We present theoretical and experimental studies of the decoherence of hyperfine ground-state superpositions due to elastic Rayleigh scattering of light off resonant with higher lying excited states. We demonstrate that under appropriate conditions, elastic Rayleigh scattering can be the dominant source of decoherence, contrary to previous discussions in the literature. We show that the elastic-scattering decoherence rate of a two-level system is given by the square of the difference between the elastic-scattering amplitudes for the two levels, and that for certain detunings of the light, the amplitudes can interfere constructively even when the elastic-scattering rates from the two levels are equal. We confirm this prediction through calculations and measurements of the total decoherence rate for a superposition of the valence electron spin levels in the ground state of ${}^9\mathrm{Be}^{+}$ in a 4.5 T magnetic field.

The $\beta$ feeding probability of ${}^{102,104,105,106,107}\mathrm{Tc}$, ${}^{105}\mathrm{Mo}$, and ${}^{101}\mathrm{Nb}$ nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the $\gamma$ component of the decay heat for ${}^{239}\mathrm{Pu}$ in the 4–3000 s range.

Event horizons of astrophysical black holes and gravitational analogues have been predicted to excite the quantum vacuum and give rise to the emission of quanta, known as Hawking radiation. We experimentally create such a gravitational analogue using ultrashort laser pulse filaments and our measurements demonstrate a spontaneous emission of photons that confirms theoretical predictions.

We study the statistics of encounters of Lévy flights by introducing the concept of vicious Lévy flights—distinct groups of walkers performing independent Lévy flights with the process terminating upon the first encounter between walkers of different groups. We show that the probability that the process survives up to time $t$ decays as $t^{-\alpha }$ at late times. We compute $\alpha$ up to the second order in $\epsilon$ expansion, where $\epsilon =\sigma -d$, $\sigma$ is the Lévy exponent, and $d$ is the spatial dimension. For $d=\sigma$, we find the exponent of the logarithmic decay exactly. Theoretical values of the exponents are confirmed by numerical simulations. Our results indicate that walkers with smaller values of $\sigma$ survive longer and are therefore more effective at avoiding each other.

Reconstructed point defects in graphene are created by electron irradiation and annealing. By applying electron microscopy and density functional theory, it is shown that the strain field around these defects reaches far into the unperturbed hexagonal network and that metal atoms have a high affinity to the nonperfect and strained regions of graphene. Metal atoms are attracted by reconstructed defects and bonded with energies of about 2 eV. The increased reactivity of the distorted $\pi$-electron system in strained graphene allows us to attach metal atoms and to tailor the properties of graphene.

An efficient method for the prediction of fundamental band gaps in solids using density functional theory (DFT) is proposed. Generalizing the Delta self-consistent-field ($\Delta \mathrm{SCF}$) method to infinite solids, the $\Delta$-sol method is based on total-energy differences and derived from dielectric screening properties of electrons. Using local and semilocal exchange-correlation functionals (local density and generalized gradient approximations), we demonstrate a 70% reduction of mean absolute errors compared to Kohn-Sham gaps on over 100 compounds with experimental gaps of 0.5–4 eV, at computational costs similar to typical DFT calculations.

We examine how the structure of the world trade network has been shaped by globalization and recessions over the last 40 years. We show that by treating the world trade network as an evolving system, theory predicts the trade network is more sensitive to recessionary shocks and recovers more slowly from them now than it did 40 years ago, due to structural changes in the world trade network induced by globalization. We also show that recession-induced change to the world trade network leads to an increased hierarchical structure of the global trade network for a few years after the recession.

We report the realization of a novel degenerate Fermi mixture with an $\mathrm{SU}(2)\times \mathrm{SU}(6)$ symmetry in a cold atomic gas. We successfully cool the mixture of the two fermionic isotopes of ytterbium ${}^{171}\mathrm{Yb}$ with the nuclear spin $I=1/2$ and ${}^{173}\mathrm{Yb}$ with $I=5/2$ below the Fermi temperature $T_F$ as $0.46T_F$ for ${}^{171}\mathrm{Yb}$ and $0.54T_F$ for ${}^{173}\mathrm{Yb}$. The same scattering lengths for different spin components make this mixture featured with the novel $\mathrm{SU}(2)\times \mathrm{SU}(6)$ symmetry. The nuclear spin components are separately imaged by exploiting an optical Stern-Gerlach effect. In addition, the mixture is loaded into a 3D optical lattice to implement the $\mathrm{SU}(2)\times \mathrm{SU}(6)$ Hubbard model. This mixture will open the door to the study of novel quantum phases such as a spinor Bardeen-Cooper-Schrieffer-like fermionic superfluid.

Silicon is promising for spin-based quantum computation because nuclear spins, a source of magnetic noise, may be eliminated through isotopic enrichment. Long spin decoherence times $T_2$ have been measured in isotope-enriched silicon but come far short of the $T_2=2T_1$ limit. The effect of nuclear spins on $T_2$ is well established. However, the effect of background electron spins from ever present residual phosphorus impurities in silicon can also produce significant decoherence. We study spin decoherence decay as a function of donor concentration, ${}^{29}\mathrm{Si}$ concentration, and temperature using cluster expansion techniques specifically adapted to the problem of a sparse dipolarly coupled electron spin bath. Our results agree with the existing experimental spin echo data in Si:P and establish the importance of background dopants as the ultimate decoherence mechanism in isotope-enriched silicon.

We show that the dynamical melting of a Mott insulator in a three-dimensional lattice leads to condensation at nonzero momenta, a phenomenon that can be used to generate strongly interacting atom lasers in optical lattices. For infinite on-site repulsion, the case considered here, the momenta at which bosons condense are determined analytically and found to have a simple dependence on the hopping amplitudes. The occupation of the condensates is shown to scale linearly with the total number of atoms in the initial Mott insulator. Our results are obtained by using a Gutzwiller-type mean-field approach, gauged against exact-diagonalization solutions of small systems.

The MiniBooNE experiment at Fermilab reports results from a search for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillations, using a data sample corresponding to $5.66\times {10}^{20}$ protons on target. An excess of $20.9\pm 14.0$ events is observed in the energy range $475<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$, which, when constrained by the observed ${\overline{\nu }}_{\mu }$ events, has a probability for consistency with the background-only hypothesis of 0.5%. On the other hand, fitting for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillations, the best-fit point has a ${\chi }^2$ probability of 8.7%. The data are consistent with ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillations in the 0.1 to $1.0{\mathrm{eV}}^2$ $\Delta m^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.

Decoherence due to contact with a hot environment typically restricts quantum phenomena to the low temperature limit, $k_{B}T/\hslash \omega \ll 1$ ($\hslash \omega$ is the typical energy of the system). Here we report the existence of a nonequilibrium state for two coupled, parametrically driven, dissipative harmonic oscillators which, contrary to generalized intuition, has stationary entanglement at high temperatures. This clarifies the role of temperature and could lighten the burden on quantum experiments requiring delicate precooling setups.

We propose a quantum optical method to do the subwavelength lithography. Our method is similar to the traditional lithography but adding a critical step before dissociating the chemical bound of the photoresist. The subwavelength pattern is achieved by inducing the multi-Rabi oscillation between the two atomic levels. The proposed method does not require multiphoton absorption and the entanglement of photons. It is expected to be realizable using current technology.

We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale.

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be $\beta =0.15\pm 0.07$, which is more than an order of magnitude larger than the damping constant $\alpha$, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.

We demonstrate feedback cooling of the motion of a single rubidium atom trapped in a high-finesse optical resonator to a temperature of about $160\mu \mathrm{K}$. Time-dependent transmission and intensity-correlation measurements prove the reduction of the atomic position uncertainty. The feedback increases the $1/e$ storage time into the 1 s regime, 30 times longer than without feedback. Feedback cooling therefore rivals state-of-the-art laser cooling, but with the advantages that it requires less optical access and exhibits less optical pumping.

A study of cooled ${}^{197}\mathrm{Au}$ projectile-fragmentation products has been performed with a storage ring. This has enabled metastable nuclear excitations with energies up to 3 MeV, and half-lives extending to minutes or longer, to be identified in the neutron-rich nuclides ${}^{183,184,186}\mathrm{Hf}$ and ${}^{186,187}\mathrm{Ta}$. The results support the prediction of a strongly favored isomer region near neutron number 116.

The level of control that one has over neutral molecules in beams dictates their possible applications. Here we experimentally demonstrate that state-selected, neutral molecules can be kept together in a few mm long packet for a distance of over one mile. This is accomplished in a circular arrangement of 40 straight electrostatic hexapoles through which the molecules propagate over 1000 times. Up to 19 packets of molecules have simultaneously been stored in this ring structure. This brings the realization of a molecular low-energy collider within reach.

In this Letter, we propose a composite nanochannel system, where half of the channel is of low surface energy, while the other half has a relatively high surface energy. Molecular dynamics simulations show that fluids in such channels can be continuously driven by a symmetric temperature gradient. In the low surface energy part, the fluid moves from high to low temperature, while the fluid migrates from low to high temperature in the high surface energy part. The mechanisms that govern the flow are explained and the conditions required to guarantee the flow and the possible applications are discussed.

We discovered the chirality of charge-density waves (CDW) in $1T\mathrm{\text{−}}{\mathrm{TiSe}}_2$ by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes $I{a}_1:I{a}_2:I{a}_3=1:0.7\pm 0.1:0.5\pm 0.1$, where $I{a}_i$ ($i=1,2,3$) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality $H_{\mathrm{CDW}}\equiv {\mathit{q}}_{\mathbf{1}}·({\mathit{q}}_{\mathbf{2}}\times {\mathit{q}}_{\mathbf{3}})$, where ${\mathit{q}}_{\mathit{i}}$ are the CDW $q$ vectors, which is independent of the symmetry of components. The nonzero $H_{\mathrm{CDW}}$—the triple-$q$ vectors do not exist in an identical plane in the reciprocal space—should induce a real-space chirality in CDW system.