603 research outputs found
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
Symmetry-Enhanced Performance of Dynamical Decoupling
We consider a system with general decoherence and a quadratic dynamical
decoupling sequence (QDD) for the coherence control of a qubit coupled to a
bath of spins. We investigate the influence of the geometry and of the initial
conditions of the bath on the performance of the sequence. The overall
performance is quantified by a distance norm . It is expected that
scales with , the total duration of the sequence, as , where and are the number of pulses of the outer
and of the inner sequence, respectively. We show both numerically and
analytically that the state of the bath can boost the performance of QDD under
certain conditions: The scaling of QDD for a given number of pulses can be
enhanced by a factor of 2 if the bath is prepared in a highly symmetric state
and if the system Hamiltonian is SU(2) invariant.Comment: 9 pages, 4 figures, published versio
Optimized Dynamical Decoupling for Time Dependent Hamiltonians
The validity of optimized dynamical decoupling (DD) is extended to
analytically time dependent Hamiltonians. As long as an expansion in time is
possible the time dependence of the initial Hamiltonian does not affect the
efficiency of optimized dynamical decoupling (UDD, Uhrig DD). This extension
provides the analytic basis for (i) applying UDD to effective Hamiltonians in
time dependent reference frames, for instance in the interaction picture of
fast modes and for (ii) its application in hierarchical
DD schemes with pulses about two perpendicular axes in spin space. to
suppress general decoherence, i.e., longitudinal relaxation and dephasing.Comment: 5 pages, no figure
Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling
Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon
dispersion and an unexpected, low-lying second mode. The proximity and
intensity of these modes suggest a frustrated coupling between the alternating
spin chains. We deduce the minimal model containing such a frustration, and
show that it gives an excellent account of the magnon dispersion, static
susceptibility and electron spin resonance absorption. We consider two-magnon
states which bind due to frustration, and demonstrate that these may provide a
consistent explanation for the second mode.Comment: RevTeX, 5 pages, 6 figures, compressed from first versio
The fate of orbitons coupled to phonons
The key feature of an orbital wave or orbiton is a significant dispersion,
which arises from exchange interactions between orbitals on distinct sites. We
study the effect of a coupling between orbitons and phonons in one dimension
using continuous unitary transformations (CUTs). Already for intermediate
values of the coupling, the orbiton band width is strongly reduced and the
spectral density is dominated by an orbiton-phonon continuum. However, we find
sharp features within the continuum and an orbiton-phonon anti-bound state
above. Both show a significant dispersion and should be observable
experimentally.Comment: 7 pages, 7 figures; strongly enlarged, comprehensive revised version
according to the referees' suggestions, in pres
Thermodynamics of Adiabatically Loaded Cold Bosons in the Mott Insulating Phase of One-Dimensional Optical Lattices
In this work we give a consistent picture of the thermodynamic properties of
bosons in the Mott insulating phase when loaded adiabatically into
one-dimensional optical lattices. We find a crucial dependence of the
temperature in the optical lattice on the doping level of the Mott insulator.
In the undoped case, the temperature is of the order of the large onsite
Hubbard interaction. In contrast, at a finite doping level the temperature
jumps almost immediately to the order of the small hopping parameter. These two
situations are investigated on the one hand by considering limiting cases like
the atomic limit and the case of free fermions. On the other hand, they are
examined using a quasi-particle conserving continuous unitary transformation
extended by an approximate thermodynamics for hardcore particles.Comment: 10 pages, 6 figure
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