174 research outputs found
Entanglement and quantum phase transitions in matrix product spin one chains
We consider a one-parameter family of matrix product states of spin one
particles on a periodic chain and study in detail the entanglement properties
of such a state. In particular we calculate exactly the entanglement of one
site with the rest of the chain, and the entanglement of two distant sites with
each other and show that the derivative of both these properties diverge when
the parameter of the states passes through a critical point. Such a point
can be called a point of quantum phase transition, since at this point, the
character of the matrix product state which is the ground state of a
Hamiltonian, changes discontinuously. We also study the finite size effects and
show how the entanglement depends on the size of the chain. This later part is
relevant to the field of quantum computation where the problem of initial state
preparation in finite arrays of qubits or qutrits is important. It is also
shown that entanglement of two sites have scaling behavior near the critical
point
Recovering quantum information through partial access to the environment
We investigate the possibility of correcting errors occurring on a
multipartite system through a feedback mechanism that acquires information from
partial access to the environment. A partial control scheme of this kind might
be useful when dealing with correlated errors. In fact, in such a case, it
could be enough to gather local information to decide what kind of global
recovery to perform. Then, we apply this scheme to the depolarizing and
correlated errors, and quantify its performance by means of the entanglement
fidelity
Equi-entangled bases in arbitrary dimensions
For the space of two identical systems of arbitrary dimensions, we introduce
a continuous family of bases with the following properties: i) the bases are
orthonormal, ii) in each basis, all the states have the same values of
entanglement, and iii) they continuously interpolate between the product basis
and the maximally entangled basis. The states thus constructed may find
applications in many areas related to quantum information science including
quantum cryptography, optimal Bell tests and investigation of enhancement of
channel capacity due to entanglement.Comment: 10 pages, 2 figures, 1 table, Accepted for publication in Phys. Rev.
Stationary states of open XX-spin chains
We study an open quantum spin chain of arbitrary length with nearest neighbor X X interactions of strength g, immersed in an external constant magnetic field Δ along the z direction, whose end spins are weakly coupled to two heat baths at different temperatures. In the so-called global approach, namely, without neglecting interspin interactions, using standard weak-coupling limit techniques, we first derive the open chain master equation written in terms of fermionic mode operators. Then, we focus on the study of the dependence of the resulting open dynamics from the ratio r ≡ g/Δ. By increasing r, some of the chain Bohr transition frequencies become negative; when this occurs, both the generator of the dissipative time evolution and its stationary states behave discontinuously. As a consequence, the asymptotic spin and heat flows also exhibit discontinuities, but in a different way: while source terms in the spin flow continuity equation show jumps, the heat flow instead is continuous but with discontinuous first derivatives with respect to r. These two behaviors might be experimentally accessible; in particular, they could discriminate between the global and the local approaches to open quantum spin chains. Indeed, the latter one, which neglects interspin interactions in the derivation of the master equation, does not show any kind of discontinuous behavior
Quantum capacity of a bosonic dephasing channel
We study the quantum capacity of a continuous-variable dephasing channel, which is a notable example of a non-Gaussian quantum channel. We prove that a single-letter formula applies. The optimal input state is found to be diagonal in the Fock basis and with a distribution that is a discrete version of a Gaussian. We discuss how its mean and variance are related to the dephasing rate and input energy. We then show that by increasing the input energy, the capacity saturates to a finite value. We also show that it decays exponentially for large values of dephasing rates
Transition behavior in the capacity of correlated-noisy channels in arbitrary dimensions
We construct a class of quantum channels in arbitrary dimensions for which
entanglement improves the performance of the channel. The channels have
correlated noise and when the level of correlation passes a critical value we
see a sharp transition in the optimal input states (states which minimize the
output entropy) from separable to maximally entangled states. We show that for
a subclass of channels with some extra conditions, including the examples which
we consider, the states which minimize the output entropy are the ones which
maximize the mutual information.Comment: 11 pages, Latex, 4 figures, Accepted for publication in Physical
Review
Symmetrization and Entanglement of Arbitrary States of Qubits
Given two arbitrary pure states and of qubits or higher
level states, we provide arguments in favor of states of the form instead of symmetric or
anti-symmetric states, as natural candidates for optimally entangled states
constructed from these states. We show that such states firstly have on the
average a high value of concurrence, secondly can be constructed by a universal
unitary operator independent of the input states. We also show that these
states are the only ones which can be produced with perfect fidelity by any
quantum operation designed for intertwining two pure states with a relative
phase. A probabilistic method is proposed for producing any pre-determined
relative phase into the combination of any two arbitrary states.Comment: 6 pages, 1 figur
Interval prediction algorithm and optimal scenario making model for wind power producers bidding strategy
Nowadays, renewable energies are important sources for supplying electric power demand and a key entity of future energy markets. Therefore, wind power producers (WPPs) in most of the power systems in the world have a key role. On the other hand, the wind speed uncertainty makes WPPs deferent power generators, which in turn causes adequate bidding strategies, that leads to market rules, and the functional abilities of the turbines to penetrate the market. In this paper, a new bidding strategy has been proposed based on optimal scenario making for WPPs in a competitive power market. As known, the WPP generation is uncertain, and different scenarios must be created for wind power production. Therefore, a prediction intervals method has been improved in making scenarios and increase the accuracy of the presence of WPPs in the balancing market. Besides, a new optimization algorithm has been proposed called the grasshopper optimization algorithm to simulate the optimal bidding problem of WPPs. A set of numerical examples, as well as a case-study based on real-world data, allows illustrating and discussing the properties of the proposed method
Photon losses depending on polarization mixedness
We introduce a quantum channel describing photon losses depending on the
degree of polarization mixedness. This can be regarded as a model of quantum
channel with correlated errors between discrete and continuous degrees of
freedom. We consider classical information over a continuous alphabet encoded
on weak coherent states as well as classical information over a discrete
alphabet encoded on single photons using dual rail representation. In both
cases we study the one-shot capacity of the channel and its behaviour in terms
of correlation between losses and polarization mixedness
On a suggestion relating topological and quantum mechanical entanglements
We analyze a recent suggestion \cite{kauffman1,kauffman2} on a possible
relation between topological and quantum mechanical entanglements. We show that
a one to one correspondence does not exist, neither between topologically
linked diagrams and entangled states, nor between braid operators and quantum
entanglers. We also add a new dimension to the question of entangling
properties of unitary operators in general.Comment: RevTex, 7 eps figures, to be published in Phys. Lett. A (2004
- …