excitations that plays a crucial role to protect the
robustness of the quantum state transfer process. Moreover, our observation
also indicates that such a scheme can also be employed to explore the intrinsic
property of the quantum system.Comment: 10 pages, 6 figure
Necessary and sufficient conditions for local creation of quantum discord
We show that a local channel cannot create quantum discord (QD) for zero QD
states of size if and only if either it is a completely decohering
channel or it is a nontrivial isotropic channel. For the qubit case this
propertiy is additionally characteristic to the completely decohering channel
or the commutativity-preserving unital channel. In particular, the exact forms
of the completely decohering channel and the commutativity-preserving unital
qubit channel are proposed. Consequently, our results confirm and improve the
conjecture proposed by X. Hu et al. for the case of and improve the
result proposed by A. Streltsov et al. for the qubit case. Furthermore, it is
shown that a local channel nullifies QD in any state if and only if it is a
completely decohering channel. Based on our results, some protocols of quantum
information processing issues associated with QD, especially for the qubit
case, would be experimentally accessible.Comment: 8 page
Novel cloning machine with supplementary information
Probabilistic cloning was first proposed by Duan and Guo. Then Pati
established a novel cloning machine (NCM) for copying superposition of multiple
clones simultaneously. In this paper, we deal with the novel cloning machine
with supplementary information (NCMSI). For the case of cloning two states, we
demonstrate that the optimal efficiency of the NCMSI in which the original
party and the supplementary party can perform quantum communication equals that
achieved by a two-step cloning protocol wherein classical communication is only
allowed between the original and the supplementary parties. From this
equivalence it follows that NCMSI may increase the success probabilities for
copying. Also, an upper bound on the unambiguous discrimination of two
nonorthogonal pure product states is derived. Our investigation generalizes and
completes the results in the literature.Comment: 22 pages; the presentation is revised, and some typos are correcte
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