272 research outputs found
Universal Quantum Computation using Exchange Interactions and Teleportation of Single-Qubit Operations
We show how to construct a universal set of quantum logic gates using control
over exchange interactions and single- and two-spin measurements only.
Single-spin unitary operations are teleported instead of being executed
directly, thus eliminating a major difficulty in the construction of several of
the most promising proposals for solid-state quantum computation, such as
spin-coupled quantum dots, donor-atom nuclear spins in silicon, and electrons
on helium. Contrary to previous proposals dealing with this difficulty, our
scheme requires no encoding redundancy. We also discuss an application to
superconducting phase qubits.Comment: 4.5 pages, including 2 figure
Two Qubit Quantum Computing in a Projected Subspace
A formulation for performing quantum computing in a projected subspace is
presented, based on the subdynamical kinetic equation (SKE) for an open quantum
system. The eigenvectors of the kinetic equation are shown to remain invariant
before and after interaction with the environment. However, the eigenvalues in
the projected subspace exhibit a type of phase shift to the evolutionary
states. This phase shift does not destroy the decoherence-free (DF) property of
the subspace because the associated fidelity is 1. This permits a universal
formalism to be presented - the eigenprojectors of the free part of the
Hamiltonian for the system and bath may be used to construct a DF projected
subspace based on the SKE. To eliminate possible phase or unitary errors
induced by the change in the eigenvalues, a cancellation technique is proposed,
using the adjustment of the coupling time, and applied to a two qubit computing
system. A general criteria for constructing a DF projected subspace from the
SKE is discussed. Finally, a proposal for using triangulation to realize a
decoherence-free subsystem based on SKE is presented. The concrete formulation
for a two-qubit model is given exactly. Our approach is novel and general, and
appears applicable to any type of decoherence. Key Words: Quantum Computing,
Decoherence, Subspace, Open System PACS number: 03.67.Lx,33.25.+k,.76.60.-kComment: 24 pages. accepted by Phys. Rev.
On Quantum Control via Encoded Dynamical Decoupling
I revisit the ideas underlying dynamical decoupling methods within the
framework of quantum information processing, and examine their potential for
direct implementations in terms of encoded rather than physical degrees of
freedom. The usefulness of encoded decoupling schemes as a tool for engineering
both closed- and open-system encoded evolutions is investigated based on simple
examples.Comment: 12 pages, no figures; REVTeX style. This note collects various
theoretical considerations complementing/motivated by the experimental
demonstration of encoded control by Fortunato et a
Design and User Satisfaction of Interactive Maps for Visually Impaired People
Multimodal interactive maps are a solution for presenting spatial information
to visually impaired people. In this paper, we present an interactive
multimodal map prototype that is based on a tactile paper map, a multi-touch
screen and audio output. We first describe the different steps for designing an
interactive map: drawing and printing the tactile paper map, choice of
multi-touch technology, interaction technologies and the software architecture.
Then we describe the method used to assess user satisfaction. We provide data
showing that an interactive map - although based on a unique, elementary,
double tap interaction - has been met with a high level of user satisfaction.
Interestingly, satisfaction is independent of a user's age, previous visual
experience or Braille experience. This prototype will be used as a platform to
design advanced interactions for spatial learning
Quantum Transport in Two-Channel Fractional Quantum Hall Edges
We study the effect of backward scatterings in the tunneling at a point
contact between the edges of a second level hierarchical fractional quantum
Hall states. A universal scaling dimension of the tunneling conductance is
obtained only when both of the edge channels propagate in the same direction.
It is shown that the quasiparticle tunneling picture and the electron tunneling
picture give different scaling behaviors of the conductances, which indicates
the existence of a crossover between the two pictures. When the direction of
two edge-channels are opposite, e.g. in the case of MacDonald's edge
construction for the state, the phase diagram is divided into two
domains giving different temperature dependence of the conductance.Comment: 21 pages (REVTeX and 1 Postscript figure
Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture
We theoretically consider coherence times for spins in two quantum computer
architectures, where the qubit is the spin of an electron bound to a P donor
impurity in Si or within a GaAs quantum dot. We show that low temperature
decoherence is dominated by spin-spin interactions, through spectral diffusion
and dipolar flip-flop mechanisms. These contributions lead to 1-100 s
calculated spin coherence times for a wide range of parameters, much higher
than former estimates based on measurements.Comment: Role of the dipolar interaction clarified; Included discussion on the
approximations employed in the spectral diffusion calculation. Final version
to appear in Phys. Rev.
Entanglement in the Quantum Heisenberg XY model
We study the entanglement in the quantum Heisenberg XY model in which the
so-called W entangled states can be generated for 3 or 4 qubits. By the concept
of concurrence, we study the entanglement in the time evolution of the XY
model. We investigate the thermal entanglement in the two-qubit isotropic XY
model with a magnetic field and in the anisotropic XY model, and find that the
thermal entanglement exists for both ferromagnetic and antiferromagnetic cases.
Some evidences of the quantum phase transition also appear in these simple
models.Comment: 7 pages, 6 figs, revised version submitted to Phys. Rev.
Geometric effects on T-breaking in p+ip and d+id superconductors
Superconducting order parameters that change phase around the Fermi surface
modify Josephson tunneling behavior, as in the phase-sensitive measurements
that confirmed order in the cuprates. This paper studies Josephson coupling
when the individual grains break time-reversal symmetry; the specific cases
considered are and , which may appear in SrRuO and
NaCoO(HO) respectively. -breaking order parameters
lead to frustrating phases when not all grains have the same sign of
time-reversal symmetry breaking, and the effects of these frustrating phases
depend sensitively on geometry for 2D arrays of coupled grains. These systems
can show perfect superconducting order with or without macroscopic
-breaking. The honeycomb lattice of superconducting grains has a
superconducting phase with no spontaneous breaking of but instead power-law
correlations. The superconducting transition in this case is driven by binding
of fractional vortices, and the zero-temperature criticality realizes a
generalization of Baxter's three-color model.Comment: 8 page
High-p_T Higgs Boson Production at Hadron Colliders to Order(alpha_s G_Fermi^3)
We study high-- Higgs boson production at hadron colliders to
order( ) in hadron collisions. In particular, we investigate
the process g+q/\bar{q}\ra q/\bar{q}+H, where or , for the
LHC (a 14~TeV, proton--proton collider). Our results are compared to
the order( ) calculation. The associated production of a
high-- Higgs boson with a --quark or anti--quark is comparable to the
order( ) calculation because of the large top quark mass and
the additional contribution of electroweak gauge and Goldstone bosons. The
associated production of light quarks, however, is not significant. We also
comment on new physics effects in the framework of the electroweak chiral
Lagrangian.Comment: Latex file,16 pages, 2 figures, uses psfig.sty, postscript file
available at ftp://cithe502.cithep.caltech.edu/pub/mrenna/cp0.ps or at
http://www.cithep.caltech.edu/~mrenn
Encoded Universality for Generalized Anisotropic Exchange Hamiltonians
We derive an encoded universality representation for a generalized
anisotropic exchange Hamiltonian that contains cross-product terms in addition
to the usual two-particle exchange terms. The recently developed algebraic
approach is used to show that the minimal universality-generating encodings of
one logical qubit are based on three physical qubits. We show how to generate
both single- and two-qubit operations on the logical qubits, using suitably
timed conjugating operations derived from analysis of the commutator algebra.
The timing of the operations is seen to be crucial in allowing simplification
of the gate sequences for the generalized Hamiltonian to forms similar to that
derived previously for the symmetric (XY) anisotropic exchange Hamiltonian. The
total number of operations needed for a controlled-Z gate up to local
transformations is five. A scalable architecture is proposed.Comment: 11 pages, 4 figure
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