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preprint2016arXiv

High-harmonic generation in alpha-quartz by the electron-hole recombination

A calculation of the high-harmonic generation (HHG) in $α$-quartz using the time-dependent density functional theory is reported. The inter-band process is attributed to the dominant in HHG above the band gap. The photon energy is set to 1.55 eV, and the cutoff energy of the plateau region is found to be limited at the 19th harmonic (30 eV). The dependence of the HHG efficiency at the cutoff energy region on laser intensity is consistent with that of the hole density in the lowest-lying valence band. Numerical results indicate that electron-hole recombination plays a crucial role in HHG in $α$-quartz. It is found that a 200 attosecond pulse train is produced using HHG around the plateau cutoff energy.

preprint2016arXiv

Tailored jump operators for purely dissipative quantum magnetism

I propose an archtitecture for the realization of dissipative quantum many-body spin models. The dissipative processes are mediated by interactions with auxiliary particles and lead to a widely tunable class of correlated quantum jump operators. These findings enable the investigation of purely dissipative spin models, where coherent dynamics is entirely absent. I provide a detailed review of a recently introduced variational method to analyze such dissipative quantum many-body systems, and I discuss a specific example in terms of a purely dissipative Heisenberg model, for which I find an additional disordered phase that is not present in the corresponding ground state phase diagram.

preprint2015arXiv

Magnetically tuned, robust and efficient filtering system for spatially multimode quantum memory in warm atomic vapors

Warm atomic vapor quantum memories are simple and robust, yet suffer from a number of parasitic processes which produce excess noise. For operating in a single-photon regime precise filtering of the output light is essential. Here we report a combination of magnetically tuned absorption and Faraday filters, both light-direction-insensitive, which stop the driving lasers and attenuate spurious fluorescence and four-wave mixing while transmitting narrowband Stokes and anti-Stokes photons generated in write-in and readout processes. We characterize both filters with respect to adjustable working parameters. We demonstrate a significant increase in the signal to noise ratio upon applying the filters seen qualitatively in measurements of correlation between the Raman-scattered photons.

preprint2016arXiv

Kibble-Zurek mechanism in the self-organization of a cold atomic cloud

When applying two counter-propagating laser beams to a cold cloud of Rubidium atoms, we observe the spontaneous formation of intensity patterns associated with a spatial structuration of the atomic spins. We study the average number of defects in these patterns as a function of the sweep time employed to cross the transition threshold. We observe a power-law decrease of the number of defects with increasing sweep time, typical of the Kibble-Zurek mechanism. The measured exponent is consistent with the prediction for a supercritical bifurcation.

preprint2016arXiv

Gravitational wave detection with optical lattice atomic clocks

We propose a space-based gravitational wave detector consisting of two spatially separated, drag-free satellites sharing ultra-stable optical laser light over a single baseline. Each satellite contains an optical lattice atomic clock, which serves as a sensitive, narrowband detector of the local frequency of the shared laser light. A synchronized two-clock comparison between the satellites will be sensitive to the effective Doppler shifts induced by incident gravitational waves (GWs) at a level competitive with other proposed space-based GW detectors, while providing complementary features. The detected signal is a differential frequency shift of the shared laser light due to the relative velocity of the satellites, and the detection window can be tuned through the control sequence applied to the atoms' internal states. This scheme enables the detection of GWs from continuous, spectrally narrow sources, such as compact binary inspirals, with frequencies ranging from ~3 mHz - 10 Hz without loss of sensitivity, thereby bridging the detection gap between space-based and terrestrial optical interferometric GW detectors. Our proposed GW detector employs just two satellites, is compa

preprint2014arXiv

Nuclear critical charge for two-electron ion in Lagrange mesh method

The Schroedinger equation for two electrons in the field of a charged fixed center $Z$ is solved with the Lagrange mesh method for charges close to the critical charge $Z_{cr}$. We confirm the value of the nuclear critical charge $Z_{cr}$ recently calculated in Estienne et al. {\em Phys. Rev. Lett. \bf 112}, 173001 (2014) to 11 decimal digits using an inhomogeneous (non-uniform) three-dimensional lattice of size $70 \times 70 \times 20$. We show that the ground state energy for H$^-$ is accurate to 14 decimals on the lattice $50 \times 50 \times 40$ in comparison with the highly accurate result by Nakashima-Nakatsuji, {\it J. Chem. Phys. \bf 127}, 224104 (2007).

preprint2016arXiv

Emergent Weyl excitations in systems of polar particles

Weyl fermions are massless chiral particles first predicted in 1929 and once thought to describe neutrinos. Although never observed as elementary particles, quasiparticles with Weyl dispersion have recently been experimentally discovered in solid-state systems causing a furore in the research community. Systems with Weyl excitations can display a plethora of fascinating phenomena and offer great potential for improved quantum technologies. Here we show that Weyl excitations generically exist in three-dimensional systems of dipolar particles with weakly broken time-reversal symmetry (for example, by a magnetic field). They emerge as a result of dipolar-interaction-induced transfer of angular momentum between the $J=0$ and $J=1$ internal particle levels. We also discuss momentum-resolved Ramsey spectroscopy methods for observing Weyl quasiparticles in cold alkaline-earth-atom systems. Our results provide a pathway for a feasible experimental realisation of Weyl quasiparticles and related phenomena in clean and controllable atomic systems.

preprint2016arXiv

Search for the effect of massive bodies on atomic spectra and constraints on Yukawa-type interactions of scalar particles

We propose a new method to search for hypothetical scalar particles that have feeble interactions with Standard-Model particles. In the presence of massive bodies, these interactions produce a non-zero Yukawa-type scalar-field magnitude. Using radio-frequency spectroscopy data of atomic dysprosium, as well as atomic clock spectroscopy data, we constrain the Yukawa-type interactions of a scalar field with the photon, electron, and nucleons for a range of scalar-particle masses corresponding to length scales $ > 10$ cm. In the limit as the scalar-particle mass $m_ϕ\to 0$, our derived limits on the Yukawa-type interaction parameters are: $Λ_γ\gtrsim 8 \times 10^{19}$ GeV, $Λ_e \gtrsim 1.3 \times 10^{19}$ GeV, and $Λ_N \gtrsim 6 \times 10^{20}$ GeV. Our measurements also constrain combinations of interaction parameters, which cannot otherwise be probed with traditional anomalous-force measurements. We suggest further measurements to improve on the current level of sensitivity.

preprint2016arXiv

Coherence Times of Bose-Einstein Condensates beyond the Shot-Noise Limit via Superfluid Shielding

We demonstrate a new way to extend the coherence time of separated Bose-Einstein condensates that involves immersion into a superfluid bath. When both the system and the bath have similar scattering lengths, immersion in a superfluid bath cancels out inhomogeneous potentials either imposed by external fields or inherent in density fluctuations due to atomic shot noise. This effect, which we call superfluid shielding, allows for coherence lifetimes beyond the projection noise limit. We probe the coherence between separated condensates in different sites of an optical lattice by monitoring the contrast and decay of Bloch oscillations. Our technique demonstrates a new way that interactions can improve the performance of quantum devices.

preprint2016arXiv

Experimental Realisation of a Thermal Squeezed State of Levitated Optomechanics

We experimentally squeeze the thermal motional state of an optically levitated nanosphere, by fast switching between two trapping frequencies. The measured phase space distribution of our particle shows the typical shape of a squeezed thermal state, from which we infer up to 2.7dB of squeezing along one motional direction. The experiment features a large number of thermal excitations, therefore remaining in the classical regime. Nevertheless, we argue that the manipulation scheme described here could be used to achieve squeezing below the zero-point level, if preceded by ground state cooling of the levitated mechanical oscillator. Additionally, a higher degree of squeezing could in principle be achieved by repeating the frequency-switching protocol multiple times.

preprint2016arXiv

Relativistic calculations of quasi-one-electron atoms and ions using Laguerre and Slater spinors

A relativistic description of the structure of heavy alkali atoms and alkali-like ions using S-spinors and L-spinors has been developed. The core wavefunction is defined by a Dirac-Fock calculation using an S-spinors basis. The S-spinor basis is then supplemented by a large set of L-spinors for the calculation of the valence wavefunction in a frozen-core model. The numerical stability of the L-spinor approach is demonstrated by computing the energies and decay rates of several low-lying hydrogen eigenstates, along with the polarizabilities of a $Z=60$ hydrogenic ion. The approach is then applied to calculate the dynamic polarizabilities of the $5s$, $4d$ and $5p$ states of Sr$^+$. The magic wavelengths at which the Stark shifts between different pairs of transitions are zero are computed. Determination of the magic wavelengths for the $5s \to 4d_{\frac32}$ and $5s \to 4d_{\frac52}$ transitions near $417$~nm (near the wavelength for the $5s \to 5p_j$ transitions) would allow a determination of the oscillator strength ratio for the $5s \to 5p_{\frac12}$ and $5s \to 5p_{\frac32}$ transitions.

preprint2016arXiv

Pressure effect in the X-ray intrinsic position resolution in noble gases and mixtures

A study of the gas pressure effect in the position resolution of an interacting X- or gamma-ray photon in a gas medium is performed. The intrinsic position resolution for pure noble gases (Argon and Xenon) and their mixtures with CO2 and CH4 were calculated for several gas pressures (1-10bar) and for photon energies between 5.4 and 60.0 keV, being possible to establish a linear match between the intrinsic position resolution and the inverse of the gas pressure in that energy range. In order to evaluate the quality of the method here described, a comparison between the available experimental data and the calculated one in this work, is done and discussed. In the majority of the cases, a strong agreement is observed.

preprint2016arXiv

Resolving a discrepancy between experimental and theoretical lifetimes in atomic negative ions

Recently the lifetime of the excited $^{2}P_{1/2}$-state of S$^-$ was measured to be $503\pm 54$ s (Bäckström et al. Phys. Rev. Lett. 114, 143003 (2015)). The earlier theoretical lifetime of $436$ s was clearly outside the experimental error bars. To investigate this discrepancy we have performed systematic and large-scale multiconfiguration Dirac-Hartree-Fock calculations for this system. After including a careful treatment of correlation and relativistic effects, we predict a well-converged value of $492$ s for this lifetime, with an uncertainty considerably less than 1%, thereby removing the apparent conflict between theory and experiment. We also show that this result corresponds to the non-relativistic limit in the $LS$-approximation for the M1 transition within this $^2P$ term. We also demonstrate the usefulness of the latter approach for $^2P$ transitions in O$^-$, Se$^-$ and Te$^-$, as well as for analogous M1 transitions within $^2D$ terms in Ni$^-$ and Pt$^-$ ions.

preprint2016arXiv

Structure and dynamics of H$_2^+$ near the dissociation threshold: a combined experimental and computational investigation

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of H$_2$ has been recorded in the vicinity of the dissociative-ionization threshold following three-photon excitation via selected rotational levels of the B $^1Σ_u^+$ ($v=19$) and $\bar{\rm H}$ $^1Σ_g^+$ ($v=11$) intermediate states. The spectra consist of transitions to bound levels of the X$^+$ $^2Σ_g^+$ state of H$_2^+$ with $v^+$ in the range 14-19 and $N^+$ in the range 0-9, of the A$^+$ $^2Σ_u^+$ state with $v^+=0$ and $N^+=0-2$, and of shape resonances corresponding to the X$^+\ (v^+=17,N^+=7)$ and X$^+\ (v^+=18,N^+=4)$ quasibound levels. Calculations of the level structure of H$_2^+$ have been carried out and the influence of adiabatic, nonadiabatic, relativistic and radiative corrections on the positions of these levels, and in the case of the shape resonances also on their widths, has been investigated. Different methods of calculating the widths and profiles of the shape resonances have been tested for comparison with the experimental observations. Slow oscillations of the dissociative-ionization yield have been observed and reflect, in first approximation, the Franck-Condon factors of the $\bar{\tex

preprint2016arXiv

Deviations from Born-Oppenheimer mass scaling in spectroscopy and ultracold molecular physics

We investigate Born-Oppenheimer breakdown (BOB) effects (beyond the usual mass scaling) for the electronic ground states of a series of homonuclear and heteronuclear alkali-metal diatoms, together with the Sr$_2$ and Yb$_2$ diatomics. Several widely available electronic structure software packages are used to calculate the leading contributions to the total isotope shift for commonly occurring isotopologs of each species. Computed quantities include diagonal Born-Oppenheimer corrections (mass shifts) and isotopic field shifts. Mass shifts dominate for light nuclei up to and including K, but field shifts contribute significantly for Rb and Sr and are dominant for Yb. We compare the {\em ab initio} mass-shift functions for Li$_2$, LiK and LiRb with spectroscopically derived ground-state BOB functions from the literature. We find good agreement in the values of the functions for LiK and LiRb at their equilibrium geometries, but significant disagreement with the shapes of the functions for all 3 systems. The differences may be due to contributions of nonadiabatic terms to the empirical BOB functions. We present a semiclassical model for the effect of BOB corrections on the binding ener

preprint2016arXiv

Difference-frequency combs in cold atom physics

Optical frequency combs provide the clockwork to relate optical frequencies to radio frequencies. Hence, combs allow to measure optical frequencies with respect to a radio frequency where the accuracy is limited only by the reference signal. In order to provide a stable link between the radio and optical frequencies, the two parameters of the frequency comb must be fixed: the carrier envelope offset frequency $f_{\rm ceo}$ and the pulse repetition-rate $f_{\rm rep}$. We have developed the first optical frequency comb based on difference frequency generation (DFG) that eliminates $f_{\rm ceo}$ by design - specifically tailored for applications in cold atom physics. An $f_{\rm ceo}$-free spectrum at 1550 nm is generated from a super continuum spanning more than an optical octave. Established amplification and frequency conversion techniques based on reliable telecom fiber technology allow generation of multiple wavelength outputs. In this paper we discuss the frequency comb design, characterization, and optical frequency measurement of Sr Rydberg states. The DFG technique allows for a compact and robust, passively $f_{\rm ceo}$ stable frequency comb significantly improving reliabilit

preprint2017arXiv

Production of strongly bound 39K bright solitons

We report on the production of 39 K matter-wave bright solitons, i.e., 1D matter-waves that propagate without dispersion thanks to attractive interactions. The volume of the soliton is studied as a function of the scattering length through three-body losses, revealing peak densities as high as $\sim 5 \times 10^{20} m^{-3}$. Our solitons, close to the collapse threshold, are strongly bound and will find applications in fundamental physics and atom interferometry.

preprint2016arXiv

Unexpectedly large difference of the electron density at the nucleus in the 4p $^2$P$_{1/2,3/2}$ fine-structure doublet of Ca$^+$

We measured the isotope shift in the $^2$S$_{1/2}$-$^2$P$_{3/2}$ (D2) transition in singly-ionized calcium ions using photon recoil spectroscopy. The high accuracy of the technique enables us to resolve the difference between the isotope shifts of this transition to the previously measured isotopic shifts of the $^2$S$_{1/2}$-$^2$P$_{1/2}$ (D1) line. This so-called splitting isotope shift is extracted and exhibits a clear signature of field shift contributions. From the data we were able to extract the small difference of the field shift coefficient and mass shifts between the two transitions with high accuracy. This J-dependence is of relativistic origin and can be used to benchmark atomic structure calculations. As a first step, we use several ab initio atomic structure calculation methods to provide more accurate values for the field shift constants and their ratio. Remarkably, the high-accuracy value for the ratio of the field shift constants extracted from the experimental data is larger than all available theoretical predictions.

preprint2016arXiv

Few-body quantum physics with strongly interacting Rydberg polaritons

We present an extension of our recent paper [Bienias et al., Phys. Rev. A 90, 053804 (2014)] in which we demonstrated the scattering properties and bound-state structure of two Rydberg polaritons, as well as the derivation of the effective low-energy many-body Hamiltonian. Here, we derive a microscopic Hamiltonian describing the propagation of Rydberg slow light polaritons in one dimension. We describe possible decoherence processes within a Master equation approach, and derive equations of motion in a Schroedinger picture by using an effective non-Hermitian Hamiltonian. We illustrate diagrammatic methods on two examples: First, we show the solution for a single polariton in an external potential by exact summation of Feynman diagrams. Secondly, we solve the two body problem in a weakly interacting regime exactly.

preprint2016arXiv

An atom-by-atom assembler of defect-free arbitrary 2d atomic arrays

Large arrays of individually controlled atoms trapped in optical tweezers are a very promising platform for quantum engineering applications. However, to date, only disordered arrays have been demonstrated, due to the non-deterministic loading of the traps. Here, we demonstrate the preparation of fully loaded, two-dimensional arrays of up to 50 microtraps each containing a single atom, and arranged in arbitrary geometries. Starting from initially larger, half-filled matrices of randomly loaded traps, we obtain user-defined target arrays at unit filling. This is achieved with a real-time control system and a moving optical tweezers that performs a sequence of rapid atom moves depending on the initial distribution of the atoms in the arrays. These results open exciting prospects for quantum engineering with neutral atoms in tunable geometries.

preprint2016arXiv

Surface-electrode trap with an integrated permanent magnet for generating a magnetic-field gradient at trapped ions

We report on a surface-electrode trap with SmCo magnets arranged in a quadrupole configuration underneath the trap electrode. Because the distance between the magnets and the trapped ions can be as little as several hundred micrometers, a large magnetic field is produced without any heat management. The magnetic-field gradient was measured using the Zeeman splitting of a single trapped $^{40}$Ca$^+$ ion at several positions, and a field gradient of 36 T/m was obtained. Such a field gradient is useful for the generation of a state-dependent force, which is important for quantum simulation and/or quantum gate operation using radio-frequency or microwave radiation.

preprint2016arXiv

Nuclear size corrections to the energy levels of single-electron atoms

A study is made of nuclear size corrections to the energy levels of single-electron atoms for the ground state of hydrogen like atoms. We consider Fermi charge distribution to the nucleus and calculate atomic energy level shift due to the finite size of the nucleus in the perturbation theory context. The exact relativistic correction based upon the available analytical calculations is compared to the result of first-order relativistic perturbation theory and the non-relativistic approximation. We find small discrepancies between our perturbative results and those obtained from exact relativistic calculation even for large nuclear charge number .

preprint2015arXiv

Optical techniques for Rydberg physics in lattice geometries

We address the technical challenges when performing quantum information experiments with ultracold Rydberg atoms in lattice geometries. We discuss the following key aspects: (i) The coherent manipulation of atomic ground states, (ii) the coherent excitation of Rydberg states, and (iii) spatial addressing of individual lattice sites. We briefly review methods and solutions which have been successfully implemented, and give examples based on our experimental apparatus. This includes an optical phase-locked loop, an intensity and frequency stabilization setup for lasers, and a nematic liquid-crystal spatial light modulator.

preprint2016arXiv

Spatially and spectrally resolved quantum path interference with chirped driving pulses

We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short ($α_s$) and long ($α_l$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.

778 works