R-3 Repository :: Browsing by Author "Hulet, Randall G."

Browsing by Author "Hulet, Randall G."

Now showing 1 - 20 of 77

1D to 3D Crossover of a Spin-Imbalanced Fermi Gas
(American Physical Society, 2016) Revelle, Melissa C.; Fry, Jacob A.; Olsen, Ben A.; Hulet, Randall G.; Rice Center for Quantum Materials
We have characterized the one-dimensional (1D) to three-dimensional (3D) crossover of a two-component spin-imbalanced Fermi gas of 6Li atoms in a 2D optical lattice by varying the lattice tunneling and the interactions. The gas phase separates, and we detect the phase boundaries using in situ imaging of the inhomogeneous density profiles. The locations of the phases are inverted in 1D as compared to 3D, thus providing a clear signature of the crossover. By scaling the tunneling rate t with respect to the pair binding energy εB, we observe a collapse of the data to a universal crossover point at a scaled tunneling value of ˜tc=0.025(7).
3D optical lattice system for ultra-cold Lithium 6
(2014-04-25) Yang, Tsung-Lin; Hulet, Randall G.; Killian, Thomas C.; Dai, Pengcheng
Optical lattice experiments have proved to be a versatile tool for studying strongly correlated quantum systems. The motivation behind these experiments is to use the cold atoms to emulate a solid state system for which the Hamiltonian is analytically or numerically non-solvable. In our experiment, we construct a 3D simple cubic lattice system which realizes the Fermi-Hubbard model and provides us the opportunity to study the antiferromagnetic (AFM) insulator phase. In order to establish evaporative cooling in the optical lattice and to enlarge the size of the AFM phase, we implement a compensation potential additional to the standard lattice beam. In this thesis, I will present detailed steps to construct and calibrate the compensated 3D optical lattice system.
A 2D optical lattice for creating a 1-dimensional Fermi gas
(2009) Paprotta, Tobias; Hulet, Randall G.
Ultracold 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.
A laser diode system and its use in a laser cooling experiment
(1992) Bradley, Curtis Charles; Hulet, Randall G.
A 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.
A New Optical Trap System for Ultracold Strontium
(2013-12-04) Huang, Ying; Killian, Thomas C.; Hulet, Randall G.; Pu, Han
Atoms can be trapped at the foci of intense laser beams, which can enable the study of interactions and dynamics of ultracold gases. In this thesis, we will describe our new trap designs. A large volume pancake-shaped optical dipole trap is initially used for loading large numbers of atoms from a Magneto-Optical Trap. A BEC with a large number of 84 Sr atoms has been achieved after evaporation in this trap. To form degenerate gas of 88 Sr we compress the mixture of 88 Sr and 87 Sr from the loading trap into a superimposed dimple trap. This combination improves the reproducibility of the experiment and shortens the time required to create quantum degenerate samples, while we are able to create 88 Sr BEC with high density. In order to generate BEC of 86 Sr, an isotope with large scattering length and extremely high three-body loss rate, we implement an optical sheet trap which has an aspect ratio of 1:10. The tight axis in the vertical direction provides strong potential to hold against gravity while the large horizontal dimension brings up the trap volume and keeps down the atomic density.
A permanent magnet trap for cold atoms
(1995) Tollett, Jeffrey John; Hulet, Randall G.
Approximately$4\times10\sp8$ ground state lithium atoms have been confined in a non-zero magnetic field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of $8\times10\sp9$ cm$\sp{-3}$ The trapped atom lifetime is 240 seconds, limited by collisions with background gas. The tightly confining fields generated through the use of permanent magnets create a trap which is an excellent environment for the study of quantum statistical effects, atomic collisions, and other ultra-low temperature phenomena.
A study of collisional trap loss in ultra-cold trapped lithium-6 and lithium-7
(1994) Ritchie, Nicholas William Miller; Hulet, Randall G.
Measurements of the trap loss rate in ultra-cold magneto-optically trapped $\sp6$Li and $\sp7$Li are presented and compared. Clear evidence is presented for two different trap loss mechanisms involving inelastic collisions between one ground state atom and one excited state atom. The fine-structure changing (FSC) mechanism, in which the excited state atom changes fine-structure level during a collision, is seen to dominate the rate at low trap laser intensities. However, as the intensity is increased, the trap becomes sufficiently deep to be able to contain the products of FSC collisions and this mechanism no longer contributes to trap loss. It is a unique aspect of Li that the energy imparted to an atom in a FSC collision is in a range that may be recaptured with experimentally attainable trap parameters. When the products of all FSC collisions remain trapped, only the radiative escape (RE) trap loss mechanism, due to the emission of a less energetic photon than the initial excitation photon, contributes to the trap loss rate. At small detunings, the rate of the RE trap loss mechanism is seen to be over two orders-of-magnitude smaller than the FSC rate. The FSC trap loss rate for $\sp6$Li and $\sp7$Li were found to be largest at smaller detunings and of comparable magnitude in $\sp6$Li and $\sp7$Li. The RE trap loss rate in $\sp6$Li was seen to be roughly four times the RE trap loss rate in $\sp7$Li. To understand better the dynamics of trap loss and magneto-optical traps in general, a sophisticated model of magneto-optical trap kinetics has been developed. This model has demonstrated that most critical factor determining the maximum velocity an atom may have and yet remained trapped is the initial atom's frame detuning, $\Delta$ - k v, where $\Delta$ is the trap laser detuning, k is the propagation vector of the trap beam most nearly anti-parallel to v, the atom's velocity. All else being equal, minimizing the magnitude of this quantity maximizes the velocity that a trap will retain. More detailed results of this model are also presented.
Adiabatic cooling of atoms by an intense standing wave
(American Physical Society, 1992) Chen, Jian; Story, J.G.; Tollett, J.J.; Hulet, Randall G.; Rice Quantum Institute
Lithium atoms channeled in the nodes of an intense standing-wave radiation field are cooled to near the recoil limit by adibatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root-mean-squared momentum of 2ħk in one dimension, where ħk is the momentum of a radiation-field photon. The data are compared with the results of a Monte Carlo simulation using a two-level atom model. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.
An All Solid-State Laser System for Cooling and Trapping Lithium
(2013-09-16) Revelle, Melissa; Hulet, Randall G.; Killian, Thomas C.; Morosan, Emilia
Ultra-cold atoms have become an essential tool in studying unique phenomena in condensed matter systems such as superconductivity and quantum phase transitions. To accomplish these experiments we use an apparatus designed to trap and cool lithium atoms down to nano-Kelvin temperatures. Recently, significant upgrades to the laser system have been made to improve performance, increase stability, minimize maintenance and improve flexibility. We are working towards two exciting projects: proving the existence of an exotic superfluid state (FFLO) and probing the crossover between one and three-dimensions in a spin-1/2 Fermi gas with a spin-imbalance.
An improved system for creating ultracold Fermi gases of lithium-6
(2004) Partridge, Guthrie Bran; Hulet, Randall G.
An apparatus has been developed to create spin mixtures of ultracold fermionic 6Li in order to investigate the behavior of strongly interacting Fermi gases, and in particular, search for a superconducting phase transition. 6Li atoms are initially confined in a dual species magneto-optical trap (MOT) along with bosonic 7Li. Due to an overlap between spectral features of the two species, a D2/D1 trap/repump scheme is used for the 6Li. The atoms are subsequently cooled to quantum degeneracy in a magnetic trap through optimized dual forced evaporation and transferred to an all optical trap where spin mixtures are created with RF sweeps and pulses. The atomic interactions are controlled through the use of magnetic Feshbach resonances and an externally applied bias magnetic field of up to 1000 G. This thesis details the construction, operation, and performance of the 6Li portion of the apparatus, as well as current results.
Bose-Einstein condensation of lithium
(1997) Bradley, Curtis Charles; Hulet, Randall G.
Bose-Einstein condensation (BEC) in ultra-cold magnetically-trapped $\sp7$Li 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 $\mu$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 $\mu$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.
Collisional interactions in an ultracold lithium gas
(2000) McAlexander, William Ian; Hulet, Randall G.
Laser cooling and atom trapping techniques have enabled atomic physicists to attain temperatures on the order of 100 nK. In this regime, a trapped gas must be described within the framework of quantum statistical mechanics. In 1995, Bose-Einstein condensation (BEC) was achieved for the first time in such a system by three independent groups. The success of attaining BEC has encouraged research in quantum degenerate Fermi gases (DFG). One of the most exciting opportunities for a DFG is the prospect for Cooper pairing in a two-component gas. The experimental techniques for obtaining degenerate gases, as well as the mechanism for Cooper pairing, hinges on understanding and characterizing ultracold collisional interactions. This thesis is a twofold study in this exciting field. The first section is an in-depth treatment of collisions in atomic lithium. Lithium has two isotopes, 6Li which is a fermion and 7Li which is a boson. This makes it an ideal candidate for studying quantum degenerate gases. In a series of experiments using a magneto-optical trap, we have spectroscopically measured the ground and excited state molecular potentials for both isotopes. From this work we have been able to obtain the most accurate potentials for lithium to date. These precise measurements have enabled us to extract the most accurate value to date for the 2P atomic radiative lifetime of lithium. Furthermore, we have developed a coupled-channel treatment of two-body collisions which has enabled us to calculate the scattering amplitude, elastic cross section, and rate of spin-exchange decay for any ground state collision in lithium as a function of temperature and magnetic field. Armed with this knowledge, we have made progress on the design and construction of an electromagnetic trap for lithium. This trap will enable the study of a degenerate Fermi gas of 6Li as well as the possibility of observing a Cooper pairing transition. The trap performance has been characterized for each isotope and current work is focussed on cooling lithium down to degeneracy. Measurements of spin-exchange decay have been made and compare well with the collisional theory that has been developed.
Collisional loss of one-dimensional fermions near a p-wave Feshbach resonance
(2021-05-13) Chang, Ya-Ting; Hulet, Randall G.
The prospect of using Majorana fermions for application to fault-tolerant quantum computation has recently received much attention. Majorana fermions can be created in $p_x + ip_y$ topological superfluids or at the ends of a 1D $p$-wave superfluid. Exploring $p$-wave pairing in ultracold atomic gases has been difficult due to severe atom loss from three-body recombination near a $p$-wave Feshbach resonance. Three-body loss is predicted to be suppressed, however, in quasi-one-dimensional (quasi-1D), due to the extended wavefunctions of the open-channel dominant Feshbach dimers. In this work, we have experimentally studied collisional loss of a quasi-1D spin-polarized Fermi gas near a $p$-wave Feshbach resonance using ultracold $^6$Li atoms confined to a 2D optical lattice. We measured the atom loss as a function of time and extracted the three-body recombination rate, $L_3$. We also analyzed the atom loss as a two-step cascade three-body recombination model, in which weakly bound dimers are formed prior to their loss arising from atom-dimer collisions. In this thesis, the comparison of the experimental results and the theoretical predictions are presented. The implications of these measurements for observing $p$-wave pairing in quasi-1D are discussed in this thesis.
Collisions of matter-wave solitons
(Nature Publishing Group, 2014) Nguyen, Jason H.V.; Dyke, Paul; Luo, De; Malomed, Boris A.; Hulet, Randall G.
Solitons are localized wave disturbances that propagate without changing shape, a result of a nonlinear interaction that compensates for wave packet dispersion. Individual solitons may collide, but a defining feature is that they pass through one another and emerge from the collision unaltered in shape, amplitude, or velocity, but with a new trajectory reflecting a discontinuous jump. This remarkable property is mathematically a consequence of the underlying integrability of the one-dimensional (1D) equations, such as the nonlinear Schrödinger equation, that describe solitons in a variety of wave contexts, including matter waves [1, 2]. Here we explore the nature of soliton collisions using Bose–Einstein condensates of atoms with attractive interactions confined to a quasi-1D waveguide. Using real-time imaging, we show that a collision between solitons is a complex event that differs markedly depending on the relative phase between the solitons. By controlling the strength of the nonlinearity we shed light on these fundamental features of soliton collisional dynamics, and explore the implications of collisions in the proximity of the crossover between one and three dimensions where the loss of integrability may precipitate catastrophic collapse.
Comparisons of approximate bases for hydrogen in a magnetic field
(American Physical Society, 1983) Zimmerman, Myron L.; Hulet, Randall G.; Kleppner, Daniel
Solutions to the diamagnetic Hamiltonian for Rydberg states of hydrogen are studied in the low-magnetic-field regime where term mixing can be ignored. Several recently proposed approximate bases are compared by calculating the overlap integrals with an accurate numerically generated basis.
Compressibility of a Fermionic Mott Insulator of Ultracold Atoms
(American Physical Society, 2015) Duarte, Pedro M.; Hart, Russell A.; Yang, Tsung-Lin; Liu, Xinxing; Paiva, Thereza; Khatami, Ehsan; Scalettar, Richard T.; Trivedi, Nandini; Hulet, Randall G.; Rice Quantum Institute
We characterize the Mott insulating regime of a repulsively interacting Fermi gas of ultracold atoms in a three-dimensional optical lattice. We use inﾠsitu imaging to extract the central density of the gas and to determine its local compressibility. For intermediate to strong interactions, we observe the emergence of a plateau in the density as a function of atom number, and a reduction of the compressibility at a density of one atom per site, indicating the formation of a Mott insulator. Comparisons to state-of-the-art numerical simulations of the Hubbard model over a wide range of interactions reveal that the temperature of the gas is of the order of, or below, the tunneling energy scale. Our results hold great promise for the exploration of many-body phenomena with ultracold atoms, where the local compressibility can be a useful tool to detect signatures of different phases or phase boundaries at specific values of the filling.
Conversion of an Atomic Fermi Gas to a Long-Lived Molecular Bose Gas
(American Physical Society, 2003) Strecker, Kevin E.; Partridge, Guthrie B.; Hulet, Randall G.; Rice Quantum Institute
We have converted an ultracold Fermi gas of Li6 atoms into an ultracold gas of Li26 molecules by adiabatic passage through a Feshbach resonance. Approximately 1.5×105 molecules in the least-bound, v=38, vibrational level of the X1Σ+g singlet state are produced with an efficiency of 50%. The molecules remain confined in an optical trap for times of up to 1 s before we dissociate them by a reverse adiabatic sweep.
Cooling Atomic Gases With Disorder
(American Physical Society, 2015) Paiva, Thereza; Khatami, Ehsan; Yang, Shuxiang; Rousseau, Valéry; Jarrell, Mark; Moreno, Juana; Hulet, Randall G.; Scalettar, Richard T.; Rice Quantum Institute
Cold atomic gases have proven capable of emulating a number of fundamental condensed matter phenomena including Bose-Einstein condensation, the Mott transition, Fulde-Ferrell-Larkin-Ovchinnikov pairing, and the quantum Hall effect. Cooling to a low enough temperature to explore magnetism and exotic superconductivity in lattices of fermionic atoms remains a challenge. We propose a method to produce a low temperature gas by preparing it in a disordered potential and following a constant entropy trajectory to deliver the gas into a nondisordered state which exhibits these incompletely understood phases. We show, using quantum Monte Carlo simulations, that we can approach the Néel temperature of the three-dimensional Hubbard model for experimentally achievable parameters. Recent experimental estimates suggest the randomness required lies in a regime where atom transport and equilibration are still robust.
Detecting π-phase superfluids with p-wave symmetry in a quasi-one-dimensional optical lattice
(American Physical Society, 2016) Liu, Bo; Li, Xiaopeng; Hulet, Randall G.; Liu, W. Vincent; Rice Quantum Institute
We propose an experimental protocol to study p-wave superfluidity in a spin-polarized cold Fermi gas tuned by an s-wave Feshbach resonance. A crucial ingredient is to add a quasi-one-dimensional optical lattice and tune the fillings of two spins to the s and p band, respectively. The pairing order parameter is confirmed to inherit p-wave symmetry in its center-of-mass motion. We find that it can further develop into a state of unexpected π-phase modulation in a broad parameter regime. Experimental signatures are predicted in the momentum distributions, density of states, and spatial densities for a realistic experimental setup with a shallow trap. The π-phase p-wave superfluid is reminiscent of the π state in superconductor-ferromagnet heterostructures but differs in symmetry and physical origin. The spatially varying phases of the superfluid gap provide an approach to synthetic magnetic fields for neutral atoms. It would represent another example of p-wave pairing, first discovered in He3 liquids.
Dipole force cooling of multilevel atoms
(1994) Sackett, Charles Ackley; Hulet, Randall G.
A theoretical discussion of laser cooling of multilevel atomic systems using intense, blue detuned laser beams is given. A method based on matrix continued fractions is presented, which enables the efficient calculation of the semi-classical force on a multilevel system. The method is applied to a three-level system driven by one standing wave and one travelling wave. The force curves obtained exhibit strong cooling features. Cooling of the three-level system is simulated using the fully quantum mechanical Monte Carlo wave function technique. The simulation predicts efficient cooling to sub-Doppler temperatures. The three-level model is related to a multilevel cooling experiment performed on $\sp7$Li. The experimental results are found to be in reasonable agreement with the three-level model. However, a comparison of the simulation and experiment for a two-level atomic system reveals significant discrepancies, which raises questions about the model and experiment.

Browsing by Author "Hulet, Randall G."

Results Per Page

Sort Options