High Order Coherent Control Sequences of Finite-Width Pulses

The performance of sequences of designed pulses of finite length $\tau$ 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 $\mathcal{O}(\tau^3)$, 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

Concatenated Control Sequences based on Optimized Dynamic Decoupling

Two recent developments in quantum control, concatenation and optimization of pulse intervals, are combined to yield a strategy to suppress unwanted couplings in quantum systems to high order. Longitudinal relaxation and transverse dephasing can be suppressed so that systems with a small splitting between their energy levels can be kept isolated from their environment. The required number of pulses grows exponentially with the desired order but is only the square root of the number needed if only concatenation is used. An approximate scheme even brings the number down to polynomial growth. The approach is expected to be useful for quantum information and for high-precision nuclear magnetic resonance.Comment: 4 pages, 1 figure, slightly modified incl. new abstract and title; to appear in Phys. Rev. Let

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

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 $\pi$ 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

Fractional and Integer Excitations in Quantum Antiferromagnetic Spin 1/2 Ladders

Spectral densities are computed in unprecedented detail for quantum antiferromagnetic spin 1/2 two-leg ladders. These results were obtained due to a major methodical advance achieved by optimally chosen unitary transformations. The approach is based on dressed integer excitations. Considerable weight is found at high energies in the two-particle sector. Precursors of fractional spinon physics occur implying that there is no necessity to resort to fractional excitations in order to describe features at higher energies.Comment: 6 pages, 4 figures included, minor text changes, improved figure