Solid-state circuit for spin entanglement generation and purification

We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.Comment: 5 pages, 2 figures; corrected minor typo

Universality of Uhrig dynamical decoupling for suppressing qubit pure dephasing and relaxation

The optimal $N$-pulse dynamical decoupling discovered by Uhrig for a spin-boson mmodel [Phys. Rev. Lett, {\bf 98}, 100504 (2007)] is proved to be universal in suppressing to $O(T^{N+1})$ the pure dephasing or the longitudinal relaxation of a qubit (or spin-1/2) coupled to a generic bath in a short-time evolution of duration $T$. It is also found that for the purpose of suppressing the longitudinal relaxation, an ideal Uhrig $\pi$-pulse sequence can be generalized to a sequence consisting of the ideal one superimposed with finite-duration pulses satisfying certain symmetry requirements.Comment: 4 pages, 1 figure

Structure Learning in a Sensorimotor Association Task

Learning is often understood as an organism's gradual acquisition of the association between a given sensory stimulus and the correct motor response. Mathematically, this corresponds to regressing a mapping between the set of observations and the set of actions. Recently, however, it has been shown both in cognitive and motor neuroscience that humans are not only able to learn particular stimulus-response mappings, but are also able to extract abstract structural invariants that facilitate generalization to novel tasks. Here we show how such structure learning can enhance facilitation in a sensorimotor association task performed by human subjects. Using regression and reinforcement learning models we show that the observed facilitation cannot be explained by these basic models of learning stimulus-response associations. We show, however, that the observed data can be explained by a hierarchical Bayesian model that performs structure learning. In line with previous results from cognitive tasks, this suggests that hierarchical Bayesian inference might provide a common framework to explain both the learning of specific stimulus-response associations and the learning of abstract structures that are shared by different task environments

Familiale Lebensführung bei Familien mit Kindern mit Autismus-Spektrum-Störungen

Die aktuelle Situation von Familien mit behinderten Kindern ist gekennzeichnet durch Veränderungen, die mit gesamtgesellschaftlichen Entwicklungen im Zusammenhang stehen. Ausgehend von einer Umbruchsituation in der Gesellschaft hin zur Lebensgestaltung von Individuen hat sich ein familialer Wandel vollzogen, der auch Familien mit Kindern mit Autismus Spektrum Störungen trifft. Die Forschung zum Thema Autismus wird intensiv und breit geführt. Jedoch gibt es bisher wenige Erkenntnisse wie Familien den heutigen Alltag inszenieren, damit sie den Bedürfnissen ihrer Kinder mit Autismus gerecht werden. Zentral bei der vorliegenden Untersuchung ist das Konzept der ‚Familialen Lebensführung‘, anhand dessen die Gestaltungsprozesse der Familie in Hinblick auf Koordination und Synchronisation des Familienlebens, der Ansprüche der einzelnen Familienmitglieder sowie die Organisation von Gemeinsamkeiten aufgezeigt werden. Diese qualitative Studie zeigt mittels episodischer Interviews und deren Auswertung nach der Methode der Qualitativen Inhaltsanalyse auf, dass z.B. durch die Notwendigkeit von Anpassung sowie dem Einsatz von bewussten Strategien zur Planung und Vereinbarkeit der Lebensführungen, sich die Prozesse hinsichtlich der Organisation und Aushandlung des Familienlebens verstärken. Die Ergebnisse führen u.a. zu der Forderung die familiale Lebensführung dieser Familien im Kontext der Unterstützungsleistungen vermehrt zu gewichten

Influence of local fullerene orientation on the electronic properties of A3C60 compounds

We have investigated sodium containing fullerene superconductors Na2AC60, A = Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T in the temperature range of 10 to 400 K. Despite the structural differences from the Rb3C60 class of fullerene superconductors, in these compounds the NMR line of the tetrahedrally coordinated alkali nuclei also splits into two lines (T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80 K. Detailed investigations of the spectrum, spin-spin and spin-lattice relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show that these two different tetrahedral sites are mixed on a microscopic scale. The T and T' sites differ in the orientation of first-neighbor C60 molecules. We present evidence that the orientations of neighboring molecules are uncorrelated. Thermally activated molecular reorientations cause an exchange between the T and T' sites and motional narrowing at high temperature. We infer the same activation energy, 3300 K, in the temperature range 125 to 300 K. The spin lattice relaxation rate is the same for T and T' down to 125 K but different below. Both the spin-lattice relaxation rate and Knight shift are strongly temperature dependent in the whole range investigated. We interpret this temperature variation by the effect of phonon excitations involving the rigid librational motion of the C60 molecules. By extending the understanding of the structure and molecular dynamics of C60 superconductors, these results may help in clarifying the effects of the structure on the superconducting properties.Comment: 13 pages, 10 figures, submitted to PR

Controlling coherence using the internal structure of hard pi pulses

The tiny difference between hard pi pulses and their delta-function approximation can be exploited to control coherence. Variants on the magic echo that work despite a large spread in resonance offsets are demonstrated using the zeroth- and first-order average Hamiltonian terms, for 13-C NMR in C60. The 29-Si NMR linewidth of Silicon has been reduced by a factor of about 70,000 using this approach, which also has potential applications in magnetic resonance microscopy and imaging of solids.Comment: 4 pages, 4 color figure

Long-lived memory for mesoscopic quantum bits

We describe a technique to create long-lived quantum memory for quantum bits in mesoscopic systems. Specifically we show that electronic spin coherence can be reversibly mapped onto the collective state of the surrounding nuclei. The coherent transfer can be efficient and fast and it can be used, when combined with standard resonance techniques, to reversibly store coherent superpositions on the time scale of seconds. This method can also allow for ``engineering'' entangled states of nuclear ensembles and efficiently manipulating the stored states. We investigate the feasibility of this method through a detailed analysis of the coherence properties of the system.Comment: 4 pages, 2 figure