We present experiments on decelerating and trapping ammonia molecules using a combination of a Stark decelerator and a traveling wave decelerator. In the traveling wave decelerator, a moving potential is created by a series of ring-shaped electrodes to which oscillating high voltages (HV) are applied. By lowering the frequency of the applied voltages, the molecules confined in the moving trap are decelerated and brought to a standstill. As the molecules are confined in a true 3D well, this kind of deceleration has practically no losses, resulting in a great improvement on the usual Stark deceleration techniques. The necessary voltages are generated by amplifying the output of an arbitrary wave generator using fast HV amplifiers, giving us great control over the trapped molecules. We illustrate this by experiments in which we adiabatically cool trapped ${\mathrm{NH}}_3$ and ${\mathrm{ND}}_3$ molecules and resonantly excite their motion.

The formation of the smallest permanent indentation in a Au(111) surface is studied by scanning tunneling microscopy and atomic force microscopy in ultrahigh vacuum. The 9.5 nm radius W(111) indenter was characterized in situ by field ion microscopy. Elastic and plastic indentations are identified both in the residual impression image and by features in their force-displacement curves such as the sink-in depth, pop-ins, and hysteresis energy. Plasticity is best identified quantitatively in the force-displacement curves by the sink-in depth. The minimum of plastic damage producible in the substrate is associated with an energy budget of $\sim 70\mathrm{eV}$.

Tungsten borides are among a distinct class of transition-metal light-element compounds with superior mechanical properties that rival those of traditional superhard materials. An in-depth understanding of these compounds, however, has been impeded by uncertainties regarding their complex crystal structures. Here, we examine a wide range of chemical compositions of tungsten borides using a recently developed global structural optimization approach. We establish thermodynamically stable structures and identify a large number of metastable phases. These results clarify and correct previous structural assignments and predict new structures for possible synthesis. Our findings provide crucial insights for understanding the rich and complex crystal structures of tungsten borides, which have broad implications for further exploration of this class of promising materials.

With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in ${\mathrm{SrTiO}}_3$ which strongly reduces the lattice polarizability and the subsequent screening. In particular, we show that the charge localization and the polar order of ${\mathrm{SrTiO}}_3$ both develop below $\sim 50\mathrm{K}$ and exhibit similar, unipolar hysteresis loops as a function of the gate voltage.

The flow of amorphous solids results from a combination of elastic deformation and local structural rearrangements, which induce nonlocal elastic deformations. These elements are incorporated into a mechanically consistent mesoscopic model of interacting elastoplastic blocks. We investigate the specific case of channel flow with numerical simulations, paying particular attention to situations of strong confinement. We find that the simple picture of plastic events embedded in an elastic matrix successfully accounts for manifestations of spatial cooperativity. Shear rate fluctuations are observed in seemingly quiescent regions, and the velocity profiles in confined flows at high applied pressure deviate from those expected in the absence of nonlocal effects, in agreement with experimental data. However, we suggest a different physical origin for the large deviations observed when walls have rough surfaces, associated with “bumps” of the particles against the asperities of the walls.

From ultracold atoms to quantum chromodynamics, reliable ab initio studies of strongly interacting fermions require numerical methods, typically in some form of quantum Monte Carlo calculation. Unfortunately, (non)relativistic systems at finite density (spin polarization) generally have a sign problem, such that those ab initio calculations are impractical. It is well-known, however, that in the relativistic case imaginary chemical potentials solve this problem, assuming the data can be analytically continued to the real axis. Is this feasible for nonrelativistic systems? Are the interesting features of the phase diagram accessible in this manner? By introducing complex chemical potentials, for real total particle number and imaginary polarization, the sign problem is avoided in the nonrelativistic case. To give a first answer to the above questions, we perform a mean-field study of the finite-temperature phase diagram of spin-$1/2$ fermions with imaginary polarization.

We prove basic theorems about the ground states of the $S=1$ Bose-Hubbard model. The results are quite universal and depend only on the coefficient $U_2$ of the spin-dependent interaction. We show that the ground state exhibits saturated ferromagnetism if $U_2<0$, is spin-singlet if $U_2>0$, and exhibits “SU(3)-ferromagnetism” if $U_2=0$, and completely determine the degeneracy in each region.

We analyze the evolution of the quantum state of networks of quantum oscillators coupled with arbitrary external environments. We show that the reduced density matrix of the network always obeys a local master equation with a simple analytical solution. We use this to study the emergence of thermodynamical laws in the long time regime demonstrating two main results: First, we show that it is impossible to build a quantum absorption refrigerator using linear networks (thus, nonlinearity is an essential resource for such refrigerators recently studied by Levy and Kosloff [Phys. Rev. Lett. 108, 070604 (2012)] and Levy et al. [Phys. Rev. B 85, 061126 (2012)]). Then, we show that the third law imposes constraints on the low frequency behavior of the environmental spectral densities.

We demonstrate an absolute magnetometer based on quantum beats in the ground state of nitrogen-vacancy centers in diamond. We show that, by eliminating the dependence of spin evolution on the zero-field splitting $D$, the magnetometer is immune to temperature fluctuation and strain inhomogeneity. We apply this technique to measure low-frequency magnetic field noise by using a single nitrogen-vacancy center located within 500 nm of the surface of an isotopically pure (99.99% ${}^{12}\mathrm{C}$) diamond. The photon-shot-noise limited sensitivity achieves $38\mathrm{nT}/\sqrt{\mathrm{Hz}}$ for 4.45 s acquisition time, a factor of $\sqrt{2}$ better than the implementation which uses only two spin levels. For long acquisition times ($>10\mathrm{s}$), we realize up to a factor of 15 improvement in magnetic sensitivity, which demonstrates the robustness of our technique against thermal drifts. Applying our technique to nitrogen-vacancy center ensembles, we eliminate dephasing from longitudinal strain inhomogeneity, resulting in a factor of 2.3 improvement in sensitivity.

We study the behavior of $n$-point functions of the primordial curvature perturbations, assuming our observed Universe is only a subset of a larger space with statistically homogeneous and isotropic perturbations. If the larger space has arbitrary $n$-point functions in a family of local type non-Gaussian statistics, sufficiently biased smaller volumes will have statistics from a “natural” version of that family with moments that are weakly non-Gaussian and ordered, regardless of the statistics of the original field. We also describe the effect of this bias on the shape of the bispectrum.

The beauty to up quark coupling constant $|V_{ub}|$ can be extracted from $B\to \rho e^{+}{\nu }_e$ combined with the form factors for $D\to K^{*}{e}^{+}{\nu }_e$ and $B\to V{\ell }^{+}{\ell }^{-}$ and $D\to \rho e^{+}{\nu }_e$. Using the entire CLEO-c $\psi (3770)\to D\overline{D}$ event sample, corresponding to an integrated luminosity of $818{\mathrm{pb}}^{-1}$ and approximately $5.4\times {10}^6$ $D\overline{D}$ events, we measure the form factors for the decays $D^0\to {\rho }^{-}{e}^{+}{\nu }_e$ and $D^{+}\to {\rho }^0{e}^{+}{\nu }_e$ for the first time and the branching fractions with improved precision. A four-dimensional unbinned maximum likelihood fit determines the form factor ratios to be $V(0)/A_1(0)=1.48\pm 0.15\pm 0.05$ and $A_2(0)/A_1(0)=0.83\pm 0.11\pm 0.04$. Assuming Cabibbo-Kobayashi-Maskawa unitarity, the known $D$ meson lifetimes, and our measured branching fractions we obtain the form factor normalizations $A_1(0)$, $A_2(0)$, and $V(0)$. We also present a measurement of the branching fraction for $D^{+}\to \omega e^{+}{\nu }_e$ with improved precision.

We investigate quantum control of a single atom in a tightly focused optical tweezer trap. We show that inevitable spatially varying polarization gives rise to significant internal-state decoherence but that this effect can be mitigated by an appropriately chosen magnetic bias field. This enables Raman sideband cooling of a single atom close to its three-dimensional ground state (vibrational quantum numbers ${\overline{n}}_x={\overline{n}}_y=0.01$, ${\overline{n}}_z=8$) even for a trap beam waist as small as $w=900\mathrm{nm}$. The small atomic wave packet with $\delta x=\delta y=24\mathrm{nm}$ and $\delta z=270\mathrm{nm}$ represents a promising starting point for future hybrid quantum systems where atoms are placed in close proximity to surfaces.

We propose a method to suppress unwanted transition channels and achieve perfect population transfer in multistate quantum systems by using composite pulse sequences. Unwanted transition paths may be present due to imperfect light polarization, misalignment of the quantization axis, spatial inhomogeneity of the trapping fields, off-resonant couplings, etc., or they may be merely unavoidable, e.g., due to perturbing excitations in molecules and solids. Compensation of separate or simultaneous deviations in polarization, pulse area, and detuning is demonstrated, even when these deviations are unknown, in three-state $V$ and $\Xi$ (ladder) systems and in a four-state $Y$ system.

We perform a high-resolution real-time readout of the motion of a single trapped and laser-cooled ${\mathrm{Ba}}^{+}$ ion. By using an interferometric setup, we demonstrate a shot-noise-limited measurement of thermal oscillations with a resolution of 4 times the standard quantum limit. We apply the real-time monitoring for phase control of the ion motion through a feedback loop, suppressing the photon recoil-induced phase diffusion. Because of the spectral narrowing in the phase-locked mode, the coherent ion oscillation is measured with a resolution of about 0.3 times the standard quantum limit.

We investigate the quantum phases of systems in which a multimode bosonic field is coupled to the transitions between two flat electronic bands. In the literature, such systems are usually modeled using a single or multimode Dicke model, leading to the prediction of superradiant quantum phase transitions for large enough couplings. We show that the physics of these systems is remarkably richer than previously expected, with the system continuously interpolating between a Dicke model exhibiting a superradiant quantum phase transition and a quantum Rabi model with no phase transition.

A light-storage experiment with a total (storage and retrieval) efficiency $\eta =56\%$ is carried out by enclosing a sample, with a single-pass absorption of 10%, in an impedance-matched cavity. The experiment is carried out using the atomic frequency comb (AFC) technique in a praseodymium-doped crystal ($0.05\%{\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$) and the cavity is created by depositing reflection coatings directly onto the crystal surfaces. The AFC technique has previously by far demonstrated the highest multimode capacity of all quantum memory concepts tested experimentally. We claim that the present work shows that it is realistic to create efficient, on-demand, long storage time AFC memories.

We consider a generalization of the Kuramoto model of coupled oscillators to the situation where communities of oscillators having essentially different natural frequencies interact. General equations describing possible resonances between the communities’ frequencies are derived. The simplest situation of three resonantly interacting groups is analyzed in detail. We find conditions for the mutual coupling to promote or suppress synchrony in individual populations and present examples where the interaction between communities leads to their synchrony or to a partially asynchronous state or to a chaotic dynamics of order parameters.

A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research.

Using a chirped pulse probe technique, we have obtained single-shot measurements of temporal evolution of ac conductivity at 1.55 eV (800 nm) during electron energy relaxation in nonequilibrium warm dense gold with energy densities up to $4.1\mathrm{MJ}/\mathrm{kg}$ ($8\times {10}^{10}\mathrm{J}/{\mathrm{m}}^3$). The results uncover important changes that have been masked in an earlier experiment. Equally significant, they provide valuable tests of an ab initio model for the calculation of electron heat capacity, electron-ion coupling, and ac conductivity in a single, first principles framework. While measurements of the real part of ac conductivity corroborate our theoretical temperature-dependent electron heat capacity, they point to an electron-ion coupling factor of $\sim 2.2\times {10}^{16}\mathrm{W}/{\mathrm{m}}^3\mathrm{K}$, significantly below that predicted by theory. In addition, measurements of the imaginary part of ac conductivity reveal the need to improve theoretical treatment of intraband contributions at very low photon energy.

Controlled electron injection into a laser-driven wakefield at a well defined space and time is reported based on particle-in-cell simulations. Key novel ingredients are an underdense plasma target with an up-ramp density profile followed by a plateau and a fairly large laser focus diameter that leads to an essentially one-dimensional (1D) regime of laser wakefield, which is different from the bubble (complete blowout) regime occurring for tightly focused drive beams. The up-ramp profile causes 1D wave breaking to occur sharply at the up-ramp-plateau transition. As a result, it generates an ultrathin (few nanometer, corresponding to attosecond duration), strongly overdense relativistic electron sheet that is injected and accelerated in the wakefield. A peaked electron energy spectrum and high charge ($\sim \mathrm{nC}$) distinguish the final sheet.