1,445 research outputs found
Spinon excitations in the XX chain: spectra, transition rates, observability
The exact one-to-one mapping between (spinless) Jordan-Wigner lattice
fermions and (spin-1/2) spinons is established for all eigenstates of the
one-dimensional s = 1=2 XX model on a lattice with an even or odd number N of
lattice sites and periodic boundary conditions. Exact product formulas for the
transition rates derived via Bethe ansatz are used to calculate asymptotic
expressions of the 2-spinon and 4-spinon parts (for large even N) as well as of
the 1-spinon and 3-spinon parts (for large odd N) of the dynamic spin structure
factors. The observability of these spectral contributions is assessed for
finite and infinite N.Comment: 19 pages, 10 figure
Optimization of Short Coherent Control Pulses
The coherent control of small quantum system is considered. For a two-level
system coupled to an arbitrary bath we consider a pulse of finite duration. We
derive the leading and the next-leading order corrections to the evolution
operator due to the non-commutation of the pulse and the bath Hamiltonian. The
conditions are computed that make the leading corrections vanish. The pulse
shapes optimized in this way are given for and pulses.Comment: 9 pages, 6 figures; published versio
Der Einfluß des nicht resorbierbaren Disaccharids Laktulose auf die postprandiale Freisetzung von gastroenteropankreatischen Peptiden
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
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
Optimized pulses for the perturbative decoupling of spin and decoherence bath
In the framework of nuclear magnetic resonance, we consider the general
problem of the coherent control of a spin coupled to a bath by means of
composite or continuous pulses of duration . We show explicity
that it is possible to design the pulse in order to achieve a decoupling of the
spin from the bath up to the third order in . The evolution of
the system is separated in the evolution of the spin under the action of the
pulse and of the bath times correction terms. We derive the correction terms
for a general time dependent axis of rotation and for a general coupling
between the spin and the environment. The resulting corrections can be made
vanish by an appropriate design of the pulse. For and pulses, we
demonstrate explicitly that pulses exist which annihilate the first and the
second order corrections even if the bath is fully quantum mechanical, i.e., it
displays internal dynamics. Such pulses will also be useful for quantum
information processing.Comment: 9 pages, 7 figures. Published versio
Transport Control in Low-Dimensional Spin-1/2 Heisenberg Systems
We analyze transport of local magnetization and develop schemes to control
transport behavior in finite spin-1/2 Heisenberg chains and spin-1/2 Heisenberg
two-leg ladders at zero temperature. By adjusting parameters in the
Hamiltonians, these quantum systems may show both integrable and chaotic
limits. We provide examples of chaotic systems leading to diffusive and to
ballistic transport. In addition, methods of coherent quantum control to induce
a transition from diffusive to ballistic transport are proposed.Comment: 8 pages, 5 figure
Manipulation of the dynamics of many-body systems via quantum control methods
We investigate how dynamical decoupling methods may be used to manipulate the
time evolution of quantum many-body systems. These methods consist of sequences
of external control operations designed to induce a desired dynamics. The
systems considered for the analysis are one-dimensional spin-1/2 models, which,
according to the parameters of the Hamiltonian, may be in the integrable or
non-integrable limits, and in the gapped or gapless phases. We show that an
appropriate control sequence may lead a chaotic chain to evolve as an
integrable chain and a system in the gapless phase to behave as a system in the
gapped phase. A key ingredient for the control schemes developed here is the
possibility to use, in the same sequence, different time intervals between
control operations.Comment: 10 pages, 3 figure
Direct Observation of Field-Induced Incommensurate Fluctuations in a One-Dimensional S=1/2 Antiferromagnet
Neutron scattering from copper benzoate, Cu(C6D5COO)2 3D2O, provides the
first direct experimental evidence for field-dependent incommensurate low
energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes
occur for wavevectors q=\pi +- dq(H) where dq(H) ~ 2 \pi M(H)/g\mu_B as
predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain.
Unexpected was a field-induced energy gap , where
as determined from specific heat measurements. At H = 7 T
(g\mu_B H/J = 0.52), the magnitude of the gap varies from 0.06 - 0.3 J
depending on the orientation of the applied field.Comment: 11 pages, 5 postscript figures, LaTeX, Submitted to PRL 3/31/97,
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Search for CP violation in D+→ϕπ+ and D+s→K0Sπ+ decays
A search for CP violation in D + → ϕπ + decays is performed using data collected in 2011 by the LHCb experiment corresponding to an integrated luminosity of 1.0 fb−1 at a centre of mass energy of 7 TeV. The CP -violating asymmetry is measured to be (−0.04 ± 0.14 ± 0.14)% for candidates with K − K + mass within 20 MeV/c 2 of the ϕ meson mass. A search for a CP -violating asymmetry that varies across the ϕ mass region of the D + → K − K + π + Dalitz plot is also performed, and no evidence for CP violation is found. In addition, the CP asymmetry in the D+s→K0Sπ+ decay is measured to be (0.61 ± 0.83 ± 0.14)%
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