Atomic Physics

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Conservation of torus-knot angular momentum in high-order harmonic generation (1810.06503v2)

Emilio Pisanty, Laura Rego, Julio San Román, Antonio Picón, Kevin M. Dorney, Henry C. Kapteyn, Margaret M. Murnane, Luis Plaja, Maciej Lewenstein, Carlos Hernández-García

2018-10-15

High-order harmonic generation stands as a unique nonlinear optical up-conversion process, mediated by a laser-driven electron recollision mechanism, which has been shown to conserve energy, momentum, and spin and orbital angular momentum. Here we present theoretical simulations which demonstrate that this process also conserves a mixture of the latter, the torus-knot angular momentum , by producing high-order harmonics with driving pulses that are invariant under coordinated rotations. We demonstrate that the charge of the emitted harmonics scales linearly with the harmonic order, and that this conservation law is imprinted onto the polarization distribution of the emitted spiral of attosecond pulses. We also demonstrate how the nonperturbative physics of high-order harmonic generation affect the torus-knot angular momentum of the harmonics, and we show that this configuration harnesses the spin selection rules to channel the full yield of each harmonic into a single mode of controllable orbital angular momentum.

Part-per-billion measurement of the electric quadrupole transition isotope shifts between Ca and Ca (1906.04105v1)

Felix W. Knollmann, Ashay N. Patel, S. Charles Doret

2019-06-10

We report a precise measurement of the isotope shifts in the SD electric quadrupole transition at 729~nm in the Ca. The measurement has been made via high-resolution laser spectroscopy of co-trapped ions, finding measured shifts of 2,771,872,467.6(7.6), 5,340,887,394.6(7.8), and 9,990,381,870.0(6.3) Hz between Caand Ca, respectively. By exciting the two isotopes simultaneously using frequency sidebands derived from a single laser systematic uncertainties resulting from laser frequency drifts are eliminated. This permits far greater precision than similar previously published measurements in other alkaline-earth systems. The resulting measurement precision provides a benchmark for tests of theoretical isotope shift calculations, and also offers a step towards probing New Physics via isotope shift spectroscopy.

QED theory of the specific mass shift in atoms (1903.09733v2)

A. V. Malyshev, I. S. Anisimova, D. V. Mironova, V. M. Shabaev, G. Plunien

2019-03-22

The quantum electrodynamics formalism to treat the interelectronic-interaction correction of first order in to the two-electron part of the nuclear recoil effect on binding energies in atoms and ions is developed. The nonperturbative in calculations of the corresponding contribution to the energies of the state in He-like and the and states in Li-like ions are performed in the range . The behavior of the two-electron part of the nuclear recoil effect beyond the lowest-order relativistic approximation as grows is studied.

Single-photon interferometry and spectroscopy with two laser frequency combs (1906.03706v1)

Nathalie Picqué, Theodor W. Hänsch

2019-06-09

We demonstrate single-photon time-domain interference in a new realm. We observe interferences in the photon counting statistics with two separate mode-locked femtosecond lasers of slightly different repetition frequencies, each emitting a comb of evenly spaced spectral lines over a wide spectral span. We exploit the interference pattern for spectroscopic diagnostics over a broad spectral range. An experimental proof-of-concept shows that the emerging technique of high-resolution dual-comb Fourier transform spectroscopy can be performed at light powers that are a billion-fold weaker than those commonly employed. Our experiments challenge the intuitive concept that a photon exists before detection and they open the prospect of precise spectroscopy over broad spectral bandwidth in light-starved conditions.

Polarisation and Transparency of Relativistically Rotating Two-Level Atoms (1809.11097v2)

Calum Maitland, Matteo Clerici, Fabio Biancalana

2018-09-28

Electromagnetism and light-matter interaction in rotating systems is a rich area of ongoing research. We study the interaction of light with a gas of non-interacting two-level atoms confined to a rotating disk. We numerically solve the optical Bloch equations to investigate the how relativistic rotation affects the atoms' polarisation and inversion. The results are used to predict the steady-state stimulated emission seen by an observer at rest with the optical source in the laboratory frame. Competing physical effects due to time dilation and motion-induced detuning strongly modify solutions to the Bloch equations when the gas's velocity becomes relativistic. We account for the non-inertial motion by including acceleration-dependent excitation and emission rates, arising from a generalised Unruh effect. The effective thermal vacuum resulting from large accelerations de-polarises the gas while driving it towards population inversion, negating coherent driving due to the external light source. The results illustrate the intuitive, special-relativistic approach of assigning instantaneously comoving frames to understand non-inertial motion's influence when only local fields are physically significant.

Cold, dense atomic ion clouds produced by cryogenic buffer gas cooling (1906.03531v1)

Nishant Bhatt, Kosuke Kato, Amar C. Vutha

2019-06-08

We produce cold and dense clouds of atomic ions (Ca, Dy) by laser ablation of metal targets and cryogenic buffer gas cooling of the resulting plasma. We measure the temperature and density of the ion clouds using laser absorption spectroscopy. We find that large ion densities can be obtained at temperatures as low as 6 K. Our method opens up new ways to study cold neutral plasmas, and to perform survey spectroscopy of ions that cannot be laser-cooled easily.

Robust Strategies for Affirming Kramers-Henneberger Atoms (1906.03461v1)

Pei-Lun He, Zhao-Han Zhang, Feng He

2019-06-08

Atoms exposed to high-frequency strong laser fields experience the ionization suppression due to the deformation of Kramers-Henneberger (KH) wave functions, which has not been confirmed yet in experiment. We propose a bichromatic pump-probe strategy to affirm the existence of KH states, which is formed by the pump pulse and ionized by the probe pulse. In the case of the single-photon ionization triggered by a vacuum ultra-violet probe pulse, the double-slit structure of KH atom is mapped to the photoelectron momentum distribution. In the case of the tunneling ionization induced by an infrared probe pulse, streaking in anisotropic Coulomb potential produces a characteristic momentum drift. Apart from bichromatic schemes, the non-Abelian geometric phase provides an alternative route to affirm the existence of KH states. Following specific loops in laser parameter space, a complete spin flipping transition could be achieved. Our proposal has advantages of being robust against focal-intensity average as well as ionization depletion, and is accessible with current laser facilities.

Spin-alignment noise in atomic vapor (1906.03163v1)

A. A. Fomin, M. Yu. Petrov, G. G. Kozlov, M. M. Glazov, I. I. Ryzhov, M. V. Balabas, V. S. Zapasskii

2019-06-07

In the conventional spin noise spectroscopy, the probe laser light monitors fluctuations of the spin orientation of a paramagnet revealed as fluctuations of its gyrotropy, i.e., circular birefringence. For spins larger than 1/2, there exists spin arrangement of a higher order --- the so-called spin alignment --- which also exhibits spontaneous fluctuations. We show theoretically and experimentally that alignment fluctuations manifest themselves as the noise of the linear birefringence. In a magnetic field, the spin-alignment fluctuations, in contrast to those of spin orientation, show up as the noise of the probe-beam ellipticity at the double Larmor frequency, with the most efficient geometry of its observation being the Faraday configuration with the light propagating along the magnetic field. We have detected the spin-alignment noise in a cesium-vapor cell probed at the wavelength of D2 line (852.3 nm). The magnetic-field and polarization dependence of the ellipticity noise are in full agreement with the developed theory.

Dicke subradiance and thermal decoherence (1906.02918v1)

P. Weiss, A. Cipris, M. O. Araújo, R. Kaiser, W. Guerin

2019-06-07

Subradiance is the cooperative inhibition of the radiation by several emitters coupled to the same electromagnetic modes. It has been predicted by Dicke in 1954 and only recently observed in cold atomic vapors. Here we address the question to what extend this cooperative effect survives outside the limit of frozen two-level systems by studying the subradiant decay in an ensemble of cold atoms as a function of the temperature. Experimentally, we observe only a slight decrease of the subradiant decay time when increasing the temperature up to several millikelvins, and in particular we measure subradiant decay rates that are much smaller than the Doppler broadening. This demonstrates that subradiance is surprisingly robust against thermal decoherence. The numerical simulations are in good agreement and allow us to extrapolate the behavior of subradiance at higher temperatures.

Thermodynamics of Bose gases from functional renormalization with a hydrodynamic low-energy effective action (1902.07135v2)

Felipe Isaule, Michael C. Birse, Niels R. Walet

2019-02-19

The functional renormalization group for the effective action is used to construct an effective hydrodynamic description of weakly interacting Bose gases. We employ a scale-dependent parametrization of the boson fields developed previously to start the renormalization evolution in a Cartesian representation at high momenta and interpolate to an amplitude-phase one in the low-momentum regime. This technique is applied to Bose gases in one, two and three dimensions, where we study thermodynamic quantities such as the pressure and energy per particle. The interpolation leads to a very natural description of the Goldstone modes in the physical limit, and compares well to analytic and Monte-Carlo simulations at zero temperature. The results show that our method improves aspects of the description of low-dimensional systems, with stable results for the superfluid phase in two dimensions and even in one dimension.

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