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arXiv2014

Approximated center-of-mass motion for systems of interacting particles with space- and velocity-dependent friction and anharmonic potential

We study the center-of-mass motion in systems of trapped interacting particles with space- and velocity-dependent friction and anharmonic traps. Our approach, based on a dynamical ansatz assuming a fixed density profile, allows us to obtain information at once for a wide range of binary interactions and interaction strengths, at linear and nonlinear levels. Our findings are first tested on different simple models by comparison with direct numerical simulations. Then, we apply the method to characterize the motion of the center of mass of a magneto-optical trap and its dependence on the number of trapped atoms. Our predictions are compared with experiments performed on a large Rb85 magneto-optical trap.

physics.atom-ph↗

arXiv2011

Laser radiation pressure slowing of a molecular beam

There is substantial interest in producing samples of ultracold molecules for possible applications in quantum computation, quantum simulation of condensed matter systems, precision measurements, controlled chemistry, and high precision spectroscopy. A crucial step to obtaining large samples of ultracold, trapped molecules is developing a means to bridge the gap between typical molecular source velocities (~150-600 m/s) and velocities for which trap loading or confinement is possible (~5-20 m/s). Here we show deceleration of a beam of neutral strontium monofluoride (SrF) molecules using radiative force. Under certain conditions, the deceleration results in a substantial flux of molecules with velocities <50 m/s. The observed slowing, from ~140 m/s, corresponds to scattering ~10000 photons. We also observe longitudinal velocity compression under different conditions. Combined with molecular laser cooling techniques, this lays the groundwork to create slow and cold molecular beams suitable for trap loading.

physics.atom-ph↗

arXiv2011

Muonium emission into vacuum from mesoporous thin films at cryogenic temperatures

We report on Muonium (Mu) emission into vacuum following μ+ implantation in mesoporous thin SiO2 films. We obtain a yield of Mu into vacuum of (38\pm4)% at 250 K temperature and (20\pm4)% at 100 K for 5 keV μ+ implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D250KMu = (1.6 \pm 0.1) \times 10-4 cm2/s and D100KMu = (4.2\pm0.5)\times10-5 cm2/s. Describing the diffusion process as quantum mechanical tunneling from pore-to-pore, we reproduce the measured temperature dependence T^3/2 of the diffusion constant. We extract a potential barrier of (-0.3 \pm 0.1) eV which is consistent with our computed Mu work-function in SiO2 of [-0.3,-0.9] eV. The high Mu vacuum yield even at low temperatures represents an important step towards next generation Mu spectroscopy experiments.

physics.atom-ph↗

arXiv2012

Comparison of Two Independent Sr Optical Clocks with 1e-17 Stability at 10^3 s

Many-particle optical lattice clocks have the potential for unprecedented measurement precision and stability due to their low quantum projection noise. However, this potential has so far never been realized because clock stability has been limited by frequency noise of optical local oscillators. By synchronously probing two 87Sr lattice systems using a laser with a thermal noise floor of 1e-15, we remove classically correlated laser noise from the intercomparison, but this does not demonstrate independent clock performance. With an improved optical oscillator that has a 1e-16 thermal noise floor, we demonstrate an order of magnitude improvement over the best reported stability of any independent clock, achieving a fractional instability of 1e-17 in 1000 s of averaging time for synchronous or asynchronous comparisons. This result is within a factor of 2 of the combined quantum projection noise limit for a 160 ms probe time with ~10^3 atoms in each clock. We further demonstrate that even at this high precision, the overall systematic uncertainty of our clock is not limited by atomic interactions. For the second Sr clock, which has a cavity-enhanced lattice, the atomic-density-dependent frequency shift is evaluated to be -3.11e-17 with an uncertainty of 8.2e-19.

physics.atom-ph↗

arXiv2013

Precise characterization of ^6Li Feshbach resonances using trap-sideband resolved RF spectroscopy of weakly bound molecules

We have performed radio-frequency dissociation spectroscopy of weakly bound ^6Li_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 RF 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 ^6Li scattering potential using coupled-channel calculations. From this new potential, we determine the pole positions of the broad ^6Li 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. For example, our results imply a corrected value for the Bertsch parameter ξmeasured by Ku et al. [Science 335, 563 (2012)], which is ξ= 0.370(5)(8).

cond-mat.quant-gas↗

arXiv2013

Fast Ion Surface Energy Loss and Straggling in the Surface Wake Fields

We have measured the stopping powers and straggling of fast, highly ionized atoms passing through thin bilayer targets made up of metals and insulators. We were surprised to find that the energy losses as well as the straggling depend on the ordering of the target and have small but significantly different values on bilayer reversal. We ascribe this newly found difference in energy loss to the surface energy loss field effect due to the differing surface wake fields as the beam exits the target in the two cases. This finding is validated with experiments using several different projectiles, velocities, and bilayer targets. Both partners of the diatomic molecular ions also display similar results. A comparison of the energy loss results with those of previous theoretical predictions for the surface wake potential for fast ions in solids supports the existence of a self-wake.

physics.atom-ph↗

arXiv2013

Fast and Quasideterministic Single Ion Source from a Dipole-Blockaded Atomic Ensemble

We present a fast and Quasideterministic protocol for the production of single ions and electrons from a cloud of laser cooled atoms. The approach is based on a two-step process where first a single Rydberg atom is photo-excited from a dipole-blockade configuration and subsequently ionized by an electric field pulse. We theoretically describe these excitation-ionization cycles via dynamical quantum maps and observe a rich behavior of the ionization dynamics as a function of laser Rabi frequency, pulse duration and particle number. Our results show that a fast sequential heralded production of single charged particles is achievable even from an unstructured and fluctuating atomic ensemble.

physics.atom-ph↗

arXiv2013

Investigating polaron transitions with polar molecules

We determine the phase diagram of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for Su-Schrieffer-Heeger coupling) or no (for breathing-mode coupling) transition. Our results indicate that the physics of realistic mixed polaron models is much richer than that of simplified models. We then show that ultracold molecules trapped in optical lattices can be used to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with external electric fields. The parameters of current experimental set-ups place them in the region where one of the transitions occurs. We propose a scheme to measure the polaron dispersion using stimulated Raman spectroscopy.

cond-mat.quant-gas↗

arXiv2013

Coherence properties of nanofiber-trapped cesium atoms

We experimentally study the ground state coherence properties of cesium atoms in a nanofiber-based two-color dipole trap, localized 200 nm away from the fiber surface. Using microwave radiation to coherently drive the clock transition, we record Ramsey fringes as well as spin echo signals and infer a reversible dephasing time $T_2^\ast=0.6$ ms and an irreversible dephasing time $T_2^\prime=3.7$ ms. By theoretically modelling the signals, we find that, for our experimental parameters, $T_2^\ast$ and $T_2^\prime$ are limited by the finite initial temperature of the atomic ensemble and the heating rate, respectively. Our results represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in an optical fiber quantum network.

quant-ph↗

arXiv2013

Demonstration of a dressed-state phase gate for trapped ions

We demonstrate a trapped-ion entangling-gate scheme proposed by Bermudez et al. [Phys. Rev. A 85, 040302 (2012)]. Simultaneous excitation of a strong carrier and a single-sideband transition enables deterministic creation of entangled states. The method works for magnetic field-insensitive states, is robust against thermal excitations, includes dynamical decoupling from qubit dephasing errors, and provides simplifications in experimental implementation compared to some other entangling gates with trapped ions. We achieve a Bell state fidelity of 0.974(4) and identify the main sources of error.

quant-ph↗

arXiv2013

Direct measurement of the van der Waals interaction between two Rydberg atoms

We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.

physics.atom-ph↗

arXiv2013

Limits on the dependence of the fine-structure constant on gravitational potential from white-dwarf spectra

We propose a new probe of the dependence of the fine structure constant, alpha, on a strong gravitational field using metal lines in the spectra of white dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, respectively, at a dimensionless gravitational potential relative to Earth of (Delta phi) ~ 5x10^(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to two orders of magnitude.

astro-ph.CO↗

arXiv2013

Precision Measurement Method for Branching Fractions of Excited P1/2 States Applied to 40Ca+

We present a method for measuring branching fractions for the decay of $J = 1/2$ atomic energy levels to lower-lying states based on time-resolved recording of the atom's fluorescence during a series of population transfers. We apply this method to measure the branching fractions for the decay of the 4$^{2}$P$_{1/2}$ state of $^{40}$\Ca to the 4$^{2}$S$_{1/2}$ and 3$^{2}$D$_{3/2}$ states to be \branching\ and \branchingsm, respectively. The measurement scheme requires that at least one of the lower-lying states be long-lived. The method is insensitive to fluctuations in laser light intensity and magnetic field and is readily applicable to various atomic species due to its simplicity. Our result distinguishes well among existing state-of-the-art theoretical models of \Ca.

physics.atom-ph↗

arXiv2013

Demonstration of Weak Measurement Based on Atomic Spontaneous Emission

We demonstrate a new type of weak measurement based on the dynamics of spontaneous emission. The pointer in our scheme is given by the Lorentzian distribution characterizing atomic exponential decay via emission of a single photon. We thus introduce weak measurement, so far demonstrated nearly exclusively with laser beams and Gaussian statistics, into the quantum regime of single emitters and single quanta, enabling the exploitation of a wide class of sources that are abundant in nature. We describe a complete analogy between our scheme and weak measurement with conventional Gaussian pointers. Instead of a shift in the mean of a Gaussian distribution, an imaginary weak value is exhibited in our scheme by a significantly slower-than-natural exponential distribution of emitted photons at the postselected polarization, leading to a large shift in their mean arrival time. The dynamics of spontaneous emission offer a broader view of the measurement process than is usually considered within the weak measurement formalism. Our scheme opens the path for the use of atoms and atomlike systems as sensitive probes in weak measurements, one example being optical magnetometry.

quant-ph↗

arXiv2013

Spinor dynamics in an antiferromagnetic spin-1 thermal Bose gas

We present experimental observations of coherent spin-population oscillations in a cold thermal, Bose gas of spin-1 sodium-23 atoms. The population oscillations in a multi-spatial-mode thermal gas have the same behavior as those observed in a single-spatial-mode antiferromagnetic spinor Bose Einstein condensate. We demonstrate this by showing that the two situations are described by the same dynamical equations, with a factor of two change in the spin-dependent interaction coefficient, which results from the change to particles with distinguishable momentum states in the thermal gas. We compare this theory to the measured spin population evolution after times up to a few hundreds of ms, finding quantitative agreement with the amplitude and period. We also measure the damping time of the oscillations as a function of magnetic field.

physics.atom-ph↗

arXiv2013

Preparation of entangled and antiferromagnetic states by dissipative Rydberg pumping

We propose and analyze an approach for preparation of high fidelity entanglement and antiferromagnetic states using Rydberg mediated interactions with dissipation. Using asymmetric Rydberg interactions the two-atom Bell singlet is a dark state of the Rydberg pumping process. Master equation simulations demonstrate Bell singlet preparation fidelity $\mcF=0.998$. Antiferromagnetic states are generated on a four spin plaquette in agreement with results found from diagonalization of the transverse field Ising Hamiltonian.

quant-ph↗

arXiv2013

Suppression of Spin-Exchange Relaxation Using Pulsed Parametric Resonance

We demonstrate that spin-exchange dephasing of Larmor precession at near-earth-scale fields is effectively eliminated by dressing the alkali-metal atom spins in a sequence of AC-coupled 2-pi pulses, repeated at the Larmor precession frequency. The contribution of spin-exchange collisions to the spectroscopic line width is reduced by a factor of the duty cycle of the pulses. We experimentally demonstrate resonant transverse pumping in magnetic fields as high as 0.1 Gauss, present experimental measurements of the suppressed spin-exchange relaxation, and show enhanced magnetometer response relative to a light-narrowed scalar magnetometer.

physics.atom-ph↗

arXiv2013

Quantum-Coherence-Enhanced Surface Plasmon Amplification by Stimulated Emission of Radiation

We investigate surface plasmon amplification in a silver nanoparticle coupled to an externally driven three-level gain medium, and show that quantum coherence significantly enhances the generation of surface plasmons. Surface plasmon amplification by stimulated emission of radiation is achieved in the absence of population inversion on the spasing transition, which reduces the pump requirements. The coherent drive allows us to control the dynamics, and holds promise for quantum control of nanoplasmonic devices.

quant-ph↗