A graphene nanoribbon with sawtooth edges has a ferromagnetic ground state. Using first-principles and tight-binding model calculations, we show that, under a transverse electrical field, the sawtooth graphene nanoribbons become a spin semiconductor whose charge carriers are not only spin polarized in energy space but also spatially separated at different edges. Low-energy excitation produces spin-up electrons localized at one edge and spin-down holes at the opposite edge, and the excitation energy of spin carries can be tuned by the electric field to reach a new state of spin gapless semiconductor. Also, the spin semiconducting states are shown to be robust against at least 10% edge disorder. These features demonstrate a good tunability of spin carriers for spintronics applications.

A scheme to generate long-range spin-spin interactions between three-level ions in a chain is presented, providing a feasible experimental route to the rich physics of well-known $\mathrm{SU}(3)$ models. In particular, we demonstrate different signatures of quantum chaos which can be controlled and observed in experiments with trapped ions.

We propose a new scheme for quantum computation using flying qubits—propagating photons in a one-dimensional waveguide interacting with matter qubits. Photon-photon interactions are mediated by the coupling to a four-level system, based on which photon-photon $\pi$-phase gates (controlled-not) can be implemented for universal quantum computation. We show that high gate fidelity is possible, given recent dramatic experimental progress in superconducting circuits and photonic-crystal waveguides. The proposed system can be an important building block for future on-chip quantum networks.

We propose the implementation of Galileo group symmetry operations or, in general, linear coordinate transformations in a quantum simulator. With an appropriate encoding, unitary gates applied to our quantum system give rise to Galilean boosts or spatial and time parity operations in the simulated dynamics. This framework provides us with a flexible toolbox that enhances the versatility of quantum simulation theory, allowing the direct access to dynamical quantities that would otherwise require full tomography. Furthermore, this method enables the study of noncausal kinematics and phenomena beyond special relativity in a quantum controllable system.

Neutrino-neutrino refraction causes self-induced flavor conversion in dense neutrino fluxes. For the first time, we include the azimuth angle of neutrino propagation as an explicit variable and find a new generic multi-azimuth-angle instability which, for simple spectra, occurs in the normal neutrino mass hierarchy. Matter suppression of this instability in supernovae requires larger densities than the traditional bimodal case. The new instability shows explicitly that solutions of the equations for collective flavor oscillations need not inherit the symmetries of initial or boundary conditions. This change of paradigm requires reconsideration of numerous results in this field.

A sufficiently damped iteration of the Kohn-Sham (KS) equations with the exact functional is proven to always converge to the true ground-state density, regardless of the initial density or the strength of electron correlation, for finite Coulomb systems. We numerically implement the exact functional for one-dimensional continuum systems and demonstrate convergence of the damped KS algorithm. More strongly correlated systems converge more slowly.

Fast electron transport in Si, driven by ultraintense laser pulses, is investigated experimentally and via 3D hybrid particle-in-cell simulations. A transition from a Gaussian-like to an annular fast electron beam profile is demonstrated and explained by resistively generated magnetic fields. The results highlight the potential to completely transform the beam transport pattern by tailoring the resistivity-temperature profile at temperatures as low as a few eV.

We investigate the temperature dependence of the Yb valence in ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{1.8}{\mathrm{Sn}}_{4.2}$ and ${\mathrm{YbMn}}_6{\mathrm{Ge}}_{1.6}{\mathrm{Sn}}_{4.4}$ using resonant inelastic x-ray scattering experiments. Yb is found to be in an intermediate valent state in the whole investigated temperature range (10–450 K). We thus prove that the unusually high magnetic ordering temperature of Yb ($\sim 60$ and 90 K for $x=4.2$ and 4.4, respectively) involves an intermediate valent Yb, an unprecedentedly observed phenomenon. Further, an anomalous increase in the Yb valence is observed upon cooling. A scenario is proposed to explain this unusual behavior. It is based on the presence of magnetically ordered Mn moments and on an Anderson Hamiltonian with a Zeeman term modeling the magnetic interactions.

We demonstrate dynamic stabilization of a strongly interacting quantum spin system realized in a spin-1 atomic Bose-Einstein condensate. The spinor Bose-Einstein condensate is initialized to an unstable fixed point of the spin-nematic phase space, where subsequent free evolution gives rise to squeezing and quantum spin mixing. To stabilize the system, periodic microwave pulses are applied that rotate the spin-nematic many-body fluctuations and limit their growth. The stability diagram for the range of pulse periods and phase shifts that stabilize the dynamics is measured and compares well with a stability analysis.

We study the structure of topological phases and their boundaries in the projected entangled-pair states (PEPS) formalism. We show how topological order in a system can be identified from the structure of the PEPS transfer operator and subsequently use these findings to analyze the structure of the boundary Hamiltonian, acting on the bond variables, which reflects the entanglement properties of the system. We find that in a topological phase, the boundary Hamiltonian consists of two parts: A universal nonlocal part which encodes the nature of the topological phase and a nonuniversal part which is local and inherits the symmetries of the topological model, which helps to infer the structure of the boundary Hamiltonian and thus possibly of the physical edge modes.

We show that the relativistic motion of a quantum system can be used to generate quantum gates. The nonuniform acceleration of a cavity is used to generate well-known two-mode quantum gates in continuous variables. Observable amounts of entanglement between the cavity modes are produced through resonances that appear by repeating periodically any trajectory.

The ultimate stability of atomic clocks is limited by the quantum noise of the atoms. To reduce this noise it has been suggested to use entangled atomic ensembles with reduced atomic noise. Potentially this can push the stability all the way to the limit allowed by the Heisenberg uncertainty relation, which is denoted the Heisenberg limit. In practice, however, entangled states are often more prone to decoherence, which may prevent reaching this performance. Here we present an adaptive measurement protocol that in the presence of a realistic source of decoherence enables us to get near-Heisenberg-limited stability of atomic clocks using entangled atoms. The protocol may thus realize the full potential of entanglement for quantum metrology despite the detrimental influence of decoherence.

Atomic clocks are typically operated by locking a local oscillator (LO) to a single atomic ensemble. In this Letter, we propose a scheme where the LO is locked to several atomic ensembles instead of one. This results in an exponential improvement compared to the conventional method and provides a stability of the clock scaling as $(\alpha N{)}^{-m/2}$ with $N$ being the number of atoms in each of the $m$ ensembles and $\alpha$ a constant depending on the protocol being used to lock the LO.

A gamma-ray excess from the Galactic center consistent with line emission around 130 GeV was recently found in the Fermi-LAT data. Although the Fermi-LAT Collaboration has not confirmed its significance, such a signal would be a clear signature of dark matter annihilation. Until now, there have been many attempts to explain the excess by dark matter. However, these efforts tend to give too-small cross sections into photons if consistency with the correct thermal relic density of dark matter is required. In this Letter, we consider a simple Yukawa interaction that can be compatible with both aspects and show which parameters are favored.

The radiative decay of $J/\psi$ into a pure gauge tensor glueball is studied in the quenched lattice QCD formalism. With two anisotropic lattices, the multipole amplitudes $E_1(0)$, $M_2(0)$, and $E_3(0)$ are obtained to be 0.114(12)(6) GeV, $-0.011(5)(1)\mathrm{GeV}$, and 0.023(8)(1) GeV, respectively. The first error comes from the statistics, the $Q^2$ interpolation, and the continuum extrapolation, while the second is due to the uncertainty of the scale parameter $r_0^{-1}=410(20)\mathrm{MeV}$. Thus, the partial decay width $\Gamma (J/\psi \to \gamma G_{2^{++}})$ is estimated to be 1.01(22)(10) keV, which corresponds to a large branch ratio $1.1(2)(1)\times {10}^{-2}$. The phenomenological implication of this result is also discussed.

We observe a generic connection between LHC Higgs data and electroweak baryogenesis: the particle that contributes to the $CP$-odd $hgg$ or $h\gamma \gamma$ vertex would provide the $CP$-violating source during a first-order phase transition. It is illustrated in the two Higgs doublet model that a common complex phase controls the lightest Higgs properties at the LHC, electric dipole moments, and the $CP$-violating source for electroweak baryogenesis. We perform a general parametrization of Higgs effective couplings and a global fit to the LHC Higgs data. Current LHC measurements prefer a nonzero phase for $\tan \beta \lesssim 1$ and electric dipole moment constraints still allow an order-one phase for $\tan \beta \sim 1$, which gives sufficient room to generate the correct cosmic baryon asymmetry. We also give some prospects in the direct measurements of $CP$ violation in the Higgs sector at the LHC.

Associated production of the Higgs boson with a top-antitop pair is a key channel to gather further information on the nature of the newly discovered boson at the LHC. Experimentally, however, its observation is very challenging due to the combination of small rates, difficult multijet final states, and overwhelming backgrounds. In the standard model, the largest number of events is expected when $h\to b\overline{b}$, giving rise to a $W^{+}{W}^{-}b\overline{b}b\overline{b}$ signature, deluged in $t\overline{t}+\mathrm{\text{jets}}$. A promising strategy to improve the sensitivity is to maximally exploit the theoretical information on the signal and background processes by means of the matrix element method. We show how, despite the complexity of the final state, the method can be efficiently applied to discriminate the signal against combinatorial and $t\overline{t}+\mathrm{\text{jets}}$ backgrounds. Remarkably, we find that a moderate integrated luminosity in the next LHC run will be enough to make the signature involving both $W$’s decaying leptonically as sensitive as the single-lepton one.

A method is explained through which a pointwise accurate approximation to the pion’s valence-quark distribution amplitude (PDA) may be obtained from a limited number of moments. In connection with the single nontrivial moment accessible in contemporary simulations of lattice-regularized QCD, the method yields a PDA that is a broad concave function whose pointwise form agrees with that predicted by Dyson-Schwinger equation analyses of the pion. Under leading-order evolution, the PDA remains broad to energy scales in excess of 100 GeV, a feature which signals persistence of the influence of dynamical chiral symmetry breaking. Consequently, the asymptotic distribution ${\phi }_{\pi }^{\mathrm{asy}}(x)$ is a poor approximation to the pion’s PDA at all such scales that are either currently accessible or foreseeable in experiments on pion elastic and transition form factors. Thus, related expectations based on ${\phi }_{\pi }^{\mathrm{asy}}(x)$ should be revised.

An ab initio calculation of the ${}^{12}\mathrm{C}$ elastic form factor and sum rules of longitudinal and transverse response functions measured in inclusive ($e$, $e^{\prime }$) scattering are reported, based on realistic nuclear potentials and electromagnetic currents. The longitudinal elastic form factor and sum rule are found to be in satisfactory agreement with available experimental data. A direct comparison between theory and experiment is difficult for the transverse sum rule. However, it is shown that the calculated transverse sum rule has large contributions from two-body currents, indicating that these mechanisms lead to a significant enhancement of the quasielastic transverse response. This fact may have implications for the anomaly observed in recent neutrino quasielastic charge-changing scattering experiments on ${}^{12}\mathrm{C}$.

Measurements and calculations of the absolute carrier-envelope-phase (CEP) effects in the photodissociation of the simplest molecule, ${\mathrm{H}}_2^{+}$, with a 4.5-fs Ti:sapphire laser pulse at intensities up to $(4\pm 2)\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ are presented. Localization of the electron with respect to the two nuclei (during the dissociation process) is controlled via the CEP of the ultrashort laser pulses. In contrast to previous CEP-dependent experiments with neutral molecules, the dissociation of the molecular ions is not preceded by a photoionization process, which strongly influences the CEP dependence. Kinematically complete data are obtained by time- and position-resolved coincidence detection. The phase dependence is determined by a single-shot phase measurement correlated to the detection of the dissociation fragments. The experimental results show quantitative agreement with ab initio 3D time-dependent Schrödinger equation calculations that include nuclear vibration and rotation.