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arXiv2014

Strongly enhanced atomic parity violation due to close levels of opposite parity

We present calculations of nuclear-spin-dependent and nuclear-spin-independent parity violating amplitudes in Ba, Ra, Ac+, Th and Pa. Parity nonconservation in these systems is greatly enhanced due to the presence of very close electronic energy levels of opposite parity, large nuclear charge, and strong nuclear enhancement of parity-violating effects. The presented amplitudes constitute several of the largest atomic parity-violating signals predicted so far. Experiments using these systems may be performed to determine values for the nuclear anapole moment, a P-odd T-even nuclear moment given rise to by parity-violating nuclear forces. Such measurements may prove to be valuable tools in the study of parity violation in the hadron sector. The considered spin-independent transitions could also be used to measure the ratio of weak charges for different isotopes of the same atom, the results of which would serve as a test of the standard model and also of neutron distributions. Barium, with seven stable isotopes, is particularly promising in this regard.

physics.atom-ph↗

arXiv2014

Relativistic Equation of Motion Coupled-Cluster Method: Application to the closed-shell atomic systems

We report our successful implementation of the full fledged relativistic equation of motion coupled cluster (EOMCC) method. This method is employed to compute the principal ionization potentials (IPs) of closed-shell rare gas atoms, He-like ions, Be-like ions along with Na+, Al+, K+, Be, and Mg. Four component Dirac spinors are used in the calculations and the one and two electron integrals are evaluated using the Dirac Coulomb Hamiltonian. Our results are in excellent agreement with those available measurements, which are taken from the National Institute of Science and Technology database (NIST). We also present results using the second order many-body perturbation theory (MBPT(2)) and random phase approximation (RPA) in the EOMCC framework. These results are compared with those of EOMCC at the level of single and double excitations in order to assess the role of the electron correlation effects in the intermediate schemes considered in our calculations .

physics.atom-ph↗

arXiv2014

Proposed method for laser spectroscopy of pionic helium atoms to determine the charged-pion mass

Metastable pionic helium ($π{\rm He}^+$) is a three-body atom composed of a helium nucleus, an electron occupying the $1s$ ground state, and a negatively charged pion $π^-$ in a Rydberg state with principal- and orbital angular momentum quantum numbers of $n\sim \ell+1\sim 16$. We calculate the spin-independent energies of the $π{\rm ^3He}^+$ and $π{\rm ^4He}^+$ isotopes in the region $n=15$--19. These include relativistic and quantum electrodynamics corrections of orders $R_{\infty}α^2$ and $R_{\infty}α^3$ in atomic units, where $R_{\infty}$ and $α$ denote the Rydberg and fine structure constants. The fine-structure splitting due to the coupling between the electron spin and the orbital angular momentum of the $π^-$, and the radiative and Auger decay rates of the states are also calculated. Some states $(n,\ell)=(16,15)$ and $(17,16)$ retain nanosecond-scale lifetimes against $π^-$ absorption into the helium nucleus. We propose to use laser pulses to induce $π^-$ transitions from these metastable states, to states with large ($\sim 10^{11}$ s$^{-1}$) Auger rates. The $π{\rm He}^{2+}$ ion that remains after Auger emission of the $1s$ electron undergoes Stark mixing with the $s$, $p$, and $d$ states during collisions with the helium atoms in the experimental target. This leads to immediate nuclear absorption of the $π^-$. The resonance condition between the laser beam and the atom is thus revealed as a sharp spike in the rates of neutrons, protons, deuterons, and tritons that emerge....(continued)

physics.atom-ph↗

arXiv2014

Effective-range approximations for resonant scattering of cold atoms

Studies of cold atom collisions and few-body interactions often require the energy dependence of the scattering phase shift, which is usually expressed in terms of an effective-range expansion. We use accurate coupled-channel calculations on $^{6}$Li, $^{39}$K and $^{133}$Cs to explore the behavior of the effective range in the vicinity of both broad and narrow Feshbach resonances. We show that commonly used expressions for the effective range break down dramatically for narrow resonances and near the zero-crossings of broad resonances. We present an alternative parametrization of the effective range that is accurate through both the pole and the zero-crossing for both broad and narrow resonances. However, the effective range expansion can still fail at quite low collision energies, particularly around narrow resonances. We demonstrate that an analytical form of an energy and magnetic field-dependent phase shift, based on multichannel quantum defect theory, gives accurate results for the energy-dependent scattering length.

cond-mat.quant-gas↗

arXiv2014

Controlling the $2p$ Hole Alignment in Neon via the $2s$-$3p$ Fano Resonance

We study the state-resolved production of neon ion after resonant photoionization of Ne via the $2s$-$3p$ Fano resonance. We find that by tuning the photon energy across the Fano resonance a surprisingly high control over the alignment of the final $2p$ hole along the polarization direction can be achieved. In this way hole alignments can be created that are otherwise very hard to achieve. The mechanism responsible for this hole alignment is the destructive interference of the direct and indirect (via the autoionizing $2s^{-1}3p$ state) ionization pathways of $2p$. By changing the photon energy the strength of the interference varies and $2p$-hole alignments with ratios up to 19:1 between $2p_0$ and $2p_{\pm 1}$ holes can be created: an effect normally only encountered in tunnel ionization using strong-field IR pulses. Including spin-orbit interaction does not change the qualitative feature and leads only to a reduction in the alignment by $2/3$. Our study is based on a time-dependent configuration-interaction singles (TDCIS) approach which solves the multichannel time-dependent Schrödinger equation.

physics.atom-ph↗

arXiv2014

Slow and velocity-tunable beams of metastable He$_2$ by multistage Zeeman deceleration

Metastable helium molecules (He$_2^*$) have been generated by striking a discharge in a supersonic expansion of helium gas from a pulsed valve. When operating the pulsed valve at room temperature, 77K, and 10K, the mean velocity of the supersonic beam was measured to be 1900m/s, 980m/s, and 530m/s, respectively. A 55-stage Zeeman decelerator operated in a phase-stable manner was then used to further reduce the beam velocity and tune it in the range between 100 and 150m/s. The internal-state distribution of the decelerated sample was established by photoionization spectroscopy.

physics.chem-ph↗

arXiv2014

Coherent and Incoherent Multiple Scattering

We compare two different models of transport of light in a disordered system with a spherical Gaussian distribution of scatterers. A coupled dipole model, keeping into account all interference effects, is compared to an incoherent model, using a random walk of particles. Besides the well known coherent backscattering effect and a well pronounced forward lobe, the incoherent model reproduces extremely well all scattering features. In an experiment with cold atoms, we use the momentum recoil imparted on the center of mass of the sample as a partial probe of the light scattering properties. We find that the force acting on the center of mass of the atoms is not well suited to exhibit the coherence effects in light propagation under multiple scattering conditions.

physics.atom-ph↗

arXiv2014

Time-dependent R-matrix theory applied to two-photon double ionization of He

We introduce a time-dependent R-matrix theory generalised to describe double ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wavefunction description in a extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wavefunction densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

physics.atom-ph↗

arXiv2014

Measuring the re-absorption cross section of a Magneto-Optical Trap

Magneto-Optical Traps have been used for several decades. Among fundamental mechanisms occuring in such traps, the magnitude of the multiple scattering is still unclear. Indeed, many experimental situations cannot be modeled easily, different models predict different values of the re-absortion cross section, and no simple experimental measurement of this cross section are available. We propose in this paper a simple measurement of this cross section through the size and the shape of the cloud of cold atoms. We apply this method to traps with a configuration where theoretical values are available, and show that the measured values are compatible with some models. We also apply the method to configurations where models are not relevant, and show that the re-absorption is sometimes much larger than the usually assumed value.

physics.atom-ph↗

arXiv2014

Dispersion relation and excitation character of a two-component Bose-Einstein condensate

We present a study for the dispersion relation and character of the excitations of a single and two component Bose Einstein Condensate (BEC). We study the single component dispersion for a finite BEC system and look at examples of quasiparticles to understand and characterize the dispersion relation. Next we present the dispersion relation for a two component BEC in both the miscible and immiscible parameter regimes. Then we present examples of the quasiparticles for both regimes.

cond-mat.quant-gas↗

arXiv2014

Observing Properties of an Interacting Homogeneous Bose--Einstein Condensate: Heisenberg-Limited Momentum Spread, Interaction Energy and Free-Expansion Dynamics

We study the properties of an atomic Bose--Einstein condensate produced in an optical-box potential, using high-resolution Bragg spectroscopy. For a range of box sizes, up to $70~μ$m, we directly observe Heisenberg-limited momentum uncertainty of the condensed atoms. We measure the condensate interaction energy with a precision of $k_B \times 100$ pK and study, both experimentally and numerically, the dynamics of its free expansion upon release from the box potential. All our measurements are in good agreement with theoretical expectations for a perfectly homogeneous condensate of spatial extent equal to the size of the box, which also establishes the uniformity of our optical-box system on a sub-nK energy scale.

cond-mat.quant-gas↗

arXiv2014

Ultrarelativistic quasiclassical wave functions in strong laser and atomic fields

The problem of an ultrarelativistic charge in the presence of an atomic and a plane-wave field is investigated in the quasiclassical regime by including exactly the effects of both background fields. Starting from the quasiclassical Green's function obtained in [Phys. Lett. B \textbf{717}, 224 (2012)], the corresponding in- and out-wave functions are derived in the experimentally relevant case of the particle initially counterpropagating with respect to the plane wave. The knowledge of these electron wave functions opens the possibility of investigating a variety of problems in strong-field QED, where both the atomic field and the laser field are strong enough to be taken into account exactly from the beginning in the calculations.

hep-ph↗

arXiv2014

Heating and ion transport in a Y-junction surface-electrode trap

We measure ion heating following transport throughout a Y-junction surface-electrode ion trap. By carefully selecting the trap voltage update rate during adiabatic transport along a trap arm, we observe minimal heating relative to the anomalous heating background. Transport through the junction results in an induced heating between 37 and 150 quanta in the axial direction per traverse. To reliably measure heating in this range, we compare the experimental sideband envelope, including up to fourth-order sidebands, to a theoretical model. The sideband envelope method allows us to cover the intermediate heating range inaccessible to the first-order sideband and Doppler recooling methods. We conclude that quantum information processing in this ion trap will likely require sympathetic cooling in order to support high fidelity gates after junction transport.

quant-ph↗

arXiv2014

Zeeman interaction in ThO $H^3Δ_1$ for the electron EDM search

The current limit on the electron's electric dipole moment, $|d_\mathrm{e}|<8.7\times 10^{-29} e {\cdotp} {\rm cm}$ (90% confidence), was set using the molecule thorium monoxide (ThO) in the $J=1$ rotational level of its $H ^3Δ_1$ electronic state [Science $\bf 343$, 269 (2014)]. This state in ThO is very robust against systematic errors related to magnetic fields or geometric phases, due in part to its $Ω$-doublet structure. These systematics can be further suppressed by operating the experiment under conditions where the $g$-factor difference between the $Ω$-doublets is minimized. We consider the $g$-factors of the ThO $H^3Δ_1$ state both experimentally and theoretically, including dependence on $Ω$-doublets, rotational level, and external electric field. The calculated and measured values are in good agreement. We find that the $g$-factor difference between $Ω$-doublets is smaller in $J=2$ than in $J=1$, and reaches zero at an experimentally accessible electric field. This means that the $H,J=2$ state should be even more robust against a number of systematic errors compared to $H,J=1$.

physics.atom-ph↗

arXiv2014

Synthetic Lorentz force in classical atomic gases via Doppler effect and radiation pressure

We theoretically predict a novel type of synthetic Lorentz force for classical (cold) atomic gases, which is based on the Doppler effect and radiation pressure. A fairly uniform and strong force can be constructed for gases in macroscopic volumes of several cubic millimeters and more. This opens the possibility to mimic classical charged gases in magnetic fields, such as those in a tokamak, in cold atom experiments.

physics.atom-ph↗

arXiv2014

Towards Quantum Information Processing at the Attosecond Timescale

Coherent processing of quantum information and attosecond science had so far little in common. We here show that recent data in high harmonic emission reveal quantum information processing at the attosecond timescale. By observing the interference pattern created by the spatiotemporal overlap of photons emitted by two interfering electron paths we generate a photon Hadamard gate and thus erase the electron-trajectory information. This allows the measurement of the relative phase in electron-trajectory quantum superpositions and establishes the era of electron-photon quantum coherence and entanglement at the attosecond timescale of high-field physics.

physics.atom-ph↗

arXiv2014

Out-of-equilibrium structures in strongly interacting Rydberg gases with dissipation

The non-equilibrium dynamics of a gas of cold atoms in which Rydberg states are off-resonantly excited is studied in the presence of noise. The interplay between interaction and off-resonant excitation leads to an initial dynamics where aggregates of excited Rydberg atoms slowly nucleate and grow, eventually reaching long-lived meta-stable arrangements which then relax further on much longer timescales. This growth dynamics is governed by an effective Master equation which permits a transparent and largely analytical understanding of the underlying physics. By means of extensive numerical simulations we study the many-body dynamics and the correlations of the resulting non-equilibrium states in various dimensions. Our results provide insight into the dynamical richness of strongly interacting Rydberg gases in noisy environments, and highlight the usefulness of these kind of systems for the exploration of soft-matter-type collective behaviour.

cond-mat.quant-gas↗

arXiv2014

SeD Radical: A probe for measurement of time variation of Fine Structure Constant($α$) and Proton to Electron Mass Ratio($μ$)

Based on the spectroscopic constants derived from highly accurate potential energy surfaces, the SeD radical is identified as a spectroscopic probe for measuring spatial and temporal variation of fundamental physical constants such as the fine-structure constant (denoted as $α=\frac{e^2}{\hbar c}$) and the proton-to-electron mass ratio (denoted as $μ=\frac{m_p}{m_e}$). The ground state of SeD ($X^2Π$), due to spin-orbit coupling, splits into two fine structure multiplets $^2Π_{\frac{3}{2}}$ and $^2Π_{\frac{1}{2}}$. The potential energy surfaces of these spin-orbit components are derived from a state of the art electronic structure method, MRCI+Q inclusive of scalar relativistic effects with the spin-orbit effects accounted through the Breit-Pauli operator. The relevant spectroscopic data are evaluated using Murrel-Sorbie fit to the potential energy surfaces. The spin-orbit splitting($ω_f$) between the two multiplets is similar in magnitude with the harmonic frequency ($ω_e$) of the diatomic molecule. The amplification factor derived from this theoretical method for this particular molecule can be as large as 350, on the lower side it can be about 34. The significantly large values of K indicate that SeD radical can be a plaussible experimental candidate for measuring variation in $α$ and $μ$.

physics.atom-ph↗