1,938 research outputs found

    Universal Quantum Computation with the Exchange Interaction

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    Experimental implementations of quantum computer architectures are now being investigated in many different physical settings. The full set of requirements that must be met to make quantum computing a reality in the laboratory [1] is daunting, involving capabilities well beyond the present state of the art. In this report we develop a significant simplification of these requirements that can be applied in many recent solid-state approaches, using quantum dots [2], and using donor-atom nuclear spins [3] or electron spins [4]. In these approaches, the basic two-qubit quantum gate is generated by a tunable Heisenberg interaction (the Hamiltonian is Hij=J(t)S⃗i⋅S⃗jH_{ij}=J(t){\vec S}_i\cdot{\vec S}_j between spins ii and jj), while the one-qubit gates require the control of a local Zeeman field. Compared to the Heisenberg operation, the one-qubit operations are significantly slower and require substantially greater materials and device complexity, which may also contribute to increasing the decoherence rate. Here we introduce an explicit scheme in which the Heisenberg interaction alone suffices to exactly implement any quantum computer circuit, at a price of a factor of three in additional qubits and about a factor of ten in additional two-qubit operations. Even at this cost, the ability to eliminate the complexity of one-qubit operations should accelerate progress towards these solid-state implementations of quantum computation.Comment: revtex, 2 figures, this version appeared in Natur

    Elevated and sustained intracellular calcium signalling is necessary for efficacious induction of the human sperm acrosome reaction

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    Progesterone and prostaglandin E1 are postulated to trigger the human sperm acrosome reaction (AR). However, their reported efficacy is very variable which likely, in part, reflects the plethora of experimental conditions and methodologies used to detect this physiologically relevant event. The purpose of this study was to develop an assay for the robust induction and objective measurement of the complete AR. Sperm from healthy volunteers or patients undertaking IVF were treated with a variety of ligands (progesterone, prostaglandin E1 or NH4Cl, alone or in combinations). AR, motility and intracellular calcium measurements were measured using flow cytometry, computer-assisted sperm analysis (CASA) and fluorimetry, respectively. The AR was significantly increased by the simultaneous application of progesterone, prostaglandin E1 and NH4Cl, following an elevated and sustained intracellular calcium concentration. However, we observed notable inter- and intra-donor sample heterogeneity of the AR induction. When studying the patient samples, we found no relationship between the IVF fertilization rate and the AR. We conclude that progesterone and prostaglandin E1 alone do not significantly increase the percentage of live acrosome-reacted sperm. This assay has utility for drug discovery and sperm toxicology studies but is not predictive for IVF success

    Modelling Oscillator synchronisation during vertebrate axis segmentation

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    he somitogenesis clock regulates the periodicity with which somites form in the posterior pre-somitic mesoderm. Whilst cell heterogeneity results in noisy oscillation rates amongst constituent cells, synchrony within the population is maintained as oscillators are entrained via juxtracine signalling mechanisms. Here we consider a population of phase-coupled oscillators and investigate how biologically motivated perturbations to the entrained state can perturb synchrony within the population. We find that the ratio of mitosis length to clock period can influence levels of desynchronisation. Moreover, we observe that random cell movement, and hence change of local neighbourhoods, increases synchronisation

    Strain-controlled criticality governs the nonlinear mechanics of fibre networks

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    Disordered fibrous networks are ubiquitous in nature as major structural components of living cells and tissues. The mechanical stability of networks generally depends on the degree of connectivity: only when the average number of connections between nodes exceeds the isostatic threshold are networks stable (Maxwell, J. C., Philosophical Magazine 27, 294 (1864)). Upon increasing the connectivity through this point, such networks undergo a mechanical phase transition from a floppy to a rigid phase. However, even sub-isostatic networks become rigid when subjected to sufficiently large deformations. To study this strain-controlled transition, we perform a combination of computational modeling of fibre networks and experiments on networks of type I collagen fibers, which are crucial for the integrity of biological tissues. We show theoretically that the development of rigidity is characterized by a strain-controlled continuous phase transition with signatures of criticality. Our experiments demonstrate mechanical properties consistent with our model, including the predicted critical exponents. We show that the nonlinear mechanics of collagen networks can be quantitatively captured by the predictions of scaling theory for the strain-controlled critical behavior over a wide range of network concentrations and strains up to failure of the material

    Qubits from tight knots and bent nano-bars

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    We propose a novel mechanism for creating a qubit based on a tight knot, that is a nano-quantum wire system so small and so cold as to be quantum coherent with respect to curvature-induced effects. To establish tight knots as legitimate candidates for qubits, we propose an effective curvature-induced potential that produces the two-level system and identify the tunnel coupling between the two local states. We propose also a different design of nano-mechanical qubit based on twisted nano-rods. We describe how both devices can be manipulated. Also we outline possible decoherence channels, detection schemes and experimental setups

    Quiet SDS Josephson Junctions for Quantum Computing

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    Unconventional superconductors exhibit an order parameter symmetry lower than the symmetry of the underlying crystal lattice. Recent phase sensitive experiments on YBCO single crystals have established the d-wave nature of the cuprate materials, thus identifying unambiguously the first unconventional superconductor. The sign change in the order parameter can be exploited to construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting a degenerate ground state and a double-periodic current-phase characteristic. Here we discuss how to make use of these special junction characteristics in the construction of a quantum computer. Combining such junctions together with a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit --- a solid state implementation of a quantum bit which remains optimally isolated from its environment.Comment: 4 pages, 2 ps-figure

    Curvature effects on collective excitations in dumbbell-shaped hollow nanotubes

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    We investigate surface-curvature induced alteration in the Tomonaga-Luttinger liquid (TLL) states of a one-dimensional (1D) deformed hollow nanotube with a dumbbell-shape. Periodic variation of the surface curvature along the axial direction is found to enhance the TLL exponent significantly, which is attributed to an effective potential field that acts low-energy electrons moving on the curved surface. The present results accounts for the experimental observation of the TLL properties of 1D metallic peanut-shaped fullerene polymers whose enveloping surface is assumed to be a dumbbell-shaped hollow tube.Comment: 4 pages, 4 figure

    Structure formation in active networks

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    Structure formation and constant reorganization of the actin cytoskeleton are key requirements for the function of living cells. Here we show that a minimal reconstituted system consisting of actin filaments, crosslinking molecules and molecular-motor filaments exhibits a generic mechanism of structure formation, characterized by a broad distribution of cluster sizes. We demonstrate that the growth of the structures depends on the intricate balance between crosslinker-induced stabilization and simultaneous destabilization by molecular motors, a mechanism analogous to nucleation and growth in passive systems. We also show that the intricate interplay between force generation, coarsening and connectivity is responsible for the highly dynamic process of structure formation in this heterogeneous active gel, and that these competing mechanisms result in anomalous transport, reminiscent of intracellular dynamics

    Fetal in vivo continuous cardiovascular function during chronic hypoxia.

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    Although the fetal cardiovascular defence to acute hypoxia and the physiology underlying it have been established for decades, how the fetal cardiovascular system responds to chronic hypoxia has been comparatively understudied. We designed and created isobaric hypoxic chambers able to maintain pregnant sheep for prolonged periods of gestation under controlled significant (10% O2) hypoxia, yielding fetal mean P(aO2) levels (11.5 ± 0.6 mmHg) similar to those measured in human fetuses of hypoxic pregnancy. We also created a wireless data acquisition system able to record fetal blood flow signals in addition to fetal blood pressure and heart rate from free moving ewes as the hypoxic pregnancy is developing. We determined in vivo longitudinal changes in fetal cardiovascular function including parallel measurement of fetal carotid and femoral blood flow and oxygen and glucose delivery during the last third of gestation. The ratio of oxygen (from 2.7 ± 0.2 to 3.8 ± 0.8; P < 0.05) and of glucose (from 2.3 ± 0.1 to 3.3 ± 0.6; P < 0.05) delivery to the fetal carotid, relative to the fetal femoral circulation, increased during and shortly after the period of chronic hypoxia. In contrast, oxygen and glucose delivery remained unchanged from baseline in normoxic fetuses. Fetal plasma urate concentration increased significantly during chronic hypoxia but not during normoxia (Δ: 4.8 ± 1.6 vs. 0.5 ± 1.4 ÎŒmol l(-1), P<0.05). The data support the hypotheses tested and show persisting redistribution of substrate delivery away from peripheral and towards essential circulations in the chronically hypoxic fetus, associated with increases in xanthine oxidase-derived reactive oxygen species.This work was supported by the British Heart Foundation.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1113/JP27109

    The No-Scale Multiverse at the LHC

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    We present a contemporary perspective on the String Landscape and the Multiverse of plausible string, M- and F-theory vacua, seeking to demonstrate a non-zero probability for the existence of a universe matching our own observed physics within the solution ensemble, arguing for the importance of No-Scale Supergravity as an essential common underpinning. Our context is a highly detailed phenomenological probe of No-Scale F-SU(5), a model representing the intersection of the F-lipped SU(5) X U(1)_X Grand Unified Theory (GUT) with extra TeV-Scale vector-like multiplets derived out of F-theory, and the dynamics of No-Scale Supergravity. We present a highly constrained "Golden" region with tan(beta) \sim 15, m_t = 173.0 - 174.4 GeV, M_1/2 = 455 - 481 GeV, and M_V = 691 - 1020 GeV, which simultaneously satisfies all known experimental constraints. We supplement this bottom-up phenomenological perspective with a top-down theoretical analysis of the one-loop effective Higgs potential, achieving a striking consonance via the dynamic determination of tan(beta) and M_1/2 at the local secondary minimization of the spontaneously broken electroweak Higgs vacuum V_min. We present the distinctive signatures of No-Scale F-SU(5) at the LHC, where a light stop and gluino are expected to generate a surplus of ultra-high multiplicity (>= 9) hadronic jet events. We propose modest alterations to the canonical background selection cut strategy which would enhance resolution of these events, while readily suppressing the contribution of all Standard Model processes, and allowing a clear differentiation from competing models of new physics. Detection by the LHC of the ultra-high jet signal would constitute a suggestive evocation of the intimately linked stringy origins of F-SU(5), and could provide a glimpse into the fundamental string moduli, and possibly even the workings of the No-Scale Multiverse.Comment: A review of recent work, submitted to the DICE 2010 Workshop proceedings, based on the invited talk by D.V.N. (20 Pages, 5 Tables, 18 Figures
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