We observe narrow band pairs of time-correlated photons of wavelengths 776 and 795 nm from nondegenerate four-wave mixing in a laser-cooled atomic ensemble of ${}^{87}\mathrm{Rb}$ using a cascade decay scheme. Coupling the photon pairs into single mode fibers, we observe an instantaneous rate of 7700 pairs per second with silicon avalanche photodetectors, and an optical bandwidth below 30 MHz. Detection events exhibit a strong correlation in time [$g^{(2)}(\tau =0)\approx 5800$] and a high coupling efficiency indicated by a pair-to-single ratio of 23%. The violation of the Cauchy-Schwarz inequality by a factor of $8.4\times {10}^6$ indicates a strong nonclassical correlation between the generated fields, while a Hanbury Brown–Twiss experiment in the individual photons reveals their thermal nature. The comparison between the measured frequency bandwidth and $1/e$ decay time of $g^{(2)}$ indicates a transform-limited spectrum of the photon pairs. The narrow bandwidth and brightness of our source makes it ideal for interacting with atomic ensembles in quantum communication protocols.

We have developed a new scanning-tunneling-microscopy–based spectroscopy technique to characterize infrared (IR) absorption of submonolayers of molecules on conducting crystals. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by IR light from a tunable laser source. Using this technique, we obtain the IR absorption spectra of [121]tetramantane and [123]tetramantane on Au(111). Significant differences between the IR spectra for these two isomers show the power of this new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy (STM) images look quite similar. Furthermore, the new technique was found to yield significantly better spectral resolution than STM-based inelastic electron tunneling spectroscopy, and to allow determination of optical absorption cross sections. Compared to IR spectroscopy of bulk tetramantane powders, infrared scanning tunneling microscopy (IRSTM) spectra reveal narrower and blueshifted vibrational peaks for an ordered tetramantane adlayer. Differences between bulk and surface tetramantane vibrational spectra are explained via molecule-molecule interactions.

Pumped heat electricity storage (PHES) has been recently suggested as a potential solution to the large-scale energy storage problem. PHES requires neither underground caverns as compressed air energy storage (CAES) nor kilometer-sized water reservoirs like pumped hydrostorage and can therefore be constructed anywhere in the world. However, since no large PHES system exists yet, and theoretical predictions are scarce, the efficiency of such systems is unknown. Here we formulate a simple thermodynamic model that predicts the efficiency of PHES as a function of the temperature of the thermal energy storage at maximum output power. The resulting equation is free of adjustable parameters and nearly as simple as the well-known Carnot formula. Our theory predicts that for storage temperatures above $400°\mathrm{C}$ PHES has a higher efficiency than existing CAES and that PHES can even compete with the efficiencies predicted for advanced-adiabatic CAES.

To compare the increasing number of optical frequency standards, highly stable optical signals have to be transferred over continental distances. We demonstrate optical-frequency transfer over a 1840-km underground optical fiber link using a single-span stabilization. The low inherent noise introduced by the fiber allows us to reach short term instabilities expressed as the modified Allan deviation of $2\times {10}^{-15}$ for a gate time $\tau$ of 1 s reaching $4\times {10}^{-19}$ in just 100 s. We find no systematic offset between the sent and transferred frequencies within the statistical uncertainty of about $3\times {10}^{-19}$. The spectral noise distribution of our fiber link at low Fourier frequencies leads to a ${\tau }^{-2}$ slope in the modified Allan deviation, which is also derived theoretically.

For the first time the emission of neutron bursts in the process of high-voltage discharge in air was observed. Experiments were carried out at an average electric field strength of $\sim 1\mathrm{MV}·{\mathrm{m}}^{-1}$ and discharge current of $\sim 10\mathrm{kA}$. Two independent methods (CR-39 track detectors and plastic scintillation detectors) registered neutrons within the range from thermal energies up to energies above 10 MeV and with an average flux density of $\gtrsim {10}^6{\mathrm{cm}}^{-2}$ per shot inside the discharge zone. Neutron generation occurs at the initial phase of the discharge and correlates with x-ray generation. The data obtained allow us to assume that during the discharge fast neutrons are mainly produced.

A high-resolution $\alpha$, x-ray, and $\gamma$-ray coincidence spectroscopy experiment was conducted at the GSI Helmholtzzentrum für Schwerionenforschung. Thirty correlated $\alpha$-decay chains were detected following the fusion-evaporation reaction ${}^{48}\mathrm{Ca}+{}^{243}\mathrm{Am}$. The observations are consistent with previous assignments of similar decay chains to originate from element $Z=115$. For the first time, precise spectroscopy allows the derivation of excitation schemes of isotopes along the decay chains starting with elements $Z>112$. Comprehensive Monte Carlo simulations accompany the data analysis. Nuclear structure models provide a first level interpretation.

We present a method for determining the phase and contrast of a single shot of an atom interferometer. The application of a phase shear across the atom ensemble yields a spatially varying fringe pattern at each output port, which can be imaged directly. This method is broadly relevant to atom-interferometric precision measurement, as we demonstrate in a 10 m ${}^{87}\mathrm{Rb}$ atomic fountain by implementing an atom-interferometric gyrocompass with 10 mdeg precision.

We present a new tool for imaging spin properties. We show that a spatially averaged spin signal, measured as a function of a scanned magnetic probe’s position, contains information about the local spin properties. In this first demonstration we map the injected spin density in GaAs by measuring spin photoluminescence with a resolution of $1.2\mu \mathrm{m}$. The ultimate limit of the technique is set by the gradient of the probe’s field, allowing for a resolution beyond the optical diffraction limit. Such probes can also be integrated with other detection methods. This generality allows the technique to be extended to buried interfaces and optically inactive materials.

We use graph theory to analyze chromatin interaction (Hi-C) data in the human genome. We show that a key functional feature of the genome—“master” replication origins—corresponds to DNA loci of maximal network centrality. These loci form a set of interconnected hubs both within chromosomes and between different chromosomes. Our results open the way to a fruitful use of graph theory concepts to decipher DNA structural organization in relation to genome functions such as replication and transcription. This quantitative information should prove useful to discriminate between possible polymer models of nuclear organization.

We argue that the observed large-scale cosmic microwave anomalies, discovered by WMAP and confirmed by the Planck satellite, are most naturally explained in the context of a marginally open universe. Particular focus is placed on the dipole power asymmetry, via an open universe implementation of the large-scale gradient mechanism of Erickcek et al. Open inflation models, which are motivated by the string landscape and which can excite “supercurvature” perturbation modes, can explain the presence of a very-large-scale perturbation that leads to a dipole modulation of the power spectrum measured by a typical observer. We provide a specific implementation of the scenario which appears compatible with all existing constraints.

The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency leads to the formation of Rydberg polaritons which are quasiparticles with tunable effective mass and nonlocal interactions. Confined to one spatial dimension their low energy physics is that of a moving-frame Luttinger liquid which, due to the nonlocal character of the repulsive interaction, can form a Wigner crystal of individual photons. We calculate the Luttinger $K$ parameter using density-matrix renormalization group simulations and find that under typical slow-light conditions kinetic energy contributions are too strong for crystal formation. However, adiabatically increasing the polariton mass by turning a light pulse into stationary spin excitations allows us to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a time-dependent Luttinger theory.

Electrically induced reorientation of nematic liquid crystal (NLC) molecules caused by dielectric anisotropy of the material is a fundamental phenomenon widely used in modern technologies. Its Achilles heel is a slow (millisecond) relaxation from the field-on to the field-off state. We present an electro-optic effect in an NLC with a response time of about 30 ns to both the field-on and field-off switching. This effect is caused by the electric field induced modification of the order parameters and does not require reorientation of the optic axis (director).

We demonstrate precise control of the coupling of each of two trapped ions to the mode of an optical resonator. When both ions are coupled with near-maximum strength, we generate ion-ion entanglement heralded by the detection of two orthogonally polarized cavity photons. The entanglement fidelity with respect to the Bell state ${\Psi }^{+}$ reaches $F\ge (91.9\pm 2.5)\%$. This result represents an important step toward distributed quantum computing with cavities linking remote atom-based registers.

This Letter describes new work on the determination of the Newtonian constant of gravitation, $G$, carried out at the BIPM since publication of the first results in 2001. The apparatus has been completely rebuilt and extensive tests carried out on the key parameters needed to produce a new value for $G$. The basic principles of the experiment remain the same, namely a torsion balance suspended from a wide, thin Cu-Be strip with two modes of operation, free deflection (Cavendish) and electrostatic servo control. The result from the new work is: $G=6.67545(18)\times {10}^{-11}{\mathrm{m}}^3{\mathrm{kg}}^{-1}{\mathrm{s}}^{-2}$ with a standard uncertainty of 27 ppm. This is 21 ppm below our 2001 result but 241 ppm above The CODATA 2010 value, which has an assigned uncertainty of 120 ppm. This confirms the discrepancy of our results with the CODATA value and highlights the wide divergence that now exists in recent values of $G$. The many changes made to the apparatus lead to the formal correlation between our two results being close to zero. Being statistically independent and statistically consistent, the two results taken together provide a unique contribution to determinations of $G$.

Results are presented from a search for the rare decays $B_s^0\to {\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}$ in $pp$ collisions at $\sqrt{s}=7$ and 8 TeV, with data samples corresponding to integrated luminosities of 5 and $20{\mathrm{fb}}^{-1}$, respectively, collected by the CMS experiment at the LHC. An unbinned maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=({3.0}_{-0.9}^{+1.0})\times {10}^{-9}$, where the uncertainty includes both statistical and systematic contributions. An excess of $B_s^0\to {\mu }^{+}{\mu }^{-}$ events with respect to background is observed with a significance of 4.3 standard deviations. For the decay $B^0\to {\mu }^{+}{\mu }^{-}$ an upper limit of $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<1.1\times {10}^{-9}$ at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.

A search for the rare decays $B_s^0\to {\mu }^{+}{\mu }^{-}$ and $B^0\to {\mu }^{+}{\mu }^{-}$ is performed at the LHCb experiment. The data analyzed correspond to an integrated luminosity of $1{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of 7 TeV and $2{\mathrm{fb}}^{-1}$ at 8 TeV. An excess of $B_s^0\to {\mu }^{+}{\mu }^{-}$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=({2.9}_{-1.0}^{+1.1})\times {10}^{-9}$ is obtained and an upper limit of $\mathcal{B}(B^0\to {\mu }^{+}{\mu }^{-})<7.4\times {10}^{-10}$ at 95% confidence level is set. These results are consistent with the standard model expectations.

The ratio of the ${\Lambda }_b^0$ baryon lifetime to that of the ${\overline{B}}^0$ meson is measured using $1.0{\mathrm{fb}}^{-1}$ of integrated luminosity in 7 TeV center-of-mass energy $pp$ collisions at the LHC. The ${\Lambda }_b^0$ baryon is observed for the first time in the decay mode ${\Lambda }_b^0\to J/\psi p{K}^{-}$, while the ${\overline{B}}^0$ meson decay used is the well known ${\overline{B}}^0\to J/\psi {\pi }^{+}{K}^{-}$ mode, where the ${\pi }^{+}{K}^{-}$ mass is consistent with that of the ${\overline{K}}^{*0}(892)$ meson. The ratio of lifetimes is measured to be $0.976\pm 0.012\pm 0.006$, in agreement with theoretical expectations based on the heavy quark expansion. Using previous determinations of the ${\overline{B}}^0$ meson lifetime, the ${\Lambda }_b^0$ lifetime is found to be $1.482\pm 0.018\pm 0.012\mathrm{ps}$. In both cases, the first uncertainty is statistical and the second systematic.

Photoabsorption by and fluorescence of the $K\alpha$ transitions in highly charged iron ions are essential mechanisms for x-ray radiation transfer in astrophysical environments. We study photoabsorption due to the main $K\alpha$ transitions in highly charged iron ions from heliumlike to fluorinelike (${\mathrm{Fe}}^{24+}$ to ${\mathrm{Fe}}^{17+}$) using monochromatic x rays around 6.6 keV at the PETRA III synchrotron photon source. Natural linewidths were determined with hitherto unattained accuracy. The observed transitions are of particular interest for the understanding of photoexcited plasmas found in x-ray binary stars and active galactic nuclei.

We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.

Quantum degeneracy is an important concept in quantum mechanics with large implications to many processes in condensed matter. Here, we show the consequences of electron energy level degeneracy on the conductance and the chemical force between two bodies at the atomic scale. We propose a novel way in which a scanning probe microscope can detect the presence of degenerate states in atomic-sized contacts even at room temperature. The tunneling conductance $G$ and chemical binding force $F$ between two bodies both tend to decay exponentially with distance in a certain distance range, usually maintaining direct proportionality $G\propto F$. However, we show that a square relation $G\propto F^2$ arises as a consequence of quantum degeneracy between the interacting frontier states of the scanning tip and a surface atom. We demonstrate this phenomenon on the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface reconstruction where the Si adatom possesses a strongly localized dangling-bond state at the Fermi level.