6,315 research outputs found
A tutorial task and tertiary courseware model for collaborative learning communities
RAED provides a computerised infrastructure to support the development and administration of Vicarious Learning in collaborative learning communities spread across multiple universities and workplaces. The system is based on the OASIS middleware for Role-based Access Control. This paper describes the origins of the model and the approach to implementation and outlines some of its benefits to collaborative teachers and learners
Classical simulation of quantum entanglement without local hidden variables
Recent work has extended Bell's theorem by quantifying the amount of communication required to simulate entangled quantum systems with classical information. The general scenario is that a bipartite measurement is given from a set of possibilities and the goal is to find a classical scheme that reproduces exactly the correlations that arise when an actual quantum system is measured. Previous results have shown that, using local hidden variables, a finite amount of communication suffices to simulate the correlations for a Bell state. We extend this in a number of ways. First, we show that, when the communication is merely required to be finite {\em on average}, Bell states can be simulated {\em without} any local hidden variables. More generally, we show that arbitrary positive operator valued measurements on systems of Bell states can be simulated with bits of communication on average (again, without local hidden variables). On the other hand, when the communication is required to be {\em absolutely bounded}, we show that a finite number of bits of local hidden variables is insufficent to simulate a Bell state. This latter result is based on an analysis of the non-deterministic communication complexity of the NOT-EQUAL function, which is constant in the quantum model and logarithmic in the classical model
Universal Leakage Elimination
``Leakage'' errors are particularly serious errors which couple states within
a code subspace to states outside of that subspace thus destroying the error
protection benefit afforded by an encoded state. We generalize an earlier
method for producing leakage elimination decoupling operations and examine the
effects of the leakage eliminating operations on decoherence-free or noiseless
subsystems which encode one logical, or protected qubit into three or four
qubits. We find that by eliminating the large class of leakage errors, under
some circumstances, we can create the conditions for a decoherence free
evolution. In other cases we identify a combination decoherence-free and
quantum error correcting code which could eliminate errors in solid-state
qubits with anisotropic exchange interaction Hamiltonians and enable universal
quantum computing with only these interactions.Comment: 14 pages, no figures, new version has references updated/fixe
Implications of Qudit Superselection rules for the Theory of Decoherence-free Subsystems
The use of d-state systems, or qudits, in quantum information processing is
discussed. Three-state and higher dimensional quantum systems are known to have
very different properties from two-state systems, i.e., qubits. In particular
there exist qudit states which are not equivalent under local unitary
transformations unless a selection rule is violated. This observation is shown
to be an important factor in the theory of decoherence-free, or noiseless,
subsystems. Experimentally observable consequences and methods for
distinguishing these states are also provided, including the explicit
construction of new decoherence-free or noiseless subsystems from qutrits.
Implications for simulating quantum systems with quantum systems are also
discussed.Comment: 13 pages, 1 figures, Version 2: Typos corrected, references fixed and
new ones added, also includes referees suggested changes and a new exampl
Closed Timelike Curves in Relativistic Computation
In this paper, we investigate the possibility of using closed timelike curves
(CTCs) in relativistic hypercomputation. We introduce a wormhole based
hypercomputation scenario which is free from the common worries, such as the
blueshift problem. We also discuss the physical reasonability of our scenario,
and why we cannot simply ignore the possibility of the existence of spacetimes
containing CTCs.Comment: 17 pages, 5 figure
Optimal classical-communication-assisted local model of n-qubit Greenberger-Horne-Zeilinger correlations
We present a model, motivated by the criterion of reality put forward by
Einstein, Podolsky, and Rosen and supplemented by classical communication,
which correctly reproduces the quantum-mechanical predictions for measurements
of all products of Pauli operators on an n-qubit GHZ state (or ``cat state'').
The n-2 bits employed by our model are shown to be optimal for the allowed set
of measurements, demonstrating that the required communication overhead scales
linearly with n. We formulate a connection between the generation of the local
values utilized by our model and the stabilizer formalism, which leads us to
conjecture that a generalization of this method will shed light on the content
of the Gottesman-Knill theorem.Comment: New version - expanded and revised to address referee comment
Decoherence-Free Subspaces for Multiple-Qubit Errors: (I) Characterization
Coherence in an open quantum system is degraded through its interaction with
a bath. This decoherence can be avoided by restricting the dynamics of the
system to special decoherence-free subspaces. These subspaces are usually
constructed under the assumption of spatially symmetric system-bath coupling.
Here we show that decoherence-free subspaces may appear without spatial
symmetry. Instead, we consider a model of system-bath interactions in which to
first order only multiple-qubit coupling to the bath is present, with
single-qubit system-bath coupling absent. We derive necessary and sufficient
conditions for the appearance of decoherence-free states in this model, and
give a number of examples. In a sequel paper we show how to perform universal
and fault tolerant quantum computation on the decoherence-free subspaces
considered in this paper.Comment: 18 pages, no figures. Major changes. Section on universal fault
tolerant computation removed. This section contained a crucial error. A new
paper [quant-ph/0007013] presents the correct analysi
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Quantum Computational Complexity in the Presence of Closed Timelike Curves
Quantum computation with quantum data that can traverse closed timelike
curves represents a new physical model of computation. We argue that a model of
quantum computation in the presence of closed timelike curves can be formulated
which represents a valid quantification of resources given the ability to
construct compact regions of closed timelike curves. The notion of
self-consistent evolution for quantum computers whose components follow closed
timelike curves, as pointed out by Deutsch [Phys. Rev. D {\bf 44}, 3197
(1991)], implies that the evolution of the chronology respecting components
which interact with the closed timelike curve components is nonlinear. We
demonstrate that this nonlinearity can be used to efficiently solve
computational problems which are generally thought to be intractable. In
particular we demonstrate that a quantum computer which has access to closed
timelike curve qubits can solve NP-complete problems with only a polynomial
number of quantum gates.Comment: 8 pages, 2 figures. Minor changes and typos fixed. Reference adde
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