7,446 research outputs found
Simulation of static and random errors on Grover's search algorithm implemented in a Ising nuclear spin chain quantum computer with few qubits
We consider Grover's search algorithm on a model quantum computer implemented
on a chain of four or five nuclear spins with first and second neighbour Ising
interactions. Noise is introduced into the system in terms of random
fluctuations of the external fields. By averaging over many repetitions of the
algorithm, the output state becomes effectively a mixed state. We study its
overlap with the nominal output state of the algorithm, which is called
fidelity. We find either an exponential or a Gaussian decay for the fidelity as
a function of the strength of the noise, depending on the type of noise (static
or random) and whether error supression is applied (the 2pi k-method) or not.Comment: 18 pages, 8 figures, extensive revision with new figure
Effective field theories for heavy quarkonium
We review recent theoretical developments in heavy quarkonium physics from
the point of view of Effective Field Theories of QCD. We discuss
Non-Relativistic QCD and concentrate on potential Non-Relativistic QCD. Our
main goal will be to derive QCD Schr\"odinger-like equations that govern the
heavy quarkonium physics in the weak and strong coupling regime. We also
discuss a selected set of applications, which include spectroscopy, inclusive
decays and electromagnetic threshold production.Comment: 162 pages, 30 figures, revised version, references added. Accepted
for publication in Reviews of Modern Physic
Order alpha^3 ln(1/alpha) Corrections to Positronium Decays
The logarithmically enhanced alpha^3 ln(1/alpha) corrections to the para- and
orthopositronium decay widths are calculated in the framework of dimensionally
regularized nonrelativistic quantum electrodynamics.In the case of
parapositronium, the correction is negative, approximately doubles the effect
of the leading logarithmic alpha^3 ln^2(1/alpha) one, and is comparable to the
nonlogarithmic O(alpha^2) one. As for orthopositronium, the correction is
positive and almost cancels the alpha^3 ln^2(1/alpha) one. The uncertainties in
the theoretical predictions for the decay widths are reduced.Comment: 10 pages (Latex); missing term added, corrected coefficient B_p used,
numerical results insignificantly change
Leading Chiral Logarithms to the Hyperfine Splitting of the Hydrogen and Muonic Hydrogen
We study the hydrogen and muonic hydrogen within an effective field theory
framework. We perform the matching between heavy baryon effective theory
coupled to photons and leptons and the relevant effective field theory at
atomic scales. This matching can be performed in a perturbative expansion in
alpha, 1/m_p and the chiral counting. We then compute the O(m_{l_i}^3
alpha^5/m_p^2 x logarithms) contribution (including the leading chiral
logarithms) to the Hyperfine splitting and compare with experiment. They can
explain about 2/3 of the difference between experiment and the pure QED
prediction when setting the renormalization scale at the rho mass. We give an
estimate of the matching coefficient of the spin-dependent proton-lepton
operator in heavy baryon effective theory.Comment: 17 pages, LaTeX, minor changes, one reference adde
The QCD Potential at
Within an effective field theory framework, we obtain an expression for the
next-to-leading term in the expansion of the singlet QCD
potential in terms of Wilson loops, which holds beyond perturbation theory. The
ambiguities in the definition of the QCD potential beyond leading order in
are discussed and a specific expression for the potential is given.
We explicitly evaluate this expression at one loop and compare the outcome with
the existing perturbative results. On general grounds we show that for quenched
QED and fully Abelian-like models this expression exactly vanishes.Comment: 19 pages, LaTeX, 1 figure. Journal version. Discussion refined,
misprints corrected, few references added; results unchange
Poincare' invariance and the heavy-quark potential
We derive and discuss the constraints induced by Poincare' invariance on the
form of the heavy-quark potential up to order 1/m^2. We present two
derivations: one uses general arguments directly based on the Poincare' algebra
and the other follows from an explicit calculation on the expression of the
potential in terms of Wilson loops. We confirm relations from the literature,
but also clarify the origin of a long-standing false statement pointed out
recently.Comment: 20 pages, 4 figure
Chaos and Complexity of quantum motion
The problem of characterizing complexity of quantum dynamics - in particular
of locally interacting chains of quantum particles - will be reviewed and
discussed from several different perspectives: (i) stability of motion against
external perturbations and decoherence, (ii) efficiency of quantum simulation
in terms of classical computation and entanglement production in operator
spaces, (iii) quantum transport, relaxation to equilibrium and quantum mixing,
and (iv) computation of quantum dynamical entropies. Discussions of all these
criteria will be confronted with the established criteria of integrability or
quantum chaos, and sometimes quite surprising conclusions are found. Some
conjectures and interesting open problems in ergodic theory of the quantum many
problem are suggested.Comment: 45 pages, 22 figures, final version, at press in J. Phys. A, special
issue on Quantum Informatio
Zemach and magnetic radius of the proton from the hyperfine splitting in hydrogen
The current status of the determination of corrections to the hyperfine
splitting of the ground state in hydrogen is considered. Improved calculations
are provided taking into account the most recent value for the proton charge
radius. Comparing experimental data with predictions for the hyperfine
splitting, the Zemach radius of the proton is deduced to be fm.
Employing exponential parametrizations for the electromagnetic form factors we
determine the magnetic radius of the proton to be fm. Both values
are compared with the corresponding ones derived from the data obtained in
electron-proton scattering experiments and the data extracted from a rescaled
difference between the hyperfine splittings in hydrogen and muonium
Running of the heavy quark production current and 1/k potential in QCD
The 1/k contribution to the heavy quark potential is first generated at one
loop order in QCD. We compute the two loop anomalous dimension for this
potential, and find that the renormalization group running is significant. The
next-to-leading-log coefficient for the heavy quark production current near
threshold is determined. The velocity renormalization group result includes the
alpha_s^3 ln^2(alpha_s) ``non-renormalization group logarithms'' of Kniehl and
Penin.Comment: 30 pages, journal versio
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