Semiclassical solitons in strongly correlated systems of ultracold bosonic atoms in optical lattices

We investigate theoretically soliton excitations and dynamics of their formation in strongly correlated systems of ultracold bosonic atoms in two and three dimensional optical lattices. We derive equations of nonlinear hydrodynamics in the regime of strong interactions and incommensurate fillings, when atoms can be treated as hard core bosons. When parameters change in one direction only we obtain Korteweg–de Vries type equation away from half-filling and modified KdV equation at half-filling. We apply this general analysis to a problem of the decay of the density step. We consider stability of one dimensional solutions to transverse fluctuations. Our results are also relevant for understanding nonequilibrium dynamics of lattice spin models.     

Controlling spin exchange interactions of ultracold atoms in optical lattices

We describe a general technique that allows one to induce and control strong interaction between spin states of neighboring atoms in an optical lattice. We show that the properties of spin exchange interactions, such as magnitude, sign, and anisotropy, can be designed by adjusting the optical potentials. We illustrate how this technique can be used to efficiently "engineer" quantum spin systems with desired properties, for specific examples ranging from scalable quantum computation to probing a model with complex topological order that supports exotic anyonic excitations.     

基于光晶格的超冷原子量子模拟

文章从超冷原子研究的视角出发,回顾了用"从下到上"的方案来开展量子模拟研究的历史。超冷原子作为宏观量子态,各个自由度精确可控,是量子模拟的绝佳平台。光晶格将冷原子物理和凝聚态物理融合起来,是其中最重要的技术之一,为超冷原子量子模拟提供了一个扎实的落脚点。近年来,关于拓扑量子模拟的研究日益兴起,成为超冷原子量子模拟新的重要方向。文章介绍这方面近期的一些工作进展。最后分享作者对超冷原子量子模拟的一些思考。     

Tunable superfluidity and quantum magnetism with ultracold polar molecules

By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally.     

Topological quantum memory interfacing atomic and superconducting qubits

We propose a scheme to manipulate a topological spin qubit which is realized with cold atoms in a one-dimensional optical lattice. In particular, by introducing a quantum opto-electro-mechanical interface, we are able to first transfer a superconducting qubit state to an atomic qubit state and then to store it into the topological spin qubit. In this way, an efficient topological quantum memory could be constructed for the superconducting qubit. Therefore, we can consolidate the advantages of both the noise resistance of the topological qubits and the scalability of the superconducting qubits in this hybrid architecture.     

Perfect quantum state transfer with spinor bosons on weighted graphs

A duality between the properties of many spinor bosons on a regular lattice and those of a single particle on a weighted graph reveals that a quantum particle can traverse an infinite hierarchy of networks with perfect probability in polynomial time, even as the number of nodes increases exponentially. The one-dimensional "quantum wire" and the hypercube are special cases in this construction, where the number of spin degrees of freedom is equal to one and the number of particles, respectively. An implementation of a near-perfect quantum state transfer across a weighted parallelepiped with ultracold atoms in optical lattices is discussed.     

Color superfluidity and "baryon" formation in ultracold fermions

We study fermionic atoms of three different internal quantum states (colors) in an optical lattice, which are interacting through attractive on site interactions, U<0. Using a variational calculation for equal color densities and small couplings, |U|<|UC|, a color superfluid state emerges with a tendency to domain formation. For |U|>|UC|, triplets of atoms with different colors form singlet fermions (trions). These phases are the analogies of the color superconducting and baryonic phases in QCD. In ultracold fermions, this transition is found to be of second order. Our results demonstrate that quantum simulations with ultracold gases may shed light on outstanding problems in quantum field theory.     

超冷原子中拓扑超流的发展现状

拓扑超流态是一种奇异物质态,它的内部受能隙保护,而在其系统边缘却可以容纳无能隙的Majorana 费米子。由于该粒子满足非阿贝尔统计,并且受拓扑保护具有良好的稳定性,用它 们携带量子化的信息,可以用于拓扑量子计算的研究。近年来,理论工作预测了各类系统中可能 存在的拓扑超流态。我们首先介绍了在各类光晶格模型中的拓扑超流, 光晶格的超冷原子具有良 好的可控性与普适性,是实现拓扑超流的理想模型系统。接下来我们介绍了自旋轨道耦合调控下 的拓扑超流,自旋轨道耦合效应是诱导拓扑相的重要条件,并且人们已经在实验上合成了人工自 旋轨道耦合,这为实验上观测拓扑超流取得了突破性的进展。随着近年来实验技术的提高,曾经 难以在实验中观测的,被人们所忽略的拓扑Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) 超流相也 成为了人们研究的热点,因此我们接下来介绍了拓扑的FFLO 超流。此外,我们还介绍了拓扑超 流其他方面的进展,包括孤子引诱的拓扑超流、三组分的拓扑超流、大陈数的拓扑超流以及拓扑 超流临界温度的提高。在实验中,如何检测与实现拓扑超流,是其研究的目的及意义所在,因 此我们在文章的最后介绍了拓扑超流的识别与实现。     

Energy spectrum and superfluidity of spin-2 ultracold bosons in optical lattices

This paper studies the superfluidity of ultracold spin-2 Bose atoms with weak interactions in optical lattices by calculating the excitation energy spectrum using the Bogoliubov approach. The energy spectra exhibit the characteristics of the superfluid-phase explicitly and it finds the nonvanishing critical speeds of superfluid. The obtained results display that the critical speeds of superfluid are different for five spin components and can be controlled by adjusting the lattice parameters in experiments. Finally it discusses the feasibilities of implementing and measuring superfluid.     

Quantum capillary waves at the superfluid-Mott-insulator interface

We discuss quantum fluctuations of the interface between a superfluid and a Mott-insulating state of ultracold atoms in a trap. The fluctuations of the boundary are due to a new type of surface modes, whose spectrum is similar--but not identical--to classical capillary waves. The corresponding quantum capillary length sets the scale for the penetration of the superfluid into the Mott-insulating regime by the proximity effect and may be on the order of several lattice spacings. It determines the typical magnitude of the interface width due to quantum fluctuations, which may be inferred from single-site imaging of ultracold atoms in an optical lattice.     

Controlling and detecting spin correlations of ultracold atoms in optical lattices

We report on the controlled creation of a valence bond state of delocalized effective-spin singlet and triplet dimers by means of a bichromatic optical superlattice. We demonstrate a coherent coupling between the singlet and triplet states and show how the superlattice can be employed to measure the singlet-fraction employing a spin-blockade effect. Our method provides a reliable way to detect and control nearest-neighbor spin correlations in many-body systems of ultracold atoms. Being able to measure these correlations is an important ingredient in studying quantum magnetism in optical lattices. We furthermore employ a SWAP operation between atoms which are part of different triplets, thus effectively increasing their bond-length. Such a SWAP operation provides an important step towards the massively parallel creation of a multiparticle entangled state in the lattice.     

Graphene-like physics in optical lattices

Graphene has attracted enormous attention over the past years in condensed matter physics. The most interesting feature of graphene is that its low-energy excitations are relativistic Dirac fermions. Such feature is the origin of many topological properties in graphene-like physics. On the other hand, ultracold quantum gas trapped in an optical lattice has become a unique setting for quantum simulation of condensed matter physics. Here, we mainly review our recent work on quantum simulation of graphene-like physics with ultracold atoms trapped in a honeycomb or square optical lattice, including the simulation of Dirac fermions and quantum Hall effect with and without Landau levels. We also present the related experimental advances.     

Coherent collisional spin dynamics in optical lattices

We report on the observation of coherent, purely collisionally driven spin dynamics of neutral atoms in an optical lattice. For high lattice depths, atom pairs confined to the same lattice site show weakly damped Rabi-type oscillations between two-particle Zeeman states of equal magnetization, induced by spin-changing collisions. Moreover, measurement of the oscillation frequency allows for precise determination of the spin-changing collisional coupling strengths, which are directly related to fundamental scattering lengths describing interatomic collisions at ultracold temperatures.     

弱磁场下三阱光学超晶格中自旋为1的超冷原子特性研究

双阱光学超晶格中的超冷原子是近期冷原子物理领域的研究热点. 本文推广提出了实现三阱光学超晶格的方案, 并采用精确对角化的方法分别研究了弱磁场下对称三阱 光学超晶格中铁磁性和反铁磁性的自旋为1的原子系统的基态, 发现二者的相图很不相同: 反铁磁性原子对应的相图中没有沿磁场方向总自旋磁量子数为±2的基态, 而铁磁性原子对应的相图中可能有. 在负的二次塞曼能量区域, 铁磁性原子的相图中只有完全极化态. 分析了可控参数影响基态的物理本质. 由于这些量子自旋态可以通过调节外磁场和光势垒的高度非常简便而精确地控制, 适合用来研究自旋纠缠. 关键词： 三阱光学超晶格 自旋为1的原子 弱磁场     

Quantum logic via the exchange blockade in ultracold collisions

A nuclear spin can act as a quantum switch that turns on or off ultracold collisions between atoms even when there is neither interaction between nuclear spins nor between the nuclear and electron spins. This "exchange blockade" is a new mechanism for implementing quantum logic gates that arises from the symmetry of composite identical particles, rather than direct coupling between qubits. We study the implementation of the entangling sqrt SWAP gate based on this mechanism for a model system of two atoms, each with ground electronic configuration 1S0, spin 1/2 nuclei, and trapped in optical tweezers. We evaluate a proof-of-principle protocol based on adiabatic evolution of a one-dimensional double Gaussian well, calculating fidelities of operation as a function of interaction strength, gate time, and temperature.     

Entanglement interferometry for precision measurement of atomic scattering properties

We report on a matter wave interferometer realized with entangled pairs of trapped 87Rb atoms. Each pair of atoms is confined at a single site of an optical lattice potential. The interferometer is realized by first creating a coherent spin superposition of the two atoms and then tuning the interstate scattering length via a Feshbach resonance. The selective change of the interstate scattering length leads to an entanglement dynamics of the two-particle state that can be detected in a Ramsey interference experiment. This entanglement dynamics is employed for a precision measurement of atomic interaction parameters. Furthermore, the interferometer allows us to separate lattice sites with one or two atoms in a nondestructive way.     

冷原子物理中的一维少体问题

作为构成量子多体系统的基本单元,一维少体系统的研究不仅可以在理论上为多体系统的量子关联及动力学等性质提供更为基本的理解,也可以为实验上制备多体系统提供更加方便和功能更加全面的方法.本文回顾了冷原子物理中一维少体系统最新的实验和理论进展.首先介绍了少体实验中实现的谐振子势阱中确定原子数的精确制备,亚稳态势阱和双阱系统中原子的隧穿,以及强相互作用下等效自旋链的实验结果.然后深度解析了理论研究方面,特别是基于精确可解模型的一些重要结果,包括亚稳态势阱中相互作用原子的隧穿概率,以及相应实验上常见势阱的能谱分析、密度分布、隧穿动力学以及强相互作用极限下的有效自旋链模型等.     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

p-Wave cold collisions in an optical lattice clock

We study ultracold collisions in fermionic ytterbium by precisely measuring the energy shifts they impart on the atoms' internal clock states. Exploiting Fermi statistics, we uncover p-wave collisions, in both weakly and strongly interacting regimes. With the higher density afforded by two-dimensional lattice confinement, we demonstrate that strong interactions can lead to a novel suppression of this collision shift. In addition to reducing the systematic errors of lattice clocks, this work has application to quantum information and quantum simulation with alkaline-earth atoms.