Browsing by Author "Pu, Han"
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Item A New Optical Trap System for Ultracold Strontium(2013-12-04) Huang, Ying; Killian, Thomas C.; Hulet, Randall G.; Pu, HanAtoms 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.Item An Optical Lattice for Ultracold Strontium(2022-10-11) Wang, Chuanyu; Killian, Thomas C.; Pu, Han; Chen, SongtaoStrongly interacting many-body systems are of great interest in many areas of physics. Ultracold atoms in optical lattices are a powerful tool for creating and studying such systems. In this thesis, the construction and characterization of an 3D optical lattice for a ultra-cold Strontium apparatus is described. A 1064nm fiber amplified laser system is used to create three orthogonal pairs of standing waves that together form the 3D lattice. By delivering 4W of power to each arm of the lattice and focusing the beams to a beam radius of about 250 microns at the location of the atoms, we can create a 3D lattice with a maximum trap depth around 70 photon recoils.Item Angular spin-orbit coupling in cold atoms(American Physical Society, 2015) DeMarco, Michael; Pu, Han; Rice Quantum InstituteWe propose coupling two internal atomic states using a pair of Raman beams operated in Laguerre-Gaussian laser modes with unequal phase windings. This generates a coupling between the atom's pseudospin and its orbital angular momentum. We analyze the single-particle properties of the system using realistic parameters and provide detailed studies of the spin texture of the ground state. Finally, we consider a weakly interacting atomic condensate subject to this angular spin-orbit coupling and show how the interatomic interactions modify the single-particle physics.Item Artificial topological models based on a one-dimensional spin-dependent optical lattice(American Physical Society, 2017) Zheng, Zhen; Pu, Han; Zou, Xubo; Guo, Guangcan; Rice Center for Quantum MaterialsTopological matter is a popular topic in both condensed matter and cold-atom research. In the past decades, a variety of models have been identified with fascinating topological features. Some, but not all, of the models can be found in materials. As a fully controllable system, cold atoms trapped in optical lattices provide an ideal platform to simulate and realize these topological models. Here we present a proposal for synthesizing topological models in cold atoms based on a one-dimensional spin-dependent optical lattice potential. In our system, features such as staggered tunneling, staggered Zeeman field, nearest-neighbor interaction, beyond-near-neighbor tunneling, etc. can be readily realized. They underlie the emergence of various topological phases. Our proposal can be realized with current technology and hence has potential applications in quantum simulation of topological matter.Item Bose-Fermi mapping and a multibranch spin-chain model for strongly interacting quantum gases in one dimension: Dynamics and collective excitations(American Physical Society, 2016) Yang, Li; Pu, Han; Rice Center for Quantum MaterialsWe show that the wave function in one spatial sector x1Item Building flat-band lattice models from Gram matrices(American Physical Society, 2020) Xu, Youjiang; Pu, Han; Rice Center for Quantum MaterialsWe propose a powerful and convenient method to systematically design flat-band lattice models, which overcomes the difficulties underlying the previous method. Especially, our method requires no elaborate calculations, applies to arbitrary spatial dimensions, and guarantees to result in a completely flat ground band. We use this method to generate several classes of lattice models, including models with both short- and long-range hoppings, both topologically trivial and nontrivial flat bands. Some of these models were previously known. Our method, however, provides important insight. For example, we have reproduced and generalized the Kapit-Mueller model [Kapit and Mueller, Phys. Rev. Lett. 105, 215303 (2010)] and demonstrated a universal scaling rule between the flat-band degeneracy and the magnetic flux that was not noticed in previous studies. We show that the flat band of this model results from the (over-)completeness properties of coherent states.Item Cavity-assisted dynamical spin-orbit coupling in cold atoms(American Physical Society, 2014) Dong, Lin; Zhou, Lu; Wu, Biao; Ramachandhran, B.; Pu, Han; Rice Quantum InstituteWe consider ultracold atoms subjected to a cavity-assisted two-photon Raman transition. The Raman coupling gives rise to effective spin-orbit interaction which couples the atom's center-of-mass motion to its pseudospin degrees of freedom. Meanwhile, the cavity photon field is dynamically affected by the atom. This feedback between the atom and photons leads to a dramatic modification of the atomic dispersion relation, and further leads to dynamical instability of the system. We propose to detect the change in the cavity photon number as a direct way to demonstrate dynamical instability.Item Designing Quantum Multicritical and Flat-Band Models via Hamiltonian Engineering(2021-04-23) Xu, Youjiang; Pu, HanAtomic, molecular, and optical (AMO) systems often feature great controllability. As such, they offer ideal platforms to explore various kinds of quantum phenomena. Designing artificial quantum systems that possess novel and exotic properties is one of the major tasks of theorists working in the AMO field. In this dissertation, we introduce our work on designing novel Hamiltonians which give rise to multicriticality or flat bands. In the first half of the dissertation, we study the multicriticality. Quantum many-body systems that support multicritical quantum phase transitions are quite rare. However, we find that, in an important generalization of the Dicke model, the superradiant quantum phase transitions can become multicritical. For a subclass of experimentally realizable schemes, multicritical conditions of arbitrary order can be expressed analytically in compact forms. As such, experiments can be readily designed to achieve quantum phase transition of desired order. The phase transition happens both in the thermodynamic limit and the classical oscillator limit. We compare the quantum fluctuation in the two cases by calculating the atom-photon entanglement entropy. We find that the order of the criticality strongly affects the critical entanglement entropy. In the second half of the dissertation, we propose a powerful and convenient method to systematically design flat-band lattice models. Flat bands often lead to exotic strongly correlated emergent quantum phenomena. We use this method to generate several classes of lattice models, including models with both short- and long-range hoppings, both ordinary and magnetic translational symmetry, both topologically trivial and non-trivial flat bands.Item Dynamic evolution of spin-1 and spin-2 dipolar BECs(2007) Tsekouras, Konstantinos; Pu, HanThis study investigates the dynamical evolution of spin-1 and spin-2 Bose-Einstein Condensates (BECs) when the dipolar interaction is taken into consideration. Using the Single-Mode Approximation (SMA) equations of motion are derived for each condensate component which are then numerically solved. The results convincingly show that the dipolar interaction has a significant effect despite being much weaker than the collision or spin-exchange interaction and that if included magnetic effects are easy to treat. Further, an analytical Mean-Field Theory formulation is used to derive and simplify equations of motion for a spin-1 BEC in 2D (pancake) configuration. Moving to a reference frame that rotates at the Larmor frequency greatly simplifies the equations and removes first-order Zeeman terms, rendering the effects of the dipolar interaction much clearer to observe in the condensate evolution.Item Dynamically Manipulating Topological Physics and Edge Modes in a Single Degenerate Optical Cavity(American Physical Society, 2017) Zhou, Xiang-Fa; Luo, Xi-Wang; Wang, Su; Guo, Guang-Can; Zhou, Xingxiang; Pu, Han; Zhou, Zheng-Wei; Rice Center for Quantum MaterialsWe propose a scheme to simulate topological physics within a single degenerate cavity, whose modes are mapped to lattice sites. A crucial ingredient of the scheme is to construct a sharp boundary so that the open boundary condition can be implemented for this effective lattice system. In doing so, the topological properties of the system can manifest themselves on the edge states, which can be probed from the spectrum of an output cavity field. We demonstrate this with two examples: a static Su-Schrieffer-Heeger chain and a periodically driven Floquet topological insulator. Our work opens up new avenues to explore exotic photonic topological phases inside a single optical cavity.Item Effective p-wave interaction and topological superfluids in s-wave quantum gases(American Physical Society, 2016) Wang, Bin; Zheng, Zhen; Pu, Han; Zou, Xubo; Guo, Guangcan; Rice Center for Quantum Materialsp-wave interaction in cold atoms may give rise to exotic topological superfluids. However, the realization of p-wave interaction in a cold atom system is experimentally challenging. Here we propose a simple scheme to synthesize effective p-wave interaction in conventional s-wave interacting quantum gases. The key idea is to load atoms into a spin-dependent optical lattice potential. Using two concrete examples involving spin-1/2 fermions, we show how the original system can be mapped into a model describing spinless fermions with nearest-neighbor p-wave interaction, whose ground state can be a topological superfluid that supports Majorana fermions under proper conditions. Our proposal has the advantage that it does not require spin-orbit coupling or loading atoms onto higher orbitals, which is the key in earlier proposals to synthesize effective p-wave interaction in s-wave quantum gases, and may provide a completely new route for realizing p-wave topological superfluids.Item Effects of spin-orbit coupling on Jaynes-Cummings and Tavis-Cummings models(American Physical Society, 2016) Zhu, Chuanzhou; Dong, Lin; Pu, Han; Rice Center for Quantum MaterialsWe consider ultracold atoms inside a ring optical cavity that supports a single plane-wave mode. The cavity field, together with an external coherent laser field, drives a two-photon Raman transition between two internal pseudospin states of the atom. This gives rise to an effective coupling between atom's pseudospin and external center-of-mass (COM) motion. For the case of a single atom inside the cavity, We show how the spin-orbit coupling modifies the static and dynamic properties of the Jaynes-Cummings (JC) model. In the case of many atoms in thermodynamic limit, we show that the spin-orbit coupling modifies the Dicke superradiance phase transition boundary and the non-superradiant normal phase may become reentrant in some regimes.Item Efficient generation of many-body singlet states of spin-1 bosons in optical superlattices(American Physical Society, 2017) Sun, Huanying; Xu, Peng; Pu, Han; Zhang, WenxianWe propose an efficient stepwise adiabatic merging (SAM) method to generate many-body singlet states in antiferromagnetic spin-1 bosons in concatenated optical superlattices with isolated double-well arrays, by adiabatically ramping up the double-well bias. With an appropriate choice of bias sweeping rate and magnetic field, the SAM protocol predicts a fidelity as high as 90% for a 16-body singlet state and even higher fidelities for smaller even-body singlet states. During their evolution, the spin-1 bosons exhibit interesting squeezing dynamics, manifested by an odd-even oscillation of the experimentally observable squeezing parameter. The generated many-body singlet states may find practical applications in precision measurement of magnetic field gradient and in quantum information processing.Item Emergence of topological and strongly correlated ground states in trapped Rashba spin-orbit-coupled Bose gases(American Physical Society, 2013) Ramachandhran, B.; Hu, Hui; Pu, HanWe theoretically study an interacting few-body system of Rashba spin-orbit-coupled two-component Bose gases confined in a harmonic trapping potential. We solve the interacting Hamiltonian at large Rashba coupling strengths using an exact-diagonalization scheme, and obtain the ground-state phase diagram for a range of interatomic interactions and particle numbers. At small particle numbers, we observe that the bosons condense to an array of topological states with n+1/2 quantum angular momentum vortex configurations, where n=0,1,2,3,.... At large particle numbers, we observe two distinct regimes: at weaker-interaction strengths, we obtain ground states with topological and symmetry properties that are consistent with mean-field theory computations; at stronger-interaction strengths, we report the emergence of strongly correlated ground states.Item Field-induced topological pair-density wave states in a multilayer optical lattice(American Physical Society, 2018) Zheng, Zhen-Fei; Guo, Guang-Can; Pu, Han; Zou, Xu-BoWe study the superfluid phases of a Fermi gas in a multilayer optical lattice system in the presence of out-of-plane Zeeman field, as well as spin-orbit (SO) coupling. We show that the Zeeman field combined with the SO coupling leads to exotic topological pair-density wave (PDW) phases in which different layers possess different superfluid order parameters, even though each layer experiences the same Zeeman field and the SO coupling. We elucidate the mechanism of the emerging PDW phases, and characterize their topological properties by calculating the associated Chern numbers.Item Gapless topological Fulde-Ferrell superfluidity induced by an in-plane Zeeman field(American Physical Society, 2014) Hu, Hui; Dong, Lin; Cao, Ye; Pu, Han; Liu, Xia-JiTopological superfluids are recently discovered quantum matter that hosts topologically protected gapless edge states known as Majorana fermions—exotic quantum particles that act as their own antiparticles and obey non-Abelian statistics. Their realizations are believed to lie at the heart of future technologies such as fault-tolerant quantum computation. To date, the most efficient scheme to create topological superfluids and Majorana fermions is based on the Sau-Lutchyn-Tewari-Das Sarma model with a Rashba-type spin-orbit coupling on the x-y plane and a large out-of-plane (perpendicular) Zeeman field along the z direction. Here we propose an alternative setup, where the topological superfluid phase is driven by applying an in-plane Zeeman field. This scheme offers a number of different features, notably Cooper pairings at finite center-of-mass momentum (i.e., Fulde-Ferrell pairing) and gapless excitations in the bulk. As a result, gapless topological quantum matter with an inhomogeneous pairing order parameter appears. It features unidirectional Majorana surface states at boundaries, which propagate in the same direction and connect two Weyl nodes in the bulk. We demonstrate the emergence of such exotic topological matter and the associated Majorana fermions in spin-orbit coupled atomic Fermi gases, and we determine its parameter space. The implementation of our scheme in semiconductor/superconductor heterostructures is briefly discussed.Item Itinerant chiral ferromagnetism in a trapped Rashba spin-orbit-coupled Fermi gas(American Physical Society, 2016) Zhang, Shang-Shun; Liu, Wu-Ming; Pu, Han; Rice Center for Quantum MaterialsWe consider a repulsive two-component Fermi gas confined in a two-dimensional isotropic harmonic potential and subject to a large Rashba spin-orbit coupling. The single-particle dispersion can be tailored by the spin-orbit-coupling term, which provides an opportunity to study itinerant ferromagnetism in this system. We show that the interplay among spin-orbit coupling, correlation effect, and mean-field repulsion leads to a competition between ferromagnetic and nonmagnetic phases. The weakly correlated nonmagnetic and the ferromagnetic phases can be well described by the mean-field Hartree-Fock theory, while the transition between the ferromagnetic and a strongly correlated nonmagnetic phase is driven by beyond-mean-field quantum correlation effect. Furthermore, the ferromagnetic phase of this system possesses a chiral current density induced by the Rashba spin-orbit coupling, whose experimental signature is investigated.Item Novel Spin-orbit Coupling in Cold Atoms(2020-01-30) Zhu, Chuanzhou; Pu, HanIn cold atom, the coupling between "spin" (atomic internal hyperfine states) and "orbit" (atomic center-of-mass motion) can be induced by Raman transition, where different hyperfine states are coupled by a pair of Raman lasers. In recent years, this synthetic spin-orbit coupling has received tremendous attention, as it leads to a variety of novel quantum phenomena in precisely controllable cold atom systems. In this thesis, we first present a comprehensive analysis of one-, two- and many- particle physics of harmonically trapped atoms with spin-orbit coupling, followed by the study of "novel spin-orbit coupling" in two different systems: (1) cold atom spinor mixtures and (2) cold atoms in an optical cavity. In the first system, we consider a spinor mixture consisting of two species of cold atoms, where the spin-orbit coupling can be transmitted from one species to the other, and we discuss novel topological properties and the supersolid stripe phase in this mixture. In the second system, we consider the coupling among three parts: the cavity photon field, the atomic internal hyperfine states, and the atomic external center-of-mass motion, and we discuss how this coupling affects familiar quantum optical phenomena, such as Rabi oscillation and Dicke superradiance phase transition. Our novel systems contribute new and practical platforms for the research field of synthetic spin-orbit coupling in cold atoms.Item Number-conserving interacting fermion models with exact topological superconducting ground states(American Physical Society, 2017) Wang, Zhiyuan; Xu, Youjiang; Pu, Han; Hazzard, Kaden R.A.We present a method to construct number-conserving Hamiltonians whose ground states exactly reproduce an arbitrarily chosen BCS-type mean-field state. Such parent Hamiltonians can be constructed not only for the usual s -wave BCS state, but also for more exotic states of this form, including the ground states of Kitaev wires and two-dimensional topological superconductors. This method leads to infinite families of locally interacting fermion models with exact topological superconducting ground states. After explaining the general technique, we apply this method to construct two specific classes of models. The first one is a one-dimensional double wire lattice model with Majorana-like degenerate ground states. The second one is a two-dimensional p x + i p y superconducting model, where we also obtain analytic expressions for topologically degenerate ground states in the presence of vortices. Our models may provide a deeper conceptual understanding of how Majorana zero modes could emerge in condensed matter systems, as well as inspire novel routes to realize them in experiment.Item Numerical studies of ultracold atomic gases(2010) Lu, Hong; Pu, HanThe experimental success in ultra-cold atomic gases, both bosonic and fermionic have boosted the theoretical studies, and especially the a lot of numerical techniques have been developed and used to describe them. In this thesis, we introduce two numerical experiments in our group on ultra-cold atomic gases. The first concerns the scalar dipolar condensate. We have developed and implemented a Split-Step Fourier scheme in imaginary time, which enable us to seek the ground state of the dipolar condensate. The second part is focused on our ongoing efforts to investigate the trapped spin polarized Fermi gas using self-consistent Bogoliubov-de Gennes (BdG) calculation.
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