Freezing distributed entanglement in spin chains

We show how to freeze distributed entanglement that has been created from the natural dynamics of spin chain systems. The technique that we propose simply requires single-qubit operations and isolates the entanglement in specific qubits at the ends of branches. Such frozen entanglement provides a useful resource, for example for teleportation or distributed quantum processing. The scheme can be applied to a wide range of systems -- including actual spin systems and alternative qubit embodiments in strings of quantum dots, molecules or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication

Entanglement distribution for a practical quantum-dot-based quantum processor architecture

We propose a quantum dot (QD) architecture for enabling universal quantum information processing. Quantum registers, consisting of arrays of vertically stacked self-assembled semiconductor QDs, are connected by chains of in-plane self-assembled dots. We propose an entanglement distributor, a device for producing and distributing maximally entangled qubits on demand, communicated through in-plane dot chains. This enables the transmission of entanglement to spatially separated register stacks, providing a resource for the realization of a sizeable quantum processor built from coupled register stacks of practical size. Our entanglement distributor could be integrated into many of the present proposals for self-assembled QD-based quantum computation (QC). Our device exploits the properties of simple, relatively short, spin-chains and does not require microcavities. Utilizing the properties of self-assembled QDs, after distribution the entanglement can be mapped into relatively long-lived spin qubits and purified, providing a flexible, distributed, off-line resource. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Convergence of the expansion of the Laplace-Borel integral in perturbative QCD improved by conformal mapping

The optimal conformal mapping of the Borel plane was recently used to accelerate the convergence of the perturbation expansions in QCD. In this work we discuss the relevance of the method for the calculation of the Laplace-Borel integral expressing formally the QCD Green functions. We define an optimal expansion of the Laplace-Borel integral in the principal value prescription and establish conditions under which the expansion is convergent.Comment: 10 pages, no figure

High orders of perturbation theory: are renormalons significant?

According to Lipatov, the high orders of perturbation theory are determined by saddle-point configurations (instantons) of the corresponding functional integrals. According to t'Hooft, some individual large diagrams, renormalons, are also significant and they are not contained in the Lipatov contribution. The history of the conception of renormalons is presented, and the arguments in favor of and against their significance are discussed. The analytic properties of the Borel transforms of functional integrals, Green functions, vertex parts, and scaling functions are investigated in the case of \phi^4 theory. Their analyticity in a complex plane with a cut from the first instanton singularity to infinity (the Le Guillou - Zinn-Justin hypothesis) is proved. It rules out the existence of the renormalon singularities pointed out by t'Hooft and demonstrates the nonconstructiveness of the conception of renormalons as a whole. The results can be interpreted as an indication of the internal consistency of \phi^4 theory.Comment: 28 pages, 8 figures include

Infrared renormalons and analyticity structure in pQCD

Relation between the infrared renormalons, the Borel resummation prescriptions, and the analyticity structure of Green functions in perturbative QCD (pQCD) is investigated. A specific recently suggested Borel resummation prescription resulted in the Principal Value and an additional power-suppressed correction that is consistent with the Operator Product Expansion. Arguments requiring the finiteness of the result for any power coefficient of the leading infrared renormalon, and the consistency in the case of the absence of that renormalon, require that this prescription be modified. The apparently most natural modification leads to the result represented by the Principal Value. The analytic structure of the amplitude in the complex coupling plane, obtained in this way, is consistent with that obtained in the literature by other methods.Comment: 6 pages, revtex4, 1 eps-figure; improved version - the paragraph containing Eqs.(18) and (19) is new, as well as the next paragraph; the Title modified; some references added; version to appear in Phys. Rev.

On the global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power-laws in time, $x/(1-x)\sim (t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration ($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi = 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We interpret these results in terms of a global second order rate equation indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b) they inhibit further hydration for $t > t_x$ and (c) the value of the associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6} liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually being furnished as a diffusion limited precipitation that the exponents $\psi = 5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure

Supercooled vortex liquid and quantitative theory of melting of the flux line lattice in type II superconductors

A metastable homogeneous state exists down to zero temperature in systems of repelling objects. Zero ''fluctuation temperature'' liquid state therefore serves as a (pseudo) ''fixed point'' controlling the properties of vortex liquid below and even around melting point. There exists Madelung constant for the liquid in the limit of zero temperature which is higher than that of the solid by an amount approximately equal to the latent heat of melting. This picture is supported by an exactly solvable large $N$ Ginzburg - Landau model in magnetic field. Based on this understanding we apply Borel - Pade resummation technique to develop a theory of the vortex liquid in type II superconductors. Applicability of the effective lowest Landau level model is discussed and corrections due to higher levels is calculated. Combined with previous quantitative description of the vortex solid the melting line is located. Magnetization, entropy and specific heat jumps along it are calculated. The magnetization of liquid is larger than that of solid by $1.8$ irrespective of the melting temperature. We compare the result with experiments on high $T_{c}$ cuprates $YBa_{2}Cu_{3}O_{7}$, $DyBCO$, low $T_{c}$ material $(K,Ba)BiO_{3}$ and with Monte Carlo simulations.Comment: 28 pages and 4 figures. Enlarged version of paper cond-mat/0107281 with many new content

Recent Field Studies of Dry Deposition to Surfaces in Plant Canopies

A variety of field techniques were used to assess the dry deposition of sulfur. In a deciduous forest canopy in eastern Tennessee, inert petri plates and adjacent chestnut oak leaves showed similar SO/sub 4//sup -2/ deposition velocities of about 0.1 cm s/sup -1/. In the same forest, statistical analysis of throughfall yielded a deposition velocity of 0.48 cm s/sup -1/ for total sulfur (SO/sub 4//sup -2/ plus SO/sub 2/). The throughfall technique appears useful for scaling individual surface measurements to larger spatial and temporal scales. On a grassy field in Illinois, flat Teflon plates, petri dishes, and dustfall buckets were exposed side by side. Measured sulfate deposition increased with increasing rim height on the collection surface, and deposition velocities ranged from 0.14 to 0.70 cm s/sup -1/. Much of the deposition to these surfaces can be attributed to large-particle SO/sub 4//sup -2/. Dry season (summer) deposition velocities of /sup 7/Be in California were found to be similar to dry deposition velocities of /sup 212/Pb in Tennessee, ranging from 0.18 to 0.35 cm s/sup -1/. These natural radionuclides attach to submicron aerosols in the atmosphere and may be useful tracers of submicron SO/sub 4//sup -2/ deposition. 9 references, 5 figures, 4 tables

High-sensitivity Gd3+-Gd3+ EPR distance measurements that eliminate artefacts seen at short distances

We would like to acknowledge EPSRC (EP/R)13705/1) for current funding on the HiPER project, and the Wellcome Trust for a multi-user equipment grant (099149/Z/12/Z) for upgrades on the Q-band system. We thank the Royal Society for an International Exchanges Grant and The Weizmann-UK Joint Research Program for allowing bilateral travel and research between the University of St Andrews and the Weizmann Institute of Science. JEL thanks the Royal Society for a University Research Fellowship. MJT thanks EPSRC for a CM-CDT studentship (EP/LO15110/1). MQ and AG thank the Deutsche Forschungsgemeinschaft (DFG) for funding within SPP 1601 (GO555/6-2).Gadolinium complexes are attracting increasing attention as spin labels for EPR dipolar distance measurements in biomolecules and particularly for in-cell measurements. It has been shown that flip-flop transitions within the central transition of the high spin Gd3+ ion can introduce artefacts in dipolar distance measurements, particularly when measuring distances less than 3–4 nm. Previous work has shown some reduction of these artefacts through increasing the frequency separation between the two frequencies required for the Double Electron-Electron Resonance (DEER) experiment. Here we use a high power (1 kW), wideband, non-resonant, system operating at 94 GHz to evaluate DEER measurement protocols using two rigid Gd(III)-rulers, consisting of two [GdIII(PyMTA)] complexes, with separations of 2.1 nm and 6.0 nm, respectively. We show that by avoiding the |−1/2⟩ → |1/2⟩ central transition completely, and placing both the pump and the observer pulses on either side of the central transition, we can now observe apparently artefact-free spectra and narrow distance distributions, even for a Gd-Gd distance of 2.1 nm. Importantly we still maintain excellent signal-to-noise ratio and relatively high modulation depths. These results have implications for in-cell EPR measurements at naturally occurring biomolecule concentrations.Publisher PDFPeer reviewe

Spin Star as Switch for Quantum Networks

Quantum state transfer is an important task in quantum information processing. It is known that one can engineer the couplings of a one-dimensional spin chain to achieve the goal of perfect state transfer. To leverage the value of these spin chains, a spin star is potentially useful for connecting different parts of a quantum network. In this work, we extend the spin-chain engineering problem to the problems with a topology of a star network. We show that a permanently coupled spin star can function as a network switch for transferring quantum states selectively from one node to another by varying the local potentials only. Together with one-dimensional chains, this result allows applications of quantum state transfer be applied to more general quantum networks.Comment: 10 pages, 2 figur