Session W45: Atom-Light Interactions: Experiment and Theory

Lattice-induced nonadiabatic frequency shifts in optical lattice clocks

We consider the frequency shift in optical lattice clocks which arises from the coupling of the electronic motion to the atomic motion within the lattice. For the simplest of three-dimensional lattice geometries this coupling is shown to affect only clocks based on blue-detuned lattices. We have estimated the size of this shift for the prospective strontium lattice clock operating at the 390-nm blue-detuned magic wavelength. The resulting fractional frequency shift is found to be on the order of $10^{-18}$ and is largely overshadowed by the electric quadrupole shift. For lattice clocks based on more complex geometries or other atomic systems, this shift could potentially be a limiting factor in clock accuracy.   

Magic wavelengths for optically trapped atoms

The ability to trap neutral atoms inside high-Q cavities in the strong coupling regime is of particular importance for quantum computation and communication schemes, where it is essential to precisely localize and control neutral atoms with minimum decoherence. In a far-detuned optical dipole trap, the potentials experienced by an atom in its ground and excited states may be of opposite sign affecting the fidelity of experiments in which excited states are temporarily occupied. ``Magic wavelengths'' are those for which such potentials are equal. Single-laser schemes offer few cases in which magic wavelengths exist for Rb [Arora et al. PRA 76, 052509 (2007).] Here we explore bichromatic schemes for state-insensitive optical trapping of the Rb. We describe the use of trapping and control lasers to minimize the variance of the potential experienced by a trapped Rb atom in ground and excited states. We have also identified wavelengths $\lambda_{\rm{zero}}$ where the ground state frequency-dependent polarizabilities in alkali-metal atoms are zero. These are relevant for cooling and trapping experiments involving mixtures such as Rb/Yb, where the vanishing lattice potential for Rb facilitates interesting applications.   

Laser frequency stabilization for narrow linewidth cooling of $^6$Li atoms

Laser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth $\Gamma$ of an atomic transition sets a lower bound on the Doppler cooling temperature $k_B T_D = \hbar \Gamma/2$. The $2s-2p$ transition in $^6$Li has a lower bound temperature of $T_D \approx 140 \, \mu$K. In contrast, the $2s-3p$ transition has a narrower linewidth and thus provides a lower temperature limit of $T_D \approx 20 \, \mu$K. We present a method for stabilizing a laser to an atomic line in a vapor cell using modulation transfer spectroscopy and a home-built lock-in amplifier. Our results demonstrate successful locking of a 323 nm laser to the $2s-3p$ transition. The stabilized laser provides a second stage of magneto-optical trapping that results in an order of magnitude increase in the phase space density before evaporating to degeneracy in an optical dipole trap.   

Demonstration of a $^{6}$Li magneto-optical trap using the $2S_{1/2}\rightarrow 3P_{3/2}$ transition

We demonstrate narrow linewidth laser cooling on the $2S_{1/2}\rightarrow 3P_{3/2}$ transition of $^{6}$Li at 323 nm. Typically, magneto-optical traps (MOTs) of alkali atoms cool on the D2 transition. The linewidth of this transition determines the Doppler limit of cooling which in the case of $^{6}$Li is 140 $\mu$K, given a 5.9 MHz transition linewidth. Due to a lack of resolved hyperfine structure that prohibits polarization gradient cooling, typical Li MOTs reach minimum temperatures near 300 $\mu$K. Cooling on the $2S_{1/2}\rightarrow 3P_{3/2}$ transition, however, allows for a Doppler limit of 20 $\mu$K since the transition linewidth is only 790 kHz. We have implemented this cooling scheme and demonstrate $^{6}$Li MOT temperatures of 65 $\mu$K. With the increased phase space density from this MOT, initial loading of the gas to an optical trap is substantially enhanced. We present our results on the characteristics of the narrow linewidth MOT and our results on the benefits of using this cooling scheme in the preparation of a degenerate gas of fermions.   

Pulsed Counter-Rotating Source of Slow, Cold Molecules

We describe the performance of a new design of the counter-rotating source (CRS) of slow molecules introduced originally by Gupta and Herschbach. The CRS produces a supersonic expansion from a nozzle near the tip of a hollow rotor spun at high speed contrary to the exit beam velocity. Thereby the lab velocity can be markedly reduced. Introducing a pulsed feeding system, cryo-cooling, and shutter system has eliminated the main problem of the original CRS apparatus, in which continuous gas flow imposed high background pressure. The new version provides intense pulses, typically of duration 0.1 ms with lab speeds as low as 40 m/s and longitudinal temperature as low as 0.5 K. This device can, in principle, decelerate (or accelerate) any molecule available as a gas; we report experiments producing slow beams of krypton, oxygen, ammonia, and nitrogen dioxide.   

Nanofriction in Cold Ion Traps

Sliding friction between crystal lattices and the physics of cold ion traps are so far non-overlapping fields. Two sliding lattices may either stick and show static friction or slip with dynamic friction; cold ions are known to form static chains, helices, or clusters, depending on trapping conditions. Based on simulations, we show that much could be learnt about friction by sliding (e.g., via an electric field) the trapped ion chains over a periodic corrugated potential. Unlike infinite chains where, according to theory, the classic Aubry transition to free sliding may take place, static pinning always shows up in trapped chains. Nonetheless we find that a properly defined static friction still vanishes Aubry-like at a symmetric-asymmetric structural transition, ubiquitous for decreasing corrugation in both straight and zig-zag trapped chains. Dynamic friction can also be addressed by ringdown oscillations of the ion trap. Long theorized static and dynamic one dimensional friction phenomena could thus become exquisitely accessible in future cold ion tribology.   

Pattern formation with trapped ions

We propose an experiment to study collective behavior in a nonlinear medium of trapped ions. Using laser cooling and heating and an anharmonic trap potential, one can turn an ion into a nonlinear van der Pol-Duffing oscillator. A chain of ions interacting electrostatically has stable plane waves for all parameters. The system also behaves like an excitable medium, since a sufficiently large perturbation generates a travelling pulse. Small chains exhibit multistability and limit cycles. We account for noise from spontaneous emission in the amplitude equation and find that the patterns are observable for realistic experimental parameters. The tunability of ion traps makes them an exciting setting to study nonequilibrium statistical physics.   

Spontaneous emission modification via quantum interference with energy shifts remained

Quantum interference in spontaneous emission from a four-level atom is investigated with the counter rotating terms and the energy shifts included. The atom has two upper levels coupled to a common lower level by the same vacuum modes and is driven by a coherent field to an auxiliary level. The effect of the counter rotating terms in coupling through the vacuum modes is taken into account by a unitary transformation method. We show how the quantum interference due to the energy shifts effects the spontaneous emission spectrum.   

Optical chirality and superchiral fields

A chiral object is any material body whose mirror image may not be superimposed on the original. Electromagnetic (EM) fields may be chiral too, with circularly polarized light as the paradigmatic example. We propose a measure of the local chirality of EM fields, which we call optical chirality. Optical chirality determines the degree of chiral asymmetry in the interaction of light with small molecules. We predict the existence of superchiral forms of light which show larger bias for exciting a single chiral enantiomer, in some regions of space, than does circularly polarized plane waves. We performed a conceptually simple experiment to probe the interaction of superchiral light with a chiral biperylene derivative. We selected this molecule for its strong intrinsic optical activity and fluorescence in the visible. The regions of enhanced chiral selectivity are too thin to detect by absorption, so we used fluorescence instead. We demonstrated experimentally a 12-fold enhancement in the chiral selectivity of superchiral fields for these chiral compounds. The demonstrated chiral enhancement is not a fundamental limit. Larger enhancement may be obtained at the expense of lower overall excitation rate. These results establish that optical chirality is a fundamental property of the electromagnetic field, with possible applications ranging from plasmonic sensors to absolute asymmetric synthesis.   

Observation of a Red-Blue Detuning Asymmetry in Matter-Wave Superradiance

We report the first experimental observation of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the detuning of the pump laser from the one-photon resonance, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned pump light the distinctive multi-order, matter-wave scattering pattern is clearly visible, whereas with blue-detuned pump light superradiance is strongly suppressed. However, in the limit of a dilute atomic gas symmetry is restored.   

Large and rapidly-responding Kerr nonlinear phase shift using a four-level~N-scheme with an active Raman gain core

We report rapidly-responding Kerr nonlinear phase shift using a long pencil-shaped cold $^{85}$Rb atom samples. Our system is a four-level~N-scheme based on both the D1 and D2 transition lines of Rb atom. The nonlinear phase shift and nonlinear optical index of the cold medium are studied using an interferometric method for various pump and phase-inducing laser intensities.   

Experimental Observation of Carrier-Envelope Phase Effects in Multi-Cycle Pulses

Using intense RF pulses interacting with the magnetic Zeeman sub-levels of Rubidium (Rb) atoms, we have experimentally and theoretically shown the CEP effects in the population transfer between two bound atomic states interacting with pulses consisting of many cycles (up to 15 cycles) of the field. It opens several exciting applications and interesting possibilities that can be easily transfer to optical range and enhance current and create new set of tools to control CEP of laser pulses. These tools allow researchers to improve laser systems that generate laser pulses with better reproducibility and accuracy and better controlled. Also the tools provide an additional handle to control the process of collisions, and the current approach of extending the duration of the pulses with measurable or controllable CEP allows researchers to extend the coherent control to a new level where they are able to study molecular collisions or electron collisions in nano-structures under the action of strong electromagnetic fields with known CEP. In particularly, the obtained results can be applied to control of chemical reactions.   

Counter-Propagating Coherent Stimulated Raman Spectroscopy for Remote Sensing in Air

We analyze phase-matching conditions in various four-wave mixing schemes for coherent nonlinear optical spectroscopy in the counter-propagating beam configuration. Coherent stimulated Raman spectroscopy satisfies the conditions and gives a signal containing specific molecular spectroscopic information. A counter-propagating broadband and a narrowband pulses are used to measure the Raman spectrum with a single shot. In addition, the nonresonant background due to the nondegenerate four-wave mixing is suppressed. Using this technique we develop a new scheme for standoff spectroscopy in atmosphere by using nitrogen molecules in air as a gain medium for remote lasing.   

Efficient Backward Emission from Optically Pumped Air

We demonstrate the generation of backwards emitted coherent light in atmosphere via optical pumping. The backwards emitted light is narrow band centered at 845 nm and is generated from the dissociation of molecular oxygen and then subsequent two photon excitation of these newly dissociated oxygen atoms. Both the dissociation and excitation of oxygen are driven by a single 226 nm $\sim $10 nanosecond pulsed pump beam. The produced 845 nm light is a pulse approximately 10 nanosecond in duration and not only exhibits threshold characteristics, but is also nearly diffraction-limited. This optically-pumped mirror-less light source which propagates back towards the pump source presents a unique opportunity to develop new techniques for remote sensing.   

Quantum lithography beyond the diffraction limit via Rabi-oscillations

We propose a quantum optical method to do the sub-wavelength lithography. Our method is similar to the traditional lithography but adding a critical step before dissociating the chemical bound of the photoresist. The subwavelength pattern is achieved by inducing the multi-Rabi-oscillation between the two atomic levels. The proposed method does not require multiphoton absorption and the entanglement of photons. This method is expected to be realizable using current technology.