Laser-cooled Atoms Research Articles

Realization of an Optomechanical Interface Between Ultracold Atoms and a Membrane

We have realized a hybrid optomechanical system by coupling ultracold atoms to a micromechanical membrane. The atoms are trapped in an optical lattice, which is formed by retroreflection of a laser beam from the membrane surface. In this setup, the lattice laser light mediates an optomechanical coupling between membrane vibrations and atomic center-of-mass motion. We observe both the effect of the membrane vibrations onto the atoms as well as the backaction of the atomic motion onto the membrane. By coupling the membrane to laser-cooled atoms, we engineer the dissipation rate of the membrane. Our observations agree quantitatively with a simple model.

Coherent population trapping in Raman-pulse atom interferometry

Raman pulse atom interferometry is an important modality for precision measurements of inertial forces and tests of fundamental physics. Typical Raman atom optics use two coherent laser fields applied at gigahertz-scale detunings from optical resonance, so that spontaneous emission produces a minor or negligible source of decoherence. An additional consequence of spontaneous emission is coherent population trapping (CPT). We show that CPT produces coherences and population differences which induce systematic effects in Raman pulse atom interferometers. We do not believe that CPT has been previously identified as an error mechanism in Raman pulse atom interferometry. We present an experimental characterization of CPT coherences and population differences induced in laser-cooled cesium atoms by application of Raman pulses at detunings near 1 GHz, commensurate with detunings used in several precision measurement experiments. We are not aware of previous demonstrations of CPT-induced population difference. We argue that CPT effects could induce phase shifts of several milliradians in magnitude for typical experimental parameters and stipulate that these errors can be suppressed by propagation direction reversal in Raman interferometer-based precision measurements.

Nanoscale focused ion beam from laser-cooled lithium atoms

We demonstrate a new type of nanoscale focused ion beam (FIB) based on photoionizing laser-cooled atoms held at millikelvin temperatures in a magneto-optical trap (MOT). This new source expands the range of available ionic species and accessible ion beam energies for FIBs, enhancing their role as one of the most important tools for nanoscale characterization and fabrication. We show examples of microscopy with lithium ions obtained by scanning the FIB and collecting the resulting secondary electrons, and characterize the beam focus by a 25–75% rise distance measurement of (26.7 ± 1.0) nm at a beam energy of 2 keV. We also examine the dependence of the focal size on MOT temperature and beam energy.

Photoassociative Production and Detection of Ultracold Polar RbCs Molecules

We have produced ultracold polar RbCs molecules via photoassociation starting from laser-cooled 85Rb and 133Cs atoms in a dual-species, forced dark magneto-optical trap. The formed electronically excited RbCs* molecules correlated to the Rb(5S1/2) + Cs(6P1/2) dissociation limit are observed by trap loss spectroscopy. Following the decay of these excited RbCs* molecules, the formed ground state molecules are directly ionized by a two-photon single-color pulse dye laser, which is a new ionization mechanism for ground state RbCs molecules and thence detected by time-of-flight mass spectroscopy.

Arbitrarily shaped high-coherence electron bunches from cold atoms

The potential to generate pulsed electron beams with charge distributions tailored in all three dimensions could revolutionize high-speed electron diffraction. A demonstration of a highly coherent pulse electron beam that can be arbitrarily tailored in two dimensions is a step towards this goal. Ultrafast electron diffractive imaging of nanoscale objects such as biological molecules1,2 and defects in solid-state devices3 provides crucial information on structure and dynamic processes: for example, determination of the form and function of membrane proteins, vital for many key goals in modern biological science, including rational drug design4. High brightness and high coherence are required to achieve the necessary spatial and temporal resolution, but have been limited by the thermal nature of conventional electron sources and by divergence due to repulsive interactions between the electrons, known as the Coulomb explosion. It has been shown that, if the electrons are shaped into ellipsoidal bunches with uniform density5, the Coulomb explosion can be reversed using conventional optics, to deliver the maximum possible brightness at the target6,7. Here we demonstrate arbitrary and real-time control of the shape of cold electron bunches extracted from laser-cooled atoms. The ability to dynamically shape the electron source itself and to observe this shape in the propagated electron bunch provides a remarkable experimental demonstration of the intrinsically high spatial coherence of a cold-atom electron source, and the potential for alleviation of electron-source brightness limitations due to Coulomb explosion6.

Measurement of the temperature of an ultracold ion source using time-dependent electric fields

We report on a measurement of the characteristic temperature of an ultracold rubidium ion source, in which a cloud of laser-cooled atoms is converted to ions by photo-ionization. Extracted ion pulses are focused on a detector with a pulsed-field technique. The resulting experimental spot sizes are compared to particle-tracking simulations, from which an effective source temperature T = (3 ± 2) mK and the corresponding transversal reduced emittance εr = 1.4 × 10−8 m rad eV are determined. Space charge effects that may affect the measurement are also discussed.

Laser-Focused Atomic Deposition for Nanascale Grating

Laser-focused atomic deposition is a technique with which nearly resonant light is used to pattern an atom beam. To solve the problem that the result of laser-cooled atoms cannot be monitored during the 30-min depositing time, we present a three-hole mechanically precollimated aperture apparatus. A 425 nm laser light standing wave is used to focus a beam of chromium atoms to fabricate the nanoscale grating. The period of the grating is 213±0.1 nm, the height is 4 nm and the full width at half miximum is 64±6 nm.

Laser-cooled atoms inside a hollow-core photonic-crystal fiber

We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of ~30,000 rubidium atoms, which creates a medium with optical depth ~180 inside the fiber. Compared to our earlier study this represents a six-fold increase in maximum achieved optical depth in this system.

Production of sodium Bose–Einstein condensates in an optical dimple trap

We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap (dT) propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage that results in efficient loading of the auxiliary dT, and a final evaporative cooling stage in the dT. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power.

Predicting and verifying transition strengths from weakly bound molecules

We investigated transition strengths from ultracold weakly bound $^{41}\mathrm{K}$$^{87}\mathrm{Rb}$ molecules produced via the photoassociation of laser-cooled atoms. An accurate potential energy curve of the excited state $(3){}^{1}{\ensuremath{\Sigma}}^{+}$ was constructed by carrying out direct potential fit analysis of rotational spectra obtained via depletion spectroscopy. Vibrational energies and rotational constants extracted from the depletion spectra of ${v}^{'}=41$--50 levels were combined with the results of the previous spectroscopic study and they were used for modifying an ab initio potential. An accuracy of 0.14% in vibrational level spacing and 0.3% in rotational constants was sufficient to predict the large observed variation in transition strengths among the vibrational levels. Our results show that transition strengths from weakly bound molecules are a good measure of the accuracy of an excited state potential.

Light pulse atom interferometry at short interrogation times

The use of cold atoms in any sensor operating in a dynamic environment requires that the measurement cycle be conducted before the atom cloud escapes the interaction region. Under multiple-g accelerations it is desirable to complete measurements in millisecond time scales, particularly when laser beams are used to interrogate the atoms. In this paper, we demonstrate high-contrast atom interferometry in a vapor cell using stimulated Raman transitions at millisecond interrogation times. Laser-cooled cesium atoms are interrogated with a sequence of three Raman pulses and the interferometer phase is read out in the same region in which the atoms are trapped. Our system achieved over 70% contrast with a Doppler insensitive interferometer and over 30% contrast with a Doppler sensitive interferometer, in an environment normally considered adverse to high-contrast atom interferometry (e.g., no retroreflector stabilization and no magnetic shielding). Demonstration of an inertially sensitive atom interferometer in this environment supports the feasibility of a high-bandwidth inertial sensor using light pulse atom interferometry. Finally, we show that Raman pulse population transfer efficiency in our system is primarily limited by nonuniformity of the Raman laser intensity across the atom cloud.

Quantum pumping of interacting bosons

Quantum pumping in an interacting one-dimensional Bose-Einstein condensate is analyzed. It is shown that a spatially periodic potential, oscillating adiabatically in time with frequency ${\ensuremath{\omega}}_{0}$, acts as a quantum pump inducing an atom current from broken spatiotemporal symmetries of the driven potential. The current generated by the pump is strongly affected by the interactions. It has a power-law dependence on the frequency with the exponent determined by the interaction, while the coupling to the pump affects the amplitudes. It depends on the phase difference between two umklapp terms of the drive, providing evidence for the full quantum character of the boson transport. The results suggest the realization of a quantum pump with laser-cooled atoms.

磁光阱与磁学阱中测量超冷Rb原子总碰撞截面

The implementation of a technique to measure total collision cross sections using laser-cooled rubidium atoms along with the introduction of room-temperature background gases, confined in both magnetooptic and magnetic traps, is proposed. Atom loss from the trap and total collision cross section can be inferred from the knowledge of the density of background gases. The measured cross sections from the magneto-optic and magnetic traps are represented, compared, and found to be significantly different. The measurements using this technique have a very small error in the range of approximately 2%-7%.

Coherent Transfer of Photoassociated Molecules into the Rovibrational Ground State

We report on the direct conversion of laser-cooled 41K and 87Rb atoms into ultracold 41K87Rb molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

Recoil-induced resonances for optical switching

We have investigated recoil-induced resonances in laser-cooled atoms for the purpose of developing an optical switch. In contrast to our earlier experiments 8, we employ a system in which the trapping beams are left on during the course of the measurement. We not only find the expected narrow resonance but also a much wider resonance. The widths of the wide and narrow resonances differ by three orders of magnitude. In addition, the two resonances show different temporal dynamics. We explore the utility of each resonance as an optical switch.

Direct Monte Carlo simulation of the sympathetic cooling of trapped molecules by ultracold argon atoms

We demonstrate that cold molecules (H2 and benzene) can be created at temperatures below 1 mK by sympathetic cooling with laser-cooled rare gas atoms on timescales of seconds. The thermalization process is studied using the direct simulation Monte Carlo (DSMC) method, which allows a detailed analysis of the atomic and molecular spatial and energy distributions as a function of time. As part of this study, ultracold elastic cross sections for Ar–Ar and Ar–C6H6 are also calculated.

Atomic fountain of laser-cooled Yb atoms for precision measurements

We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain. Atoms in a collimated thermal beam are first cooled and captured in a magneto-optical trap (MOT) operating on the strongly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{1}P}_{1}$ transition at 399 nm (blue line). They are then transferred to a MOT on the weakly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{3}P}_{1}$ transition at 556 nm (green line). Cold atoms from the green MOT are launched against gravity at a velocity of around 2.5 m/s using a pair of green beams. We trap more than ${10}^{7}$ atoms in the blue MOT and transfer up to $70%$ into the green MOT. The temperature for the odd isotope $^{171}\mathrm{Yb}$ is $~1 \mathrm{mK}$ in the blue MOT, and reduces by a factor of 40 in the green MOT.

Colloquium: Trapped ions as quantum bits: Essential numerical tools

Trapped, laser-cooled atoms and ions are quantum systems which can be experimentally controlled with an as yet unmatched degree of precision. Due to the control of the motion and the internal degrees of freedom, these quantum systems can be adequately described by a well known Hamiltonian. In this colloquium, we present powerful numerical tools for the optimization of the external control of the motional and internal states of trapped neutral atoms, explicitly applied to the case of trapped laser-cooled ions in a segmented ion-trap. We then delve into solving inverse problems, when optimizing trapping potentials for ions. Our presentation is complemented by a quantum mechanical treatment of the wavepacket dynamics of a trapped ion. Efficient numerical solvers for both time-independent and time-dependent problems are provided. Shaping the motional wavefunctions and optimizing a quantum gate is realized by the application of quantum optimal control techniques. The numerical methods presented can also be used to gain an intuitive understanding of quantum experiments with trapped ions by performing virtual simulated experiments on a personal computer. Code and executables are supplied as supplementary online material (http://kilian-singer.de/ent).

Sensitive gravity-gradiometry with atom interferometry: progress towards an improved determination of the gravitational constant

We present here the current status of our high-sensitivity gravity-gradiometer based on atom interferometry. In our apparatus, two clouds of laser-cooled rubidium atoms are launched in a fountain configuration and simultaneously interrogated by a Raman-pulse interferometry sequence. The system has recently been upgraded and its stability re-evaluated. We also discuss the recent progress of the experiment towards a precise determination of the Newtonian gravitational constant G. The signal-to-noise ratio and the long-term stability of the gravity gradiometer demonstrated interesting perspectives for pushing the G measurement precision below the 100 ppm level.

Ion Acoustic Waves in Ultracold Neutral Plasmas

We photoionize laser-cooled atoms with a laser beam possessing spatially periodic intensity modulations to create ultracold neutral plasmas with controlled density perturbations. Laser-induced fluorescence imaging reveals that the density perturbations oscillate in space and time, and the dispersion relation of the oscillations matches that of ion acoustic waves, which are long-wavelength, electrostatic, density waves.