1,721,061 research outputs found
Photoassociation in a quantum degenerate gas of lithium-7
No full textAn experiment studying the effects of photoassociation in a quantum degenerate gas of 7Li bosons has been performed in a permanent magnet trap. A saturation in the one-photon photoassociation rate and a shift in the resonance due to the applied light field from the 2S 1/2 ground state to the 2P1/2 v ' = 83 excited molecular vibrational state have been measured and compared with theory. Limitations in the ability of the permanent magnet apparatus to study photoassociation in a Bose-Einstein condensate have prompted the development of a magneto-optical trap and an electro-magnetic trap. These new traps will assist in the process of creating a large BEC where the effects of photoassociation will be studied
Measuring the weak value in an optical experiment
No full textThe first experimental realization of a measurement of the weak value of a variable, a concept recently introduced by Aharonov, Albert and Vaidman, is presented. Weak measurements (measurements of a weak value) address the situation in which the separation of the eigenvalues caused by a weakly interacting measurement device is small compared to the width of the distribution of the individual eigenvalues. By appropriate choice of the pre- and post-selected state it is possible that the overlapping eigenvalues will interfere producing a value outside the range of eigenvalues. We demonstrate that the weak value is a practical method of amplifying and resolving the separation between overlapping eigenvalues in an optical experiment proposed by Duck, Stevenson and Sudarshan. In this experiment a birefringent crystal spatially separates two linear polarization components of a Gaussian laser beam by a distance much smaller than the beam waist
Precise determination of the 2P radiative atomic lifetime of lithium using photoassociative spectroscopy
No full textSpectroscopy of the high-lying vibrational levels of the 1\sp3\Sigma\sbsp{g}{+} and 1\sp1\Sigma\sbsp{u}{+} states of both \rm\sp6Li\sb2 and \rm\sp7Li\sb2 has been accomplished via photoassociation of ultracold atomic lithium confined in a magneto-optical trap. Both these molecular states correlate to 2S\sb{1/2} plus 2P\sb{1/2} free atomic states. The long range part of these molecular potentials depends on the 2P atomic radiative lifetime. Accurate models were constructed for both the 1\sp3\Sigma\sbsp{g}{+} and 1\sp1\Sigma\sbsp{u}{+} potentials. By fitting these models to the experimentally measured vibrational levels observed in \rm\sp6Li\sb2 and \rm\sp7Li\sb2, we have been able to extract a value for the 2P lifetime of lithium. Fitting to the absolute values of the data gives a value of 26.99 0.16 ns whereas fitting to differences in the spectral features gives a value of 27.04 0.18 ns. While both values agree with previous work, improvements made in the potential models would enable this analysis to resolve the current discrepancy between experiment and theory
Solitons and Breathers in Bose-Einstein Condensates
No full textSolitons are non-dispersive wave packets that balance the wave dispersion with a focusing nonlinearity. They maintain their shape and velocity after collisions with other solitons, owing to the integrability of the underlying nonlinear partial differential equations. They appear in various physical systems, such as water waves, light waves in optical fiber, and matter waves. In our laboratory, we study solitons and their interactions in a Bose-Einstein condensate with an attractive nonlinearity, trapped in a quasi-one-dimensional waveguide. We investigate the collisions of a pair of solitons and explore the relative phase implications on the boundary of integrability. We study soliton train formation with the quench of the nonlinearity strength, and interactions between neighboring solitons in the train. We form a composite soliton, known as a breather, where multiple solitons overlap and the wavefunction ``breathes'' in time. We also explore the response of an elongated condensate to an external modulation of the nonlinearity. These experiments characterize the mean-field descriptions of solitons and Bose-Einstein condensates and explore the boundaries between the mean-field theory and quantum many-body theory
Bose-Einstein condensation of lithium
No full textBose-Einstein condensation (BEC) in ultra-cold magnetically-trapped \sp7Li vapor was experimentally observed and quantitative measurements of condensate number were made. Compared to other BEC experiments, lithium is unique due to its negative s-wave scattering length, corresponding to effectively attractive interactions. Due to this attraction, condensates are expected to undergo mechanical collapse if the condensate number exceeds a critical value. In this experiment, an upper limit of about 1000 condensate atoms is found, in agreement with theoretical predictions.
In the experiment, the atoms are confined by a set of six permanent magnets in the Ioffe configuration. Optical forces are used to slow and guide atoms from a thermal atomic beam into the magnetic trap. With about 10\sp8 atoms loaded into the trap, the vapor is laser-cooled to near 200 K and then evaporatively cooled by application of a resonantly-tuned microwave field. Evaporative cooling produces a million-fold increase in phase-space density, reaching quantum-degenerate conditions with about 10\sp5 atoms at temperatures near 300 K. After cooling, the trapped atom distribution is observed by in situ imaging via an optical probe. Calculated atom distributions are fit to the image data.
In initial data, the imaging resolution was insufficient to see the spatially-narrow condensate peak, but as phase-space densities approached the expected phase transition, the images suddenly became distorted. Initial fits to the data suggested as many as 10\sp5 condensate atoms, in strong disagreement with theoretical predictions. An imaging model, accounting for imperfections in the imaging optics, shows that the sudden appearance of the distortions is a consequence of BEC, and that these distortions led to the initial over-estimation of cloud phase-space density and condensate number.
Improved imaging was obtained using large probe detunings, a Phase-Contrast Polarization Imaging (PCPI) technique, and near-diffraction-limited imaging optics. The PCPI method exploits the birefringence of the trapped atoms. From the resulting images, quantitative estimates of condensate number are obtained and compared with theory
A laser diode system and its use in a laser cooling experiment
No full textA system to control and stabilize the output of visible laser diodes was developed and used to measure the velocity distribution of lithium atoms in a laser cooling experiment. Circuitry was designed and built for controlling the diode temperature and current, and optical feedback from a grating was used to further tune the laser and to narrow its lineshape. In the experiment, atoms from a thermal lithium beam were slowed to near zero velocity using a multi-frequency relay chirp technique
Laser Frequency Stabilization for Narrow Linewidth Cooling of Lithium-6
No full textLaser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth of an atomic transition sets a lower bound on the Doppler cooling temperature. The 2s-2p transition in lithium-6 has a Doppler temperature of ~140 μK In contrast, the 2s-3p transition has a narrower linewidth and thus provides a lower temperature limit of ~20 μK. 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 a factor of ∼5 increase in the phase space density before evaporating to degeneracy in an optical dipole trap.NSF REU program (Physics and Chemistry and DMR)No embarg
A 2D optical lattice for creating a 1-dimensional Fermi gas
No full textUltracold atomic gases can be prepared in laboratory environments with unrivaled control and purity. In one and a half decades the field has evolved from the first Bose condensates in dilute alkali gases to multiple species mixtures and degenerate Fermi systems. Combining quantum degenerate fermions and optical lattices enables the simulation of relevant condensed matter systems. By observing the final state of an atomic sample in a tailored optical potential the system can be used as an analog quantum computer to evaluate Hamiltonians that are computationally impossible to tackle on classical computers. A versatile optical dipole trap has been constructed and characterized. The trap can be converted to an optical lattice, allowing for the investigation of the phase diagram of the two-component, one-dimensional, imbalanced Fermi gas
Dynamics of Bose-Einstein condensation in lithium-7
No full textBose-Einstein condensation (BEC) of 7Li has been investigated. Because the effective interaction between 7Li atoms is attractive, the condensate occupation number N 0 is limited to ∼ 1250 atoms, and when this limit is exceeded, the condensate becomes unstable with respect to mechanical collapse. The interplay of this limit and the natural growth of the condensate during BEC leads to complicated dynamical behavior, which has been studied both theoretically and experimentally. It has been modeled by solving the quantum Boltzmann equation, in conjunction with results from the nonlinear Schrodinger equation. It is found that N0 oscillates rapidly as the condensate alternately fills and collapses, and that the oscillations can persist for many cycles before the gas comes to equilibrium. Experimental evidence for these oscillations was obtained by repeatedly producing a condensate and measuring N 0. The results were seen to vary randomly from one measurement to the next, which is to be expected as the timing of the oscillations is intrinsically stochastic. The distribution of N0 values occurring was measured, and provides quantitative information on the condensate growth and collapse. The equilibration process itself was also observed, by quenching the gas into degeneracy and observing its relaxation.
In order to carry out these experiments, sensitive measurement and analysis techniques were developed which enabled N0 to be determined in situ with an accuracy of +/-20% and a precision of +/-60 atoms. The theoretical tools used to study quantum degenerate gases were also applied to the important experimental technique of evaporative cooling, which led to substantial optimization and improvements. As part of this study, the rate constant for dipolar relaxation was measured to be 1.05 +/- 0.1 x 10-14 cm3/s, in agreement with theoretical predictions
Design of a high numerical aperture vacuum chamber for 3D optical lattice experiments
No full textThe next generation of 3D optical lattice experiments requires a new vacuum chamber design. In the past, a 3D optical lattice apparatus in our lab was used to create and observe antiferromagnetic (AFM) coorelations within a 6Li-based degenerate Fermi gas. In that experiment, implementing anti-trapping beams overlapped with the standard lattice beams allowed the potential landscape to be made smoother, in an effort to increase the spatial extent of the AFM phase. Nevertheless, preparing atoms to be even cooler, and further extending the region of interest requires newer solutions. One method is to use the anti-trapping beams in a slightly different manner. By utilizing a higher numerical aperture optical beam line for anti-trapping, finer adjustments can be made in the potential. Combined with digital-micromirror based holography techniques. the anti-trapping beams can be custom-shaped to carve even finer details into the optical lattice. With sufficient control over beamshaping, one could perform better entropy re-distribution within the atom cloud. In this way, the central AFM region can be made colder by pushing entropy out to the wings. In this thesis, I will detail the work done in analyzing and designing a new chamber to fulfill the previously mentioned features
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