Wind turbines generate electricity from turbulent wind. Large fluctuations, and, more importantly, frequent wind gusts cause a highly fluctuating electrical power feed into the grid. Such effects are the hallmark of high-frequency turbulence. Here we show evidence that it is the complex structure of turbulence that dominates the power output for one single wind turbine as well as for an entire wind farm. We illustrate the highly intermittent, peaked nature of wind power fed into the grid. Multifractal scaling is observed, as described initially by Kolmogorov’s 1962 theory of turbulence. In parallel, we propose a stochastic model that converts wind speed signals into power output signals with appropriate multifractal statistics. As more and more wind turbines become integrated into our electric grids, a proper understanding of this intermittent power source must be worked out to ensure grid stability in future networks. Thus, our results stress the need for a profound understanding of the physics of turbulence and its impact on wind energy.

Single-photon entangled states, i.e., states describing two optical paths sharing a single photon, constitute the simplest form of entanglement. Yet they provide a valuable resource in quantum information science. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped, and purified with linear optics. The main drawback of such entanglement is the difficulty in measuring it. Here, we present and experimentally test an entanglement witness allowing one to say whether a given state is path entangled and also that entanglement lies in the subspace, where the optical paths are each filled with one photon at most, i.e., refers to single-photon entanglement. It uses local homodyning only and relies on no assumption about the Hilbert space dimension of the measured system. Our work provides a simple and trustworthy method for verifying the proper functioning of future quantum networks.

Classical adiabatic invariants in actual adiabatic processes possess intrinsic dynamical fluctuations. The magnitude of such intrinsic fluctuations is often thought to be negligible. This widely believed physical picture is contested here. For adiabatic following of a moving stable fixed-point solution facing a pitchfork bifurcation, we show that intrinsic dynamical fluctuations in an adiabatic process can assist in a deterministic selection between two symmetry-connected fixed-point solutions, with the outcome independent of the duration of the adiabatic process. Using a classical model Hamiltonian also relevant to a two-mode quantum system, we further demonstrate the formation of an adiabatic hysteresis loop in purely Hamiltonian mechanics and the generation of a Berry phase via changing one single-valued parameter only.

The photon creation and annihilation operators are cornerstones of the quantum description of the electromagnetic field. They signify the isomorphism of the optical Hilbert space to that of the harmonic oscillator and the bosonic nature of photons. We perform complete experimental characterization (quantum process tomography) of these operators. By measuring their effect on coherent states by means of homodyne tomography, we obtain their process tensor in the Fock basis, which explicitly shows the “raising” and “lowering” properties of these operators with respect to photon number states. This is the first experimental demonstration of complete tomography of nondeterministic quantum processes.

For the first time a single trapped antiproton ($\overline{p}$) is used to measure the $\overline{p}$ magnetic moment ${\mathit{\mu }}_{\overline{p}}$. The moment ${\mathit{\mu }}_{\overline{p}}={\mu }_{\overline{p}}\mathit{S}/(\hslash /2)$ is given in terms of its spin $\mathit{S}$ and the nuclear magneton (${\mu }_N$) by ${\mu }_{\overline{p}}/{\mu }_N=-2.792845\pm 0.000012$. The 4.4 parts per million (ppm) uncertainty is 680 times smaller than previously realized. Comparing to the proton moment measured using the same method and trap electrodes gives ${\mu }_{\overline{p}}/{\mu }_p=-1.000000\pm 0.000005$ to 5 ppm, for a proton moment ${\mathit{\mu }}_p={\mu }_p\mathit{S}/(\hslash /2)$, consistent with the prediction of the $CPT$ theorem.

We measure the branching fraction of $B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau }$ using the full ${\rm Y}(4S)$ data sample containing $772\times {10}^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. Events with $B\overline{B}$ pairs are tagged by reconstructing one of the $B$ mesons decaying into hadronic final states, and $B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau }$ candidates are detected in the recoil. We find evidence for $B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau }$ with a significance of 3.0 standard deviations including systematic errors and measure a branching fraction $\mathcal{B}(B^{-}\to {\tau }^{-}{\overline{\nu }}_{\tau })=[{0.72}_{-0.25}^{+0.27}(\mathrm{stat})\pm 0.11(\mathrm{syst})]\times {10}^{-4}$.

We demonstrate that fast removal of many electrons uncovers initial correlations of atoms in a finite sample through a pronounced peak in the kinetic-energy spectrum of the exploding ions. This maximum is the result of an intricate interplay between the composition of the system from discrete particles and its boundary. The formation of the peak can be described analytically, accounting for correlations beyond a mean-field reference model. It can be experimentally detected with short and intense light pulses from 4th-generation light sources.

Single photons are a vital resource for optical quantum information processing. Efficient and deterministic single photon sources do not yet exist, however. To date, experimental demonstrations of quantum processing primitives have been implemented using nondeterministic sources combined with heralding and/or postselection. Unfortunately, even for eight photons, the data rates are already so low as to make most experiments impracticable. It is well known that quantum memories, capable of storing photons until they are needed, are a potential solution to this “scaling catastrophe.” Here, we analyze in detail the benefits of quantum memories for producing multiphoton states, showing how the production rates can be enhanced by many orders of magnitude. We identify the quantity $\eta B$ as the most important figure of merit in this connection, where $\eta$ and $B$ are the efficiency and time-bandwidth product of the memories, respectively.

With an eye toward extending optical wave-mixing techniques to the x-ray regime, we present the first experimental demonstration of a two-color x-ray free-electron laser at the Linac Coherent Light Source. We combine the emittance-spoiler technique with a magnetic chicane in the undulator section to control the pulse duration and relative delay between two intense x-ray pulses and we use differently tuned canted pole undulators such that the two pulses have different wavelengths as well. Two schemes are shown to produce two-color soft x-ray pulses with a wavelength separation up to $\sim 1.9\%$ and a controllable relative delay up to 40 fs.

We perform radio-frequency dissociation spectroscopy of weakly bound ${}^6\mathrm{Li}_2$ Feshbach molecules using low-density samples of about 30 molecules in an optical dipole trap. Combined with a high magnetic field stability, this allows us to resolve the discrete trap levels in the radio-frequency dissociation spectra. This novel technique allows the binding energy of Feshbach molecules to be determined with unprecedented precision. We use these measurements as an input for a fit to the ${}^6\mathrm{Li}$ scattering potential using coupled-channel calculations. From this new potential, we determine the pole positions of the broad ${}^6\mathrm{Li}$ Feshbach resonances with an accuracy better than $7\times {10}^{-4}$ of the resonance widths. This eliminates the dominant uncertainty for current precision measurements of the equation of state of strongly interacting Fermi gases. As an important consequence, our results imply a corrected value for the Bertsch parameter $\xi$ measured by Ku et al. [Science 335, 563 (2012)], which is $\xi =0.370(5)(8)$.

The irreversible damage at cracks during the fatigue of crystalline solids is well known. Here we report on in situ high-energy x-ray evidence of reversible fatigue behavior in a nanocrystalline NiFe alloy both in the plastic zone and around the crack tip. In the plastic zone, the deformation is fully recoverable as the crack propagates, and the plastic deformation invokes reversible interactions of dislocation and twinning in the nanograins. But around the crack tip lies a regime with reversible grain lattice reorientation promoted by a change of local stress state. These observations suggest unprecedented fatigue deformation mechanisms in nanostructured systems that are not addressed theoretically.

Low-energy structures of alanates are currently known to be described by patterns of isolated, nearly ideal tetrahedral [${\mathrm{AlH}}_4$] anions and metal cations. We discover that the novel polymeric motif recently proposed for ${\mathrm{LiAlH}}_4$ plays a dominant role in a series of alanates, including ${\mathrm{LiAlH}}_4$, ${\mathrm{NaAlH}}_4$, ${\mathrm{KAlH}}_4$, $\mathrm{Mg}({\mathrm{AlH}}_4{)}_2$, $\mathrm{Ca}({\mathrm{AlH}}_4{)}_2$, and $\mathrm{Sr}({\mathrm{AlH}}_4{)}_2$. In particular, most of the low-energy structures discovered for the whole series are characterized by networks of corner-sharing [${\mathrm{AlH}}_6$] octahedra, forming wires and/or planes throughout the materials. Finally, for $\mathrm{Mg}({\mathrm{AlH}}_4{)}_2$ and $\mathrm{Sr}({\mathrm{AlH}}_4{)}_2$, we identify two polymeric phases to be lowest in energy at low temperatures.

We have studied the phase transition of argon in completely filled mesopores. Our effective medium analysis of ultrasonic measurements clearly indicates a continuous phase transition of argon in completely filled pores over a broad temperature range of about 45 K. With decreasing temperature, the amount of frozen argon increases and below about 30 K all adsorbed argon (including the first few layers near the pore wall) is frozen with a shear modulus about equal to the bulk shear modulus. It is remarkable, that in a system showing such a pronounced confinement effect—a continuous phase transition over 45 K—the bulk properties are preserved. A comparison with temperature cycles with one and two adsorbed layers shows, that due to the presence of solid argon in the center of the pores the first few layers are already frozen at a higher temperature (30 K) compared to single layers (20 K). The transfer of our technique from the simple model system that we present in this Letter to other more complex adsorbates and different porous samples should help to enlighten the phase behavior under confinement in further studies.

Electron capture during forward bias and reemission at zero bias by divacancies in the depletion region of a silicon diode structure at room temperature have been studied for the first time using monoenergetic positrons. The positron response increases essentially linearly with electron current, as a result of increased positron trapping by negatively charged divacancies. The measurements indicate that $\le 1\%$ of the divacancies become negatively charged in the steady state at a forward bias of 1 V. Changes in the mean positron response when applying a square wave bias to the sample (1 V forward bias and 0 V, duty cycle $1:4$, times at 0 V in the range $0.1–100\mu \mathrm{s}$), were consistent with a rapid conversion of doubly to singly charged divacancies (in $\sim {10}^1\mathrm{ns}$), followed by slower defilling of the singly charged divacancies with a time constant of $\sim {10}^1\mu \mathrm{s}$. These ac measurements allow determination of the relative populations of singly and doubly charged divacancies. The results provide confirmation of consistency between the positron’s response to the silicon divacancy and previously extracted capture and emission kinetics determined through charge transient measurements and assigned to the same defect. The possibility of combining these two, orthogonal techniques suggest a promising new and powerful approach to defect spectroscopy in which the structure and electrical properties of a defect may be determined in a single measurement.

Combining the local spin density approximation $(\mathrm{LSDA})+U$ and an analysis of superexchange interactions beyond density functional theory, we describe the magnetic ground state of Cr-doped ${\mathrm{TiO}}_2$, an intensively studied and debated dilute magnetic oxide. In parallel, we correct our $\mathrm{LSDA}+U$ ($+$ superexchange) ground state through $GW$ corrections ($GW@\mathrm{LSDA}+U$) that reproduce the position of the impurity states and the band gaps in satisfying agreement with experiments. Because of the different topological coordinations of Cr-Cr bonds in the ground states of rutile and anatase, superexchange interactions induce either ferromagnetic or antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction leads to a new mechanism which stabilizes a (nonrobust) ferromagnetic ground state, in keeping with experimental evidence, without the need to invoke $F$-center exchange. The interplay between structural defects and vacancies in contributing to the superexchange is also unveiled.

We propose a phase diagram for ${\mathrm{Fe}}_x{\mathrm{Bi}}_2{\mathrm{Te}}_3$ ($0\le x\le 0.1$) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic to antiferromagnetic gives rise to topological phase transitions, where the paramagnetic topological insulator of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ turns into a band insulator with ferromagnetic-cluster glassy behavior around $x\sim 0.025$, and it further evolves to a topological insulator with valence-bond glassy behavior, which spans over the region from $x\sim 0.03$ up to $x\sim 0.1$. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

We directly measure the band structure of a buried two dimensional electron gas (2DEG) using angle resolved photoemission spectroscopy. The buried 2DEG forms 2 nm beneath the surface of $p$-type silicon, because of a dense delta-type layer of phosphorus $n$-type dopants which have been placed there. The position of the phosphorous layer is beyond the probing depth of the photoemission experiment but the observation of the 2DEG is nevertheless possible at certain photon energies where emission from the states is resonantly enhanced. This permits direct access to the band structure of the 2DEG and its temperature dependence.

We report on microwave-driven coherent electron transfer between two coupled donors embedded in a silicon nanowire. By increasing the microwave frequency we observe a transition from incoherent to coherent driving revealed by the emergence of a Landau-Zener-Stückelberg quantum interference pattern of the measured current through the donors. This interference pattern is fitted to extract characteristic parameters of the double-donor system. In particular we estimate a charge dephasing time of $0.3\pm 0.1\mathrm{ns}$, comparable to other types of charge-based two-level systems. The demonstrated coherent coupling between two dopants is an important step towards donor-based quantum computing devices in silicon.

By means of ac magnetic-susceptibility measurements, we find evidence for a new magnetic phase of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ below about 140 mK in zero magnetic field. In magnetic fields parallel to [111], this phase is characterized by frequency- and amplitude-dependent susceptibility and extremely slow spin dynamics. In the zero-temperature limit, it extends to about 67 mT (the internal field $H_{\mathrm{int}}\simeq 52\mathrm{mT}$), at which it makes transition to another phase. The field dependence of the susceptibility of this second phase, which extends to about 0.60 T ($H_{\mathrm{int}}\simeq 0.54\mathrm{T}$) in the zero-temperature limit, indicates the presence of a weak magnetization plateau below about 50 mK, as has been predicted by a single-tetrahedron four-spin model, suggesting that the second phase is a quantum kagome ice.

The exchange coupling of a single spin localized at the central ion of Cu-tetraazaporphyrin on a magnetite(100) surface has been studied using x-ray magnetic circular dichroism (XMCD). Sum rule analysis of the XMCD spectra results in Cu spin and orbital magnetic moments as a function of the applied external field at low temperatures (20 K). The exchange coupling is positive for magnetization direction perpendicular to the surface (ferromagnetic) while it is negative for in-plane magnetization direction (antiferromagnetic). We attribute the anisotropy of the Heisenberg exchange coupling to an orbitally dependent exchange Hamiltonian.