Spin squeezing and pairwise entanglement for symmetric multiqubit states

We show that spin squeezing implies pairwise entanglement for arbitrary symmetric multiqubit states. If the squeezing parameter is less than or equal to 1, we demonstrate a quantitative relation between the squeezing parameter and the concurrence for the even and odd states. We prove that the even states generated from the initial state with all qubits being spin down, via the one-axis twisting Hamiltonian, are spin squeezed if and only if they are pairwise entangled. For the states generated via the one-axis twisting Hamiltonian with an external transverse field for any number of qubits greater than 1 or via the two-axis counter-twisting Hamiltonian for any even number of qubits, the numerical results suggest that such states are spin squeezed if and only if they are pairwise entangled.Comment: 6 pages. Version 3: Small corrections were mad

Dynamic splitting of a Bose-Einstein Condensate

We study the dynamic process of splitting a condensate by raising a potential barrier in the center of a harmonic trap. We use a two-mode model to describe the phase coherence between the two halves of the condensate. Furthermore, we explicitly consider the spatial dependence of the mode funtions, which varies depending on the potential barrier. This allows to get the tunneling coupling between the two wells and the on-site energy as a function of the barrier height. Moreover we can get some insight on the collective modes which are excited by raising the barrier. We describe the internal and external degrees of freedom by variational ansatz. We distinguish the possible regimes as a function of the characteristic parameters of the problem and identify the adiabaticity conditions.Comment: 17 pages, 8 figure

Small Fermi surface in the one-dimensional Kondo lattice model

We study the one-dimensional Kondo lattice model through the density matrix renormalization group (DMRG). Our results for the spin correlation function indicate the presence of a small Fermi surface in large portions of the phase diagram, in contrast to some previous studies that used the same technique. We argue that the discrepancy is due to the open boundary conditions, which introduce strong charge perturbations that strongly affect the spin Friedel oscillations.Comment: 5 pages, 7 figure

The role of exploitation in the establishment of mutualistic microbial symbioses

Evolutionary theory suggests that the conditions required for the establishment of mutualistic symbioses through mutualism alone are highly restrictive, often requiring the evolution of complex stabilising mechanisms. Exploitation, whereby initially the host benefits at the expense of its symbiotic partner and mutual benefits evolve subsequently through trade-offs, offers an arguably simpler route to the establishment of mutualistic symbiosis. In this review, we discuss the theoretical and experimental evidence supporting a role for host exploitation in the establishment and evolution of mutualistic microbial symbioses, including data from both extant and experimentally evolved symbioses. We conclude that exploitation rather than mutualism may often explain the origin of mutualistic microbial symbioses

Comparison of independent evolutionary origins reveals both convergence and divergence in the metabolic mechanisms of symbiosis

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record Through the merger of previously independent lineages, symbiosis promotes the acquisition of new traits and exploitation of inaccessible ecological niches [1, 2], driving evolutionary innovation and important ecosystem functions [3–6]. The transient nature of establishment makes study of symbiotic origins difficult, but experimental comparison of independent origins could reveal the degree of convergence in the underpinning mechanisms [7, 8]. We compared the metabolic mechanisms of two independent origins of Paramecium bursaria-Chlorella photosymbiosis [9–11] using a reciprocal metabolomic pulse-chase method. This showed convergent patterns of nutrient exchange and utilization for host-derived nitrogen in the Chlorella genotypes [12, 13] and symbiont-derived carbon in the P. bursaria genotypes [14, 15]. Consistent with a convergent primary nutrient exchange, partner-switched host-symbiont pairings were functional. Direct competition of hosts containing native or recombined symbionts against isogenic symbiont-free hosts showed that the fitness benefits of symbiosis for hosts increased with irradiance but varied by genotype. Global metabolism varied more between the Chlorella than the P. bursaria genotypes and suggested divergent mechanisms of light management. Specifically, the algal symbiont genotypes either produced photo-protective carotenoid pigments at high irradiance or more chlorophyll, resulting in corresponding differences in photosynthetic efficiency and non-photochemical quenching among host-symbiont pairings. These data suggest that the multiple origins of P. bursaria-Chlorella symbiosis use a convergent nutrient exchange, whereas other photosynthetic traits linked to functioning of photosymbiosis have diverged. Although convergence enables partner switching among diverse strains, phenotypic mismatches resulting from divergence of secondary symbiotic traits could mediate host-symbiont specificity in nature. Sørensen et al. compare multiple independent evolutionary origins of Paramecium-Chlorella symbiosis to reveal the underpinning metabolic mechanisms. Although the independent origins use a convergent nutrient exchange, they have diverged in traits linked to photosynthesis, which could mediate host-symbiont specificity in nature.Natural Environment Research CouncilBiotechnology and Biological Sciences Research Counci

Dynamically turning off interactions in a two component condensate

We propose a mechanism to change the interaction strengths of a two component condensate. It is shown that the application of pi/2 pulses allows to alter the effective interspecies interaction strength as well as the effective interaction strength between particles of the same kind. This mechanism provides a simple method to transform spatially stable condensates into unstable once and vice versa. It also provides a means to store a squeezed spin state by turning off the interaction for the internal states and thus allows to gain control over many body entangled states.Comment: 7 pages 5 figures, symbols changed, minor changes, to appear in Phys. Rev.

Dispersive Manipulation of Paired Superconducting Qubits

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum information processing (QIP) on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias.Comment: 5 pages, incl. 1 figur

Series study of the One-dimensional S-T Spin-Orbital Model

We use perturbative series expansions about a staggered dimerized ground state to compute the ground state energy, triplet excitation spectra and spectral weight for a one-dimensional model in which each site has an S=\case 1/2 spin ${\bf S}_i$ and a pseudospin ${\bf T}_i$, representing a doubly degenerate orbital. An explicit dimerization is introduced to allow study of the confinement of spinon excitations. The elementary triplet represents a bound state of two spinons, and is stable over much of the Brillouine zone. A special line is found in the gapped spin-liquid phase, on which the triplet excitation is dispersionless. The formation of triplet bound states is also investigated.Comment: 9 pages, 9 figure

Encoded Recoupling and Decoupling: An Alternative to Quantum Error Correcting Codes, Applied to Trapped Ion Quantum Computation

A recently developed theory for eliminating decoherence and design constraints in quantum computers, ``encoded recoupling and decoupling'', is shown to be fully compatible with a promising proposal for an architecture enabling scalable ion-trap quantum computation [D. Kielpinski et al., Nature 417, 709 (2002)]. Logical qubits are encoded into pairs of ions. Logic gates are implemented using the Sorensen-Molmer (SM) scheme applied to pairs of ions at a time. The encoding offers continuous protection against collective dephasing. Decoupling pulses, that are also implemented using the SM scheme directly to the encoded qubits, are capable of further reducing various other sources of qubit decoherence, such as due to differential dephasing and due to decohered vibrational modes. The feasibility of using the relatively slow SM pulses in a decoupling scheme quenching the latter source of decoherence follows from the observed 1/f spectrum of the vibrational bath.Comment: 12 pages, no figure