Efficient quantum computation within a disordered Heisenberg spin-chain

We show that efficient quantum computation is possible using a disordered Heisenberg spin-chain with `always-on' couplings. Such disorder occurs naturally in nanofabricated systems. Considering a simple chain setup, we show that an arbitrary two-qubit gate can be implemented using just three relaxations of a controlled qubit, which amounts to switching the on-site energy terms at most twenty-one times.Comment: To appear in Phys. Rev.

Semimetalic graphene in a modulated electric potential

The $\pi$-electronic structure of graphene in the presence of a modulated electric potential is investigated by the tight-binding model. The low-energy electronic properties are strongly affected by the period and field strength. Such a field could modify the energy dispersions, destroy state degeneracy, and induce band-edge states. It should be noted that a modulated electric potential could make semiconducting graphene semimetallic, and that the onset period of such a transition relies on the field strength. There exist infinite Fermi-momentum states in sharply contrast with two crossing points (Dirac points) for graphene without external fields. The finite density of states (DOS) at the Fermi level means that there are free carriers, and, at the same time, the low DOS spectrum exhibits many prominent peaks, mainly owing to the band-edge states.Comment: 12pages, 5 figure

Site-resolved imaging of a fermionic Mott insulator

The complexity of quantum many-body systems originates from the interplay of strong interactions, quantum statistics, and the large number of quantum-mechanical degrees of freedom. Probing these systems on a microscopic level with single-site resolution offers important insights. Here we report site-resolved imaging of two-component fermionic Mott insulators, metals, and band insulators using ultracold atoms in a square lattice. For strong repulsive interactions we observe two-dimensional Mott insulators containing over 400 atoms. For intermediate interactions, we observe a coexistence of phases. From comparison to theory we find trap-averaged entropies per particle of $1.0\,k_{\mathrm{B}}$. In the band-insulator we find local entropies as low as $0.5\,k_{\mathrm{B}}$. Access to local observables will aid the understanding of fermionic many-body systems in regimes inaccessible by modern theoretical methods.Comment: 6+7 page

Influence of retardation effects on 2D magnetoplasmon spectrum

Within dissipationless limit the magnetic field dependence of magnetoplasmon spectrum for unbounded 2DEG system found to intersect the cyclotron resonance line, and, then approaches the frequency given by light dispersion relation. Recent experiments done for macroscopic disc-shape 2DEG systems confirm theory expectations.Comment: 2 pages,2 figure

Chiral fermions on the lattice and index relations

Comparing recent lattice results on chiral fermions and old continuum results for the index puzzling questions arise. To clarify this issue we start with a critical reconsideration of the results on finite lattices. We then work out various aspects of the continuum limit. After determining bounds and norm convergences we obtain the limit of the anomaly term. Collecting our results the index relation of the quantized theory gets established. We then compare in detail with the Atiyah-Singer theorem. Finally we analyze conventional continuum approaches.Comment: 34 pages; a more detaild introduction and a subsection with remarks on literature adde

Latent Causal Socioeconomic Health Index

This research develops a model-based LAtent Causal Socioeconomic Health (LACSH) index at the national level. We build upon the latent health factor index (LHFI) approach that has been used to assess the unobservable ecological/ecosystem health. This framework integratively models the relationship between metrics, the latent health, and the covariates that drive the notion of health. In this paper, the LHFI structure is integrated with spatial modeling and statistical causal modeling, so as to evaluate the impact of a continuous policy variable (mandatory maternity leave days and government's expenditure on healthcare, respectively) on a nation's socioeconomic health, while formally accounting for spatial dependency among the nations. A novel visualization technique for evaluating covariate balance is also introduced for the case of a continuous policy (treatment) variable. We apply our LACSH model to countries around the world using data on various metrics and potential covariates pertaining to different aspects of societal health. The approach is structured in a Bayesian hierarchical framework and results are obtained by Markov chain Monte Carlo techniques.Comment: 31 pages. arXiv admin note: substantial text overlap with arXiv:1911.0051

Orbital Ferromagnetism and Quantum Collapse in Stellar Plasmas

The possibility of quantum collapse and characteristics of nonlinear localized excitations is examined in dense stars with Landau orbital ferromagnetism in the framework of conventional quantum magnetohydrodynamics (QMHD) model including Bohm force and spin-orbit polarization effects. Employing the concepts of effective potential and Sagdeev pseudopotential, it is confirmed that the quantum collapse and Landau orbital ferromagnetism concepts are consistent with the magnetic field and mass-density range present in some white dwarf stars. Furthermore, the value of ferromagnetic-field found in this work is about the same order of magnitude as the values calculated earlier. It is revealed that the magnetosonic nonlinear propagations can behave much differently in the two distinct non-relativistic and relativistic degeneracy regimes in a ferromagnetic dense astrophysical object. Current findings should help to understand the origin of the most important mechanisms such as gravitational collapse and the high magnetic field present in many compact stars.Comment: To appear in journal Physics of Plasma