34 research outputs found
Optimal conclusive teleportation of quantum states
Quantum teleportation of qudits is revisited. In particular, we analyze the
case where the quantum channel corresponds to a non-maximally entangled state
and show that the success of the protocol is directly related to the problem of
distinguishing non-orthogonal quantum states. The teleportation channel can be
seen as a coherent superposition of two channels, one of them being a maximally
entangled state thus, leading to perfect teleportation and the other,
corresponding to a non-maximally entangled state living in a subspace of the
d-dimensional Hilbert space. The second channel leads to a teleported state
with reduced fidelity. We calculate the average fidelity of the process and
show its optimality.Comment: 8 pages, revtex, no figure
Vacuum induced Spin-1/2 Berry phase
We calculate the Berry phase of a spin-1/2 particle in a magnetic field considering the quantum nature of the field. The phase reduces to the standard Berry phase in the semiclassical limit. An eigenstate of the particle acquires a phase in the vacuum which is undetectable. We show how to generate a vacuum induced Berry phase which can be detected considering two quantized modes of the field
Holonomic quantum computation in the presence of decoherence
We present a scheme to study non-abelian adiabatic holonomies for open
Markovian systems. As an application of our framework, we analyze the
robustness of holonomic quantum computation against decoherence. We pinpoint
the sources of error that must be corrected to achieve a geometric
implementation of quantum computation completely resilient to Markovian
decoherence.Comment: I. F-G. Now publishes under name I. Fuentes-Schuller Published
versio
Spin-1/2 geometric phase driven by decohering quantum fields
We calculate the geometric phase of a spin-1/2 system driven by a one and two
mode quantum field subject to decoherence. Using the quantum jump approach, we
show that the corrections to the phase in the no-jump trajectory are different
when considering an adiabatic and non-adiabatic evolution. We discuss the
implications of our results from both the fundamental as well as quantum
computational perspective.Comment: 4 page
General formalism of Hamiltonians for realizing a prescribed evolution of a qubit
We investigate the inverse problem concerning the evolution of a qubit
system, specifically we consider how one can establish the Hamiltonians that
account for the evolution of a qubit along a prescribed path in the projected
Hilbert space. For a given path, there are infinite Hamiltonians which can
realize the same evolution. A general form of the Hamiltonians is constructed
in which one may select the desired one for implementing a prescribed
evolution. This scheme can be generalized to higher dimensional systems.Comment: 6 page
Geometric phase in open systems
We calculate the geometric phase associated to the evolution of a system
subjected to decoherence through a quantum-jump approach. The method is general
and can be applied to many different physical systems. As examples, two main
source of decoherence are considered: dephasing and spontaneous decay. We show
that the geometric phase is completely insensitive to the former, i.e. it is
independent of the number of jumps determined by the dephasing operator.Comment: 4 pages, 2 figures, RevTe
Universal quantum computation by holonomic and nonlocal gates with imperfections
We present a nonlocal construction of universal gates by means of holonomic
(geometric) quantum teleportation. The effect of the errors from imperfect
control of the classical parameters, the looping variation of which builds up
holonomic gates, is investigated. Additionally, the influence of quantum
decoherence on holonomic teleportation used as a computational primitive is
studied. Advantages of the holonomic implementation with respect to control
errors and dissipation are presented.Comment: 5 pages, 2 figures, REVTEX, title changed, typos correcte
The Close Environment of Seyfert Galaxies and Its Implication for Unification Models
This paper presents a statistical analysis of the circumgalactic environment
of nearby Seyfert galaxies based on a computer-aided search of companion
galaxies on the Digitized Sky Survey (DSS). An intrinsic difference between the
environment of Seyfert 1 and Seyfert 2 galaxies, suggested by previous work, is
confirmed as statistically significant. For Seyfert 2 galaxies we find a
significant excess of large companions (diameter of companion >= 10 Kpc) within
a search radius <= 100 Kpc of projected linear distance, as well as within a
search radius equal to three times the diameter \ds of each Seyfert galaxy. For
Seyfert 1 galaxies there is no clear evidence of any excess of companion
galaxies neither within 100 Kpc, nor within 3\ds. For all samples the number of
companions suggests a markedly non-Poissonian distribution for galaxies on
scales <= 100 Kpc. This difference in environment is not compatible with the
simplest formulation of the Unification Model for Seyferts: both types 1 and 2
should be intrinsicaly alike, the only difference being due to orientation of
an obscuring torus. We propose an alternative formulation.Comment: 1 figure, accepted for publication in Astrophysical Journal Letter
Topological Features in Ion Trap Holonomic Computation
Topological features in quantum computing provide controllability and noise
error avoidance in the performance of logical gates. While such resilience is
favored in the manipulation of quantum systems, it is very hard to identify
topological features in nature. This paper proposes a scheme where holonomic
quantum gates have intrinsic topological features. An ion trap is employed
where the vibrational modes of the ions are coherently manipulated with lasers
in an adiabatic cyclic way producing geometrical holonomic gates. A crucial
ingredient of the manipulation procedures is squeezing of the vibrational
modes, which effectively suppresses exponentially any undesired fluctuations of
the laser amplitudes, thus making the gates resilient to control errors.Comment: 9 pages, 4 figures, REVTE