Precision quantum sensors using cold atom interferometers with long interrogation times are often limited by the ballistic expansion of atomic samples after release from traps, manifesting by means of laser beam wavefront uncertainties. In this study we utilize near-resonant light interacting with an ultracold atomic sample for collective-mode excitation of a 87Rb Bose Einstein condensate (BEC) in a magnetic trap. The collective motion is initiated after abruptly modifying the atom-atom interaction energy by reduction of the BEC atom number density via photon scattering using the two-photon transition from $5S_{1/2}$ to $5D_{5/2}$ . We show that the two-photon transition can induce matter-wave lensing of the atomic cloud with minimal center-of-mass perturbation providing an optimal ultra-cold atomic sample for atom-based quantum sensors such as quantum gravimeters and accelerometers.
Precision quantum sensors using cold atom interferometers with long interrogation times are often limited by the ballistic expansion of atomic samples after release from traps, manifesting by means of laser beam wavefront uncertainties. In this study we utilize near-resonant light interacting with an ultracold atomic sample for collective-mode excitation of a 87Rb Bose Einstein condensate (BEC) in a magnetic trap. The collective motion is initiated after abruptly modifying the atom-atom interaction energy by reduction of the BEC atom number density via photon scattering using the two-photon transition from 5S1/2 to 5D5/2 . We show that the two-photon transition can induce matter-wave lensing of the atomic cloud with minimal center-of-mass perturbation providing an optimal ultra-cold atomic sample for atom-based quantum sensors such as quantum gravimeters and accelerometers.
Over the past half century, atomic ensembles have been used to create sensors, clocks, and quantum information systems. As these devices become more compact, and as the number of atoms increases to improve the sensitivity for detection, the atomic samples are increasing in density and optical depth. As such, the spectroscopic properties of the atomic media are modified due to interactions among the particles in the ensemble. We report investigation of near-resonance light scattering from a cold atomic sample of 87Rb. Initially prepared in a magneto-optical trap, the atoms are loaded into a far-off-resonance optical dipole trap (FORT) in which the ensemble has a temperature near 100 mK and initial Gaussian radii of approximately 3 mm and 280 mm in the transverse and longitudinal directions, respectively. With atomic densities in the range of 1010 - 1013 atoms/cm3, measurements are made on the F=2 -> F'=3 nearly closed hyperfine transition. The experimental geometry consists of projecting a near-resonance collimated laser beam onto the entire volume of the FORT and detecting the diffusely scattered light. The measured scattered light intensity as a function of detuning, atomic number, and sample size suggests that collective light scattering depends on the optical depth of the system.
We study necessary conditions for the observation of Bose–Einstein condensation in a magnetically trapped sample of atomic cesium gas. These constraints are due to interatomic collisions in the sample. We show that the prospects for observing Bose–Einstein condensation are favorable for a gas of ground-state Cs atoms in the highest state of the lowest hyperfine manifold. An interesting aspect of the calculations is that the scattering length for this f = 3, m f = −3 hyperfine state shows pronounced resonance structures as a function of applied magnetic field leading to variations of two orders of magnitude. Most importantly, the scattering length can change sign near the resonances. This suggests a controllable means to change the behavior of the Bose condensate because for negative values a condensate is unstable and other (quantum-)collective effects might be observed. The origin of the resonances is understood from the bound singlet and triplet rovibrational Cs 2 states which are perturbed due to the hyperfine and Zeeman interactions. It is a long standing goal to achieve quantum-collective effects in atomic Fermi- or Bose gases. The prominent reasons are that for a relatively simple system with low density, a microscopic theoretical treatment of the phase transition is still feasible, and to have an experimental testing ground for more complicated quantum-coherent effects such as superfluidity in 4 He and superconductivity in metals. Here, we focus on atomic species which behave as (composite) bosons.
Dual-species atomic sources in space promise to be the testbed for a multitude of searches in quantum gas physics such as a precise test of the Universality of Free Fall (UFF), few-body physics, cold molecules and quantum bubbles. These experiments demand exquisite control over the expansion energies of both condensed ensembles as well as over their differential center-of-mass dynamics. We propose a trap-quenched collimation technique featuring in-trap excitations of collective modes compatible with state-of-the-art atom-chip setups. Using NASA's Cold Atom Laboratory aboard the International Space Station, we demonstrate it on a single-species 87Rb condensate. By controlling the center-of-mass release dynamics, we observe free expansion times up to 700 ms and measure a two-dimensional expansion energy of kB⋅78±9pK in the imaging plane. A detailed model of the magnetically-induced dynamics indicates that this corresponds to a two-dimensional expansion energy of about kB⋅15−5+12pK along two of the condensate's eigenaxes. Finally, we theoretically study this trap-quenched collimation scheme for a 41K-87Rb mixture, predicting a simultaneous collimation that meets the expansion energy requirements for a state-of-the-art UFF test at the 10−15 accuracy level.
The precision of atom interferometers, targeted for example in the Hannover Very Long Baseline Atom Interferometer (VLBAI) facility, imposes stringent requirements in several respects. They concern the control of center-of-mass motion and expansion of the wave packets by the matter-wave source as well as the number of atoms. By reducing the expansion, systematic errors, appearing e.g. through wavefront aberrations, can be lowered. These requirements can be matched by employing ultracold quantum gases or even quantum degenerate gases. A promising method to create those ensembles is evaporative cooling in a spatially modulated optical dipole trap. Here, the utilization of time-averaged potentials enables the fast creation of ultracold atomic ensembles with large number of atoms. Both, the higher number of atoms and the increased repetition rate, enhance the performance of the interferometer due to a lower quantum projection noise, which scales with 1/sqrt(N), and a larger bandwidth of the sensor due to faster sampling. The shaping of the matter-waves by techniques such as matter-wave lensing or Delta-Kick collimation is also feasible due to the dynamic control of the trapping potential. In this thesis the implementation and application of dynamic time-averaged optical potentials created via center position modulation of dipole trap beams is demonstrated. By evaporative cooling in these potentials, 1.9(0.4) x 10^5 condensed atoms with an expansion temperature of 29.2(1.3) nK were achieved after 3 s of evaporation. Up to 4.2(0.1) x 10^5 condensed atoms could be observed with slower evaporation of 5 s. Subsequent matter-wave lensing is carried out yielding expansion rates as low as 553(49) μms^-1 resulting in an effective temperature of 3.2(0.6) nK in two dimensions. This lens can be applied at any stage of evaporative cooling, thus short-cutting the generation of ultracold effective temperatures. In this thesis the limitations of optical matter-wave lensing in the current setup are revealed and ways to improve the performance are discussed. The fast generation of ultracold atomic ensembles will enhance the performance of the dual-species atom interferometer, which represents the experiment apparatus for this thesis and strives for a test of the Universality of Free Fall with an uncertainty on the order of 10^-9. The results of this thesis were used to test numerical simulations which were utilized to show the perspective of generating up to 10^6 collimated condensed atoms within 1 s of cycle time in the rubidium source system of Hannover’s VLBAI.
In contrast to light, matter-wave optics of quantum gases deals with interactions even in free space and for ensembles comprising millions of atoms. We exploit these interactions in a quantum degenerate gas as an adjustable lens for coherent atom optics. By combining an interaction-driven quadrupole-mode excitation of a Bose-Einstein condensate (BEC) with a magnetic lens, we form a time-domain matter-wave lens system. The focus is tuned by the strength of the lensing potential and the oscillatory phase of the quadrupole mode. By placing the focus at infinity, we lower the total internal kinetic energy of a BEC comprising 101(37) thousand atoms in three dimensions to 3/2 k_{B}·38_{-7}^{+6} pK. Our method paves the way for free-fall experiments lasting ten or more seconds as envisioned for tests of fundamental physics and high-precision BEC interferometry, as well as opens up a new kinetic energy regime.
The sensitivity of atom interferometers depends on their ability to realize long pulse separation times and prevent loss of contrast by limiting the expansion of the atomic ensemble within the interferometer beam through matter-wave collimation. Here we investigate the impact of atomic interactions on collimation by applying a lensing protocol to a 39K Bose-Einstein condensate at different scattering lengths. Tailoring interactions, we measure energies corresponding to 340±12 pK in one direction. Our results are supported by an accurate simulation, which allows us to extrapolate a 2D ballistic expansion energy of 438±77 pK. Based on our findings we propose an advanced scenario, which enables 3D expansion energies below 16 pK by implementing an additional pulsed delta-kick. Our results pave the way to realize ensembles with more than 1×105 atoms and 3D energies in the two-digit pK range in typical dipole trap setups without the need for micro-gravity or long baseline environments.