4,895 research outputs found
Robustness of Highly Entangled Multi-Qubit States Under Decoherence
We investigate the decay of entanglement, due to decoherence, of multi-qubit
systems that are initially prepared in highly (in some cases maximally)
entangled states. We assume that during the decoherence processes each qubit of
the system interacts with its own, independent environment. We determine, for
systems with a small number of qubits and for various decoherence channels, the
initial states exhibiting the most robust entanglement. We also consider a
restricted version of this robustness optimization problem, only involving
states equivalent under local unitary transformations to the |GHZ> state.Comment: 16 pages, 3 figures. Changes in Sec.
Efficient generation of random multipartite entangled states using time optimal unitary operations
We review the generation of random pure states using a protocol of repeated
two qubit gates. We study the dependence of the convergence to states with Haar
multipartite entanglement distribution. We investigate the optimal generation
of such states in terms of the physical (real) time needed to apply the
protocol, instead of the gate complexity point of view used in other works.
This physical time can be obtained, for a given Hamiltonian, within the
theoretical framework offered by the quantum brachistochrone formalism. Using
an anisotropic Heisenberg Hamiltonian as an example, we find that different
optimal quantum gates arise according to the optimality point of view used in
each case. We also study how the convergence to random entangled states depends
on different entanglement measures.Comment: 5 pages, 2 figures. New title, improved explanation of the algorithm.
To appear in Phys. Rev.
The Mariner 5 flight path and its determination from tracking data
Mariner 5 flight path and its determination from tracking dat
Multi-Qubit Systems: Highly Entangled States and Entanglement Distribution
A comparison is made of various searching procedures, based upon different
entanglement measures or entanglement indicators, for highly entangled
multi-qubits states. In particular, our present results are compared with those
recently reported by Brown et al. [J. Phys. A: Math. Gen. 38 (2005) 1119]. The
statistical distribution of entanglement values for the aforementioned
multi-qubit systems is also explored.Comment: 24 pages, 3 figure
A genuine maximally seven-qubit entangled state
Contrary to A.Borras et al.'s [1] conjecture, a genuine maximally seven-qubit
entangled state is presented. We find a seven-qubit state whose marginal
density matrices for subsystems of 1,2- qubits are all completely mixed and for
subsystems of 3-qubits is almost completely mixed
Brachistochrone of Entanglement for Spin Chains
We analytically investigate the role of entanglement in time-optimal state
evolution as an appli- cation of the quantum brachistochrone, a general method
for obtaining the optimal time-dependent Hamiltonian for reaching a target
quantum state. As a model, we treat two qubits indirectly cou- pled through an
intermediate qubit that is directly controllable, which represents a typical
situation in quantum information processing. We find the time-optimal unitary
evolution law and quantify residual entanglement by the two-tangle between the
indirectly coupled qubits, for all possible sets of initial pure quantum states
of a tripartite system. The integrals of the motion of the brachistochrone are
determined by fixing the minimal time at which the residual entanglement is
maximized. Entan- glement plays a role for W and GHZ initial quantum states,
and for the bi-separable initial state in which the indirectly coupled qubits
have a nonzero value of the 2-tangle.Comment: 9 pages, 4 figure
Quantum teleportation and state sharing using a genuinely entangled six qubit state
The usefulness of the genuinely entangled six qubit state that was recently
introduced by Borras et al. is investigated for the quantum teleportation of an
arbitrary three qubit state and for quantum state sharing (QSTS) of an
arbitrary two qubit state. For QSTS, we explicitly devise two protocols and
construct sixteen orthogonal measurement basis which can lock an arbitrary two
qubit information between two parties.Comment: 6 pages, 7 table
Generalized Criterion of Maximally Multi-Qubit Entanglement
We first present a generalized criterion for maximally entangled states of 2,
3, 4, 5, 6, 8 and in theory to arbitrary-number qubits. By this criterion, some
known highly entangled multi-qubit states are examined and a new genuine
eight-qubit maximally entangle state is obtained. For the 4, 7 and 8 qubits
system in which no maximally multi-qubit entangled states (MMES) is thought to
exist before, we find that the proved and most suspected MMESes, are not
completely mixed in subsystem with a critical-number qubits, below which the
subsystems are all completely mixed. We believe that the new criterion and MMES
can play important role in quantum information technology, such as the
teleportation and dense coding
Time-optimal CNOT between indirectly coupled qubits in a linear Ising chain
We give analytical solutions for the time-optimal synthesis of entangling
gates between indirectly coupled qubits 1 and 3 in a linear spin chain of three
qubits subject to an Ising Hamiltonian interaction with equal coupling plus
a local magnetic field acting on the intermediate qubit. The energy available
is fixed, but we relax the standard assumption of instantaneous unitary
operations acting on single qubits. The time required for performing an
entangling gate which is equivalent, modulo local unitary operations, to the
between the indirectly coupled qubits 1 and 3 is
, i.e. faster than a previous estimate based on a similar
Hamiltonian and the assumption of local unitaries with zero time cost.
Furthermore, performing a simple Walsh-Hadamard rotation in the Hlibert space
of qubit 3 shows that the time-optimal synthesis of the (which acts as the identity when the control qubit 1 is in the state
, while if the control qubit is in the state the target
qubit 3 is flipped as ) also requires the same
time .Comment: 9 pages; minor modification
Highly entangled multi-qubit states with simple algebraic structure
Recent works by Brown et al and Borras et al have explored numerical
optimisation procedures to search for highly entangled multi-qubit states
according to some computationally tractable entanglement measure. We present an
alternative scheme based upon the idea of searching for states having not only
high entanglement but also simple algebraic structure. We report results for 4,
5, 6, 7 and 8 qubits discovered by this approach, showing that many of such
states do exist. In particular, we find a maximally entangled 6-qubit state
with an algebraic structure simpler than the best results known so far. For the
case of 7, we discover states with high, but not maximum, entanglement and
simple structure, as well as other desirable properties. Some preliminary
results are shown for the case of 8 qubits.Comment: 15 pages, 1 figur
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