Simulations of Information Transport in Spin Chains

Transport of quantum information in linear spin chains has been the subject of much theoretical work. Experimental studies by nuclear spin systems in solid-state by NMR (a natural implementation of such models) is complicated since the dipolar Hamiltonian is not solely comprised of nearest-neighbor XY-Heisenberg couplings. We present here a similarity transformation between the XY-Heisenberg Hamiltonian and the grade raising Hamiltonian, an interaction which is achievable with the collective control provided by radio-frequency pulses in NMR. Not only does this second Hamiltonian allows us to simulate the information transport in a spin chain, but it also provides a means to observe its signature experimentally

On Simulating Liouvillian Flow From Quantum Mechanics Via Wigner Functions

The interconnection between quantum mechanics and probabilistic classical mechanics for a free relativistic particle is derived in terms of Wigner functions (WF) for both Dirac and Klein-Gordon (K-G) equations. Construction of WF is achieved by first defining a bilocal 4-current and then taking its Fourier transform w.r.t. the relative 4-coordinate. The K-G and Proca cases also lend themselves to a closely parallel treatment provided the Kemmer- Duffin beta-matrix formalism is employed for the former. Calculation of WF is carried out in a Lorentz-covariant fashion by standard `trace' techniques. The results are compared with a recent derivation due to Bosanac.Comment: 9 pages, Latex; email: [email protected]

Universal Control of Nuclear Spins Via Anisotropic Hyperfine Interactions

We show that nuclear spin subsystems can be completely controlled via microwave irradiation of resolved anisotropic hyperfine interactions with a nearby electron spin. Such indirect addressing of the nuclear spins via coupling to an electron allows us to create nuclear spin gates whose operational time is significantly faster than conventional direct addressing methods. We experimentally demonstrate the feasibility of this method on a solid-state ensemble system consisting of one electron and one nuclear spin.Comment: RevTeX4, 8 pages, 8 figure

Universal Dephasing Control During Quantum Computation

Dephasing is a ubiquitous phenomenon that leads to the loss of coherence in quantum systems and the corruption of quantum information. We present a universal dynamical control approach to combat dephasing during all stages of quantum computation, namely, storage, single- and two-qubit operators. We show that (a) tailoring multi-frequency gate pulses to the dephasing dynamics can increase fidelity; (b) cross-dephasing, introduced by entanglement, can be eliminated by appropriate control fields; (c) counter-intuitively and contrary to previous schemes, one can increase the gate duration, while simultaneously increasing the total gate fidelity.Comment: 4 pages,3 figure

Spin diffusion of correlated two-spin states in a dielectric crystal

Reciprocal space measurements of spin diffusion in a single crystal of calcium fluoride (CaF$_2$) have been extended to dipolar ordered states. The experimental results for the component of the spin diffusion parallel with the external field are $D_{D}^{||}=29 \pm 3 \times 10^{-12}$ cm$^{2}$/s for the [001] direction and $D_{D}^{||}=33 \pm 4 \times 10^{-12}$ cm$^{2}$/s for the [111] direction. The diffusion rates for dipolar order are significantly faster than those for Zeeman order and are considerably faster than predicted by simple theoretical models. It is suggested that constructive interference in the transport of the two spin state is responsible for this enhancement. As expected the anisotropy in the diffusion rates is observed to be significantly less for dipolar order compared to the Zeeman case.Comment: 4 pages, 2 figures. Resubmitted to PRL - new figure added / discussion expande

Investigating the generalizability of EEG-based Cognitive Load Estimation Across Visualizations

We examine if EEG-based cognitive load (CL) estimation is generalizable across the character, spatial pattern, bar graph and pie chart-based visualizations for the nback~task. CL is estimated via two recent approaches: (a) Deep convolutional neural network, and (b) Proximal support vector machines. Experiments reveal that CL estimation suffers across visualizations motivating the need for effective machine learning techniques to benchmark visual interface usability for a given analytic task

Chain motion and viscoelasticity in highly entangled solutions of semiflexible rods

Brownian dynamics simulations are used to study highly entangled solutions of semiflexible polymers. Bending fluctuations of semiflexible rods are signficantly affected by entanglement only above a concentration $c^{**}$, where $c^{**}\sim 10^{3}L^{-3}$ for chains of similar length $L$ and persistence length. For $c > c^{**}$, the tube radius $R_{e}$ approaches a dependence $R_{e} \propto c^{-3/5}$, and the linear viscoelastic response develops an elastic contribution that is absent for $c < c^{**}$. Experiments on isotropic solutions of $F$-actin span concentrations near $c^{**}$ for which the predicted asymptotic scaling of the plateau modulus $G \propto c^{7/5}$ is not yet valid.Comment: 4 pages, 5 figures, submitted to PR

Observation of a 2D Bose-gas: from thermal to quasi-condensate to superfluid

We present experimental results on a Bose gas in a quasi-2D geometry near the Berezinskii, Kosterlitz and Thouless (BKT) transition temperature. By measuring the density profile, \textit{in situ} and after time of flight, and the coherence length, we identify different states of the gas. In particular, we observe that the gas develops a bimodal distribution without long range order. In this state, the gas presents a longer coherence length than the thermal cloud; it is quasi-condensed but is not superfluid. Experimental evidence indicates that we observe the superfluid transition (BKT transition).Comment: 5 pages, 6 figure