969 research outputs found
Interaction Quenches of Fermi Gases
It is shown that the jump in the momentum distribution of Fermi gases evolves
smoothly for small and intermediate times once an interaction between the
fermions is suddenly switched on. The jump does not vanish abruptly. The loci
in momentum space where the jumps occur are those of the noninteracting Fermi
sea. No relaxation of the Fermi surface geometry takes place.Comment: 4 pages, 1 figur
Generalization of short coherent control pulses: extension to arbitrary rotations
We generalize the problem of the coherent control of small quantum systems to
the case where the quantum bit (qubit) is subject to a fully general rotation.
Following the ideas developed in Pasini et al (2008 Phys. Rev. A 77, 032315),
the systematic expansion in the shortness of the pulse is extended to the case
where the pulse acts on the qubit as a general rotation around an axis of
rotation varying in time. The leading and the next-leading corrections are
computed. For certain pulses we prove that the general rotation does not
improve on the simpler rotation with fixed axis.Comment: 6 pages, no figures; published versio
Understanding panel conditioning: an examination of social desirability bias in self-reported height and weight in panel surveys using experimental data
Typically reliant on self-reports from panel data, a growing body of literature suggests that relative body weight can have negative effects on labour market outcomes. Given the interest in the effects of relative weight in the social sciences, this paper addresses the question of whether repeated interviewing affects the quality of these data. A theory that focuses on the sensitivity of the questions rather than the survey context is proposed. Examining experimental panel data from Understanding Society using quantile-regression, the findings for women are consistent with the argument that conditioning reduces social desirability effects. The ameliorative effects of panel conditioning on social desirability bias in self-reported height and bodyweight appear to strengthen the association between relative weight and employment for men, but not women, however
Perturbation Theory by Flow Equations: Dimerized and Frustrated S=1/2 Chain
The flow equation method (Wegner 1994) is used as continuous unitary
transformation to construct perturbatively effective Hamiltonians. The method
is illustrated in detail for dimerized and frustrated antiferromagnetic S=1/2
chains. The effective Hamiltonians conserve the number of elementary
excitations which are S=1 magnons for the dimerized chains. The sectors of
different number of excitations are clearly separated. Easy-to-use results for
the gap, the dispersion and the ground state energies of the chains are
provided.Comment: 18 pages, 15 figures included, to appear in Eur. Phys. J. B;
Electronic data will be made available on appearance of articl
High Order Coherent Control Sequences of Finite-Width Pulses
The performance of sequences of designed pulses of finite length is
analyzed for a bath of spins and it is compared with that of sequences of
ideal, instantaneous pulses. The degree of the design of the pulse strongly
affects the performance of the sequences. Non-equidistant, adapted sequences of
pulses, which equal instantaneous ones up to , outperform
equidistant or concatenated sequences. Moreover, they do so at low energy cost
which grows only logarithmically with the number of pulses, in contrast to
standard pulses with linear growth.Comment: 6 pages, 5 figures, new figures, published versio
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