Physics

We report on the first coherent excitation of the highly forbidden 2 S 1/2 → 2 F 7/2 electric octupole (E3) transition in a single trapped 172 Yb + ion, an isotope without nuclear spin. Using the transition in 171 Yb + as a reference, we determine the transition frequency to be 642116784950887.6(2.4) Hz. We map out the magnetic field environment using the forbidden 2 S 1/2 → 2 D 5/2 electric quadrupole (E2) transition and determine its frequency to be 729476867027206.8(4.4) Hz. Our results are a factor of 1× 10 5 ( 3× 10 5 ) more accurate for the E2 (E3) transition compared to previous measurements. The results open up the way to search for new physics via precise isotope shift measurements and improved tests of local Lorentz invariance using the metastable 2 F 7/2 state of Yb + .

Recent technological advances allowed the coherent optical manipulation of high-energy electron wavepackets with attosecond precision. Here we theoretically investigate the collision of optically-modulated pulsed electron beams with atomic targets and reveal a quantum interference associated with different momentum components of the incident broadband electron pulse, which coherently modulates both the elastic and inelastic scattering cross sections. We show that the quantum interference has a high spatial sensitivity at the level of Angstroms, offering potential applications in high-resolution ultrafast electron microscopy. Our findings are rationalized by a simple model.

We introduce a scheme to coherently suppress second-rank tensor frequency shifts in atomic clocks, relying on the continuous rotation of an external magnetic field during the free atomic state evolution in a Ramsey sequence. The method retrieves the unperturbed frequency within a single interrogation cycle and is readily applicable to various atomic clock systems. For the frequency shift due to the electric quadrupole interaction, we experimentally demonstrate suppression by more than two orders of magnitude for the 2 S 1/2 → 2 D 3/2 transition of a single trapped 171 Yb + ion. The scheme provides particular advantages in the case of the 171 Yb + 2 S 1/2 → 2 F 7/2 electric octupole (E3) transition. For an improved estimate of the residual quadrupole shift for this transition, we measure the excited state electric quadrupole moments Θ( 2 D 3/2 )=1.95(1) e a 2 0 and Θ( 2 F 7/2 )=−0.0297(5) e a 2 0 with e the elementary charge and a 0 the Bohr radius, improving the measurement uncertainties by one order of magnitude.

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19...40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C 2 H + 2 cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4? -collection geometry of velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

The collective response of an atomic ensemble is shaped by its macroscopic environment. We demonstrate this effect in the near-resonant transmission of light through a thermal rubidium vapor confined in a planar nanocavity. Our model reveals density-dependent line shifts and broadenings beyond continuous electrodynamics models that oscillate with cavity width and have been observed in recent experiments. We predict that the amplitudes of these oscillations can be controlled by coatings that modify the cavity's Finesse.

We experimentally study resonant light scattering by a one-dimensional randomly filled chain of cold two-level atoms. By a local measurement of the light scattered along the chain, we observe constructive interferences in light-induced dipole-dipole interactions between the atoms. They lead to a shift of the collective resonance despite the average interatomic distance being larger than the wavelength of the light. This result demonstrates that strong collective effects can be enhanced by structuring the geometrical arrangement of the ensemble. We also explore the high intensity regime where atoms cannot be described classically. We compare our measurement to a mean-field, nonlinear coupled-dipole model accounting for the saturation of the response of a single atom.

We study collisional loss of a quasi-one-dimensional (1D) spin-polarized Fermi gas near a p -wave Feshbach resonance in ultracold 6 Li atoms. We measure the location of the p -wave resonance in quasi-1D and observe a confinement-induced shift and broadening. We find that the three-body loss coefficient L 3 as a function of the quasi-1D confinement has little dependence on confinement strength. We also analyze the atom loss with a two-step cascade three-body loss model in which weakly bound dimers are formed prior to their loss arising from atom-dimer collisions. Our data are consistent with this model. We also find a possible suppression in the rate of dimer relaxation with strong quasi-1D confinement. We discuss the implications of these measurements for observing p -wave pairing in quasi-1D.

We observe spin transfer within a non-degenerate heteronuclear spinor atomic gas comprised of a small 7 Li population admixed with a 87 Rb bath, with both elements in their F=1 hyperfine spin manifolds and at temperatures of 10's of μ K. Prepared in a non-equilibrium initial state, the 7 Li spin distribution evolves through incoherent spin-changing collisions toward a steady-state distribution. We identify and measure the cross-sections of all three types of spin-dependent heteronuclear collisions, namely the spin-exchange, spin-mixing, and quadrupole-exchange interactions, and find agreement with predictions of heteronuclear 7 Li- 87 Rb interactions at low energy. Moreover, we observe that the steady state of the 7 Li spinor gas can be controlled by varying the composition of the 87 Rb spin bath with which it interacts.

We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study their inelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules in the spin-stretched component of the first rotationally excited state, they collide inelastically with a rate coefficient of k 2 =(6.6±1.5)? 10 ??1 cm 3 /s at temperatures near 100~ μ K. We attribute this to rotation-changing collisions. When the molecules are in the ground rotational state we see no inelastic loss and set an upper bound on the spin relaxation rate coefficient of k 2 <5.8? 10 ??2 cm 3 /s with 95% confidence. We compare these measurements to the results of a single-channel loss model based on quantum defect theory. The comparison suggests a short-range loss parameter close to unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.

We observe bimodal fluorescence patterns from atoms in a fast atomic beam when the laser excitation occurs in the presence of a magnetic field and the atoms sample only a portion of the laser profile. The behavior is well explained by competition between the local intensity of the laser, which tends to generate a coherent-population-trapping (CPT) dark state in the J=1 to J ′ =0 system, and the strength of an applied magnetic field that can frustrate the CPT process. This work is relevant for understanding and optimizing the detection process for clocks or other coherent systems utilizing these transitions and could be applicable to in situ calibration of the laser-atom interaction, such as the strength of the magnetic field or laser intensity at a specific location.