The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over $P$ states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a ${}^4\mathrm{He}$ ionization energy at $h\times 5945204212(6)\mathrm{MHz}$, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system.

The process of electron localization on a cluster of 32 water molecules at 20, 50, and 300 K is unraveled using ab initio molecular dynamics simulations. In warm, liquid clusters, the excess electron relaxes from an initial diffuse and weakly bound structure to an equilibrated, strongly bound species within 1.5 ps. In contrast, in cold, glassy clusters the relaxation processes is not completed and the electron becomes trapped in a metastable surface state with an intermediate binding energy. These results question the validity of extrapolations of the properties of solvated electrons from cold clusters of increasing size to the liquid bulk.

Integration of fiber optics may play a critical role in the development of quantum information processors based on trapped ions and atoms by enabling scalable collection and delivery of light and coupling trapped ions to optical microcavities. We trap ions in a surface-electrode Paul trap that includes an integrated optical fiber for detecting 280-nm fluorescence photons. The collection numerical aperture is 0.37, and total collection efficiency is 2.1%. The ion can be positioned between 80 and from the tip of the fiber by use of an adjustable rf pseudopotential.

We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise $CPT$ test on trapped antihydrogen is a long-standing goal.

We report on injection locking of optically excited mechanical oscillations of a single, trapped ion. The injection locking dynamics are studied by analyzing the oscillator spectrum with a spatially selective Fourier transform technique and the oscillator phase with stroboscopic imaging. In both cases we find excellent agreement with theory inside and outside the locking range. We attain injection locking with forces as low as $5(1)\times {10}^{-24}\mathrm{N}$ so this system appears promising for the detection of ultraweak oscillating forces.

We use universality in two-body dipolar physics to study three-body recombination. We present results for the universal structure of weakly bound two-dipole states that depend only on the $s$-wave scattering length ($a$). We study threshold three-body recombination rates into weakly bound dimer states as a function of the scattering length. A Fermi golden rule analysis is used to estimate rates for different events mediated by the dipole-dipole interaction and a phenomenological contact interaction. The three-body recombination rate in the limit where $a\gg D$ contains terms which scale as $a^4$, $a^2{D}^2$, and $D^4$, where $D$ is the dipolar length. When $a\ll D$, the three-body recombination rate scales as $D^4$.

Sequential multiple photoionization of the prototypical molecule ${\mathrm{N}}_2$ is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for single-pulse imaging applications.

We describe an experiment demonstrating the radarlike technique of “chirp and compress.” Chirped biphotons are generated using a quasi-phase-matched nonlinear crystal where the phase-matched frequency varies linearly with position. Sum frequency generation is used to measure the amplitude of the biphoton wave function. As compared to a nonchirped crystal, compression and an increase in summing efficiency of a factor of 5 is observed.

We use mixed quantum-classical molecular dynamics simulations and ultrafast transient hole-burning spectroscopy to build a molecular-level picture of the motions of solvent molecules around Na atoms in liquid tetrahydrofuran. We find that even at room temperature, the solvation of Na atoms occurs in discrete steps, with the number of solvent molecules nearest the atom changing one at a time. This explains why the rate of solvent relaxation differs for different initial nonequilibrium states, and reveals how the solvent helps determine the identity of atomic species in liquids.

We provide a full quantum mechanical analysis of a weak energy measurement of a driven mechanical resonator. We demonstrate that measurements too weak to resolve individual mechanical Fock states can nonetheless be used to detect the nonclassical energy fluctuations of the driven mechanical resonator, i.e., “phonon shot noise”. We also show that the third moment of the oscillator’s energy fluctuations provides a far more sensitive probe of quantum effects than the second moment, and that measuring the third moment via the phase shift of light in an optomechanical setup directly yields the type of operator ordering postulated in the theory of full-counting statistics.

We present experimental observation of electromagnetically induced transparency (EIT) on a single macroscopic artificial “atom” (superconducting quantum system) coupled to open 1D space of a transmission line. Unlike in an optical media with many atoms, the single-atom EIT in 1D space is revealed in suppression of reflection of electromagnetic waves, rather than absorption. The observed almost 100% modulation of the reflection and transmission of propagating microwaves demonstrates full controllability of individual artificial atoms and a possibility to manipulate the atomic states. The system can be used as a switchable mirror of microwaves and opens a good perspective for its applications in photonic quantum information processing and other fields.

The helium pair potential was computed including relativistic and quantum electrodynamics contributions as well as improved accuracy adiabatic ones. Accurate asymptotic expansions were used for large distances $R$. Error estimates show that the present potential is more accurate than any published to date. The computed dissociation energy and the average $R$ for the ${}^4\mathrm{He}_2$ bound state are $1.62\pm 0.03\mathrm{mK}$ and $47.1\pm 0.5\AA$. These values can be compared with the measured ones: ${1.1}_{-0.2}^{+0.3}\mathrm{mK}$ and $52\pm 4\AA$ [R. E. Grisenti et al., Phys. Rev. Lett. 85, 2284 (2000)].

Currently, laser cooling schemes are fundamentally based on the weak coupling regime. This requirement sets the trap frequency as an upper bound to the cooling rate. In this work we present a numerical study that shows the feasibility of cooling in the strong-coupling regime which then allows cooling rates that are faster than the trap frequency with experimentally feasible parameters. The scheme presented here can be applied to trapped atoms or ions as well as to mechanical oscillators. It can also cool medium sized ion chains close to the ground state.

Cooperative scattering of light by an extended object such as an atomic ensemble or a dielectric sphere is fundamentally different from scattering from many pointlike scatterers such as single atoms. Homogeneous distributions tend to scatter cooperatively, whereas fluctuations of the density distribution increase the disorder and suppress cooperativity. In an atomic cloud, the amount of disorder can be tuned via the optical thickness, and its role can be studied via the radiation force exerted by the light on the atomic cloud. Monitoring cold ${}^{87}\mathrm{Rb}$ atoms released from a magneto-optical trap, we present the first experimental signatures of radiation force reduction due to cooperative scattering. The results are in agreement with an analytical expression interpolating between the disorder and the cooperativity-dominated regimes.

Long-lived $|m|=1$ positronium (Ps) atoms are produced in vacuum when high density bursts of positrons with net polarization $p_0$ are implanted into a porous silica film in a 2.3 T magnetic field. We observe a decrease in the $|m|=1$ population as the density of the incident positron beam is increased due to quenching interactions between oppositely polarized Ps atoms within the target. Saturation of this density dependent quenching indicates that the initial positron spin polarization $p_0=28\pm 1\%$, and demonstrates the long term (${10}^2\mathrm{s}$) survival of positron polarization in a Surko-type buffer gas trap. We conclude that, at high Ps densities, the minority spin component is essentially eliminated and the remaining Ps is almost entirely ($\sim 96\%$) polarized, as required for the formation of a Ps Bose-Einstein condensate.

We observe the capture and field ionization of individual atoms near the side wall of a single suspended nanotube. Extremely large cross sections for ionization from an atomic beam are observed at modest voltages due to the nanotube’s small radius and extended length. The effects of the field strength on both the atomic capture and the ionization process are clearly distinguished in the data, as are prompt and delayed ionizations related to the locations at which they occur. Efficient and sensitive neutral atom detectors can be based on the nanotube capture and wall ionization processes.

Optical interferometry is by far the most sensitive displacement measurement technique available, with sensitivities at the ${10}^{-20}\mathrm{m}/\sqrt{\mathrm{Hz}}$ level in the large-scale gravitational-wave interferometers currently in operation. Second-generation interferometers will experience a tenfold improvement in sensitivity and be mainly limited by quantum noise, close to the standard quantum limit (SQL), once considered as the ultimate displacement sensitivity achievable by interferometry. In this Letter, we experimentally demonstrate one of the techniques envisioned to go beyond the SQL: amplification of a signal by radiation-pressure backaction in a detuned cavity.

A simple quantum-defect model gives analytic expressions for the complex scattering length and threshold collision rates of ultracold molecules. If the probability of reaction in the short-range part of the collision is high, the model gives universal rate constants for $s$- and $p$-wave collisions that are independent of short-range dynamics. This model explains the magnitudes of the recently measured rate constants for collisions of two ultracold ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules, or an ultracold ${}^{40}\mathrm{K}$ atom with the ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecule [S. Ospelkaus et al., Science 327, 853 (2010)].

Quantum parameter estimation has many applications, from gravitational wave detection to quantum key distribution. The most commonly used technique for this type of estimation is quantum filtering, using only past observations. We present the first experimental demonstration of quantum smoothing, a time-symmetric technique that uses past and future observations, for quantum parameter estimation. We consider both adaptive and nonadaptive quantum smoothing, and show that both are better than their filtered counterparts. For the problem of estimating a stochastically varying phase shift on a coherent beam, our theory predicts that adaptive quantum smoothing (the best scheme) gives an estimate with a mean-square error up to $2\sqrt{2}$ times smaller than nonadaptive filtering (the standard quantum limit). The experimentally measured improvement is $2.24\pm 0.14$.

We have investigated the hyperfine structure of the transition between the $5d^{7}6s^2$ ${}^{4}F_{9/2}^e$ ground state and the $5d^{6}6s^{2}6p$ ${}^{6}D_J^o$ excited state in the negative osmium ion by high-resolution collinear laser spectroscopy. This transition is unique because it is the only known electric-dipole transition in atomic anions and might be amenable to laser cooling. From the observed hyperfine structure in ${}^{187}\mathrm{Os}^{-}$ and ${}^{189}\mathrm{Os}^{-}$ the yet unknown total angular momentum of the bound excited state was found to be $J=9/2$. The hyperfine structure constants of the ${}^{4}F_{9/2}^e$ ground state and the ${}^{6}D_{9/2}^o$ excited state were determined experimentally and compared to multiconfiguration Dirac-Fock calculations. Using the knowledge of the ground and excited state angular momenta, the full energy level diagram of ${}^{192}\mathrm{Os}^{-}$ in an external magnetic field was calculated, revealing possible laser cooling transitions.