自旋相干态法研究自旋轨道耦合BEC系统的基态性质

文章采用自旋相干态的方法研究自旋轨道耦合玻色爱因斯坦凝聚(BEC)系统的基态性质,在自旋相干态表象下把系统的哈密顿量对角化,由变分法求出系统的基态能谱、原子布居数和光子数,从而得到系统的量子相变,与Holstein-Primakoff方法所得的结果一致.     

山西大学成功实现铷原子的玻色-爱因斯坦凝聚

山西大学量子光学与光量子器件国家重点实验室张靖教授研究小组于2007年7月7日成功实现了铷原子的玻色爱因斯坦凝聚（BEC）。实验结果显示，冷原子云在温度降低到约为500nK时开始发生相变，经过进一步蒸发冷却，最终得到了约为6×10^4个原子的纯净BEC。目前该小组正将铷原子和费米原子咏同时装入磁阱中，蒸发冷却铷原子来协同冷却钾原子，最终同时实现玻色气体玻色爱因斯坦凝聚和费米气体量子简并。     

玻色-爱因斯坦凝聚中的色散关系

超流性是玻色-爱因斯坦凝聚宏观量子效应的重要体现,分别用运动方程法和哈密顿量对角化方法,通过傅里叶变换导出了玻色-爱因斯坦凝聚超流体的色散关系,并给出了凝聚体中声子速度的表达式.结果表明,在长波极限下,色散关系与波数近似成线性关系.     

简谈理想玻色原子气体的BEC

本文描述了理想玻色气体的玻色一爱因斯坦凝聚（BEC）现象,讨论生成BEC现象的临界温度条件,从量子统计解释了BEC现象．对三维体系,指出了当分子间的平均距离火于分子热运动的德波罗意波长是就能产生BEC现象．从讨论BEC态的热容量可知当温度低于临界温度时,正常态的热容量产生突变即存在相变．     

自旋轨道耦合BEC系统中类Dicke模型的相变

本文研究了自旋轨道耦合玻色爱因斯坦凝聚(BEC)系统的基态性质,并把它映射为量子光学中我们所熟悉的Dicke模型(N个二能级原子与光场的相互作用).采用自旋相干态的方法得出系统的基态能量、光子数和自旋极化随拉曼耦合强度的变化关系,从而探测了系统超辐射相和正常相之间的量子相变.     

玻色-爱因斯坦凝聚态间的光学介导纠缠

着重研究了一种在玻色-爱因斯坦凝聚态(Bose-Einstein Condensate,BEC)之间产生光学介导纠缠的方案.该方案使用量子非破坏性哈密顿量,并将BEC置于马赫-曾德(Mach-Zehnder)构型中.结果表明,通过对光进行测量,可以诱导产生纠缠态.还特别分析了纠缠态在退相干作用下的效应.研究表明,该纠缠态的行为表现对于原子与光的作用时间较为敏感:当相互作用时间■时,该纠缠态相对稳定;当相互作用时间■时,该纠缠态则相对脆弱.     

具有局域E2对称性的推广玻色-哈巴德模型

利用局域的E2生成元构造了一种具有准严格解的推广玻色-哈巴德模型.与原来的玻色-哈巴德模型类似,推广玻色-哈巴德模型可用于描述格点玻色系统从超流相到Mott绝缘相的量子相变.介绍了模型的准严格解及其显著特点,给出了一维推广玻色-哈巴德模型填充数ρ=1时的相图,并计算了N=M=14时体系的基态纠缠度随控制参量的变化特征.     

玻色爱因斯坦凝聚的Husimi函数计算

以双势阱玻色-爱因斯坦凝聚体系为例,给出双势阱BEC的Husimi函数计算方法,并根据数值计算结果,讨论在外加周期调制下,由Husimi函数描述的BEC系统中的隧穿过程.     

三腿梯子上硬核玻色-哈伯德模型的量子蒙特卡罗研究

运用集团平均场和密度矩阵重整化群方法得到三腿梯子中玻色-哈伯德模型基于z方向密度交错排布的超固体相,比单层三角晶格多一种超固体.然而平均场超流序参量没有考虑超流的方向,这种超固体的本质有待于进一步探索.本文运用可靠的有向圈随机序列展开量子蒙特卡罗方法模拟了三腿梯子上硬核玻色-哈伯德模型,通过测量层内和层间的结构因子和超流刚硬度,发现系统具有密度ρ=1/2的固体,增加量子隧穿后,z方向不存在超固体.计算结果有助于冷原子实验寻找新的量子相.     

玻色-爱因斯坦凝聚体量子模拟实验技术进展

玻色爱因斯坦凝聚体（BEC）的最初理论在1925年由爱因斯坦和玻色提出,直到1994年才通过激光冷却原子气体的方法在实验室实现,并于2001年获得诺贝尔物理学奖。BEC具有宏观量子特性并且具有较长的相干时间,因此是开展量子模拟研究的重要平台。随着光阱、光晶格、微结构光阱和量子气体显微镜等技术的发展,基于BEC的量子模拟器不仅能够用于研究凝聚态物理中已有的基本模型,而且能够探索自然界中不存在的新奇量子物理现象和量子物态。甚至在非平衡和动力学演化研究方面,BEC量子模拟器可以用来研究当前物理实验中无法观测的快速量子物理现象。     

光晶格中自旋3/2超冷费米原子的Mott绝缘态及其量子相变

我们首先简要介绍了超冷原子和光晶格,然后利用推广的单带Hubbard模型讨论了二维正方光晶格中自旋3/2费米型超冷原子体系中的几种Mott绝缘态,其中该系统的自旋自由度具有较大SO(5)对称性.对于对称性破缺的自旋四极矩有序态和自旋偶极-八极矩共存有序态,我们详细研究了系统的低能集体激发模式-Goldstone玻色激发的具体行为.在SU(4)对称点附近,我们发现强烈的量子涨落可能会演生出一个SU(4)π通量的自旋液体态.对此,我们还分析了这两种奇异量子态之间可能存在的量子相变.     

A Mott insulator of fermionic atoms in an optical lattice

Strong interactions between electrons in a solid material can lead to surprising properties. A prime example is the Mott insulator, in which suppression of conductivity occurs as a result of interactions rather than a filled Bloch band. Proximity to the Mott insulating phase in fermionic systems is the origin of many intriguing phenomena in condensed matter physics, most notably high-temperature superconductivity. The Hubbard model, which encompasses the essential physics of the Mott insulator, also applies to quantum gases trapped in an optical lattice. It is therefore now possible to access this regime with tools developed in atomic physics. However, an atomic Mott insulator has so far been realized only with a gas of bosons, which lack the rich and peculiar nature of fermions. Here we report the formation of a Mott insulator of a repulsively interacting two-component Fermi gas in an optical lattice. It is identified by three features: a drastic suppression of doubly occupied lattice sites, a strong reduction of the compressibility inferred from the response of double occupancy to an increase in atom number, and the appearance of a gapped mode in the excitation spectrum. Direct control of the interaction strength allows us to compare the Mott insulating regime and the non-interacting regime without changing tunnel-coupling or confinement. Our results pave the way for further studies of the Mott insulator, including spin-ordering and ultimately the question of d-wave superfluidity.     

Single-spin addressing in an atomic Mott insulator

Ultracold atoms in optical lattices provide a versatile tool with which to investigate fundamental properties of quantum many-body systems. In particular, the high degree of control of experimental parameters has allowed the study of many interesting phenomena, such as quantum phase transitions and quantum spin dynamics. Here we demonstrate how such control can be implemented at the most fundamental level of a single spin at a specific site of an optical lattice. Using a tightly focused laser beam together with a microwave field, we were able to flip the spin of individual atoms in a Mott insulator with sub-diffraction-limited resolution, well below the lattice spacing. The Mott insulator provided us with a large two-dimensional array of perfectly arranged atoms, in which we created arbitrary spin patterns by sequentially addressing selected lattice sites after freezing out the atom distribution. We directly monitored the tunnelling quantum dynamics of single atoms in the lattice prepared along a single line, and observed that our addressing scheme leaves the atoms in the motional ground state. The results should enable studies of entropy transport and the quantum dynamics of spin impurities, the implementation of novel cooling schemes, and the engineering of quantum many-body phases and various quantum information processing applications.     

有限势垒量子阱中极化子结合能的压力效应

计入流体静压力效应,同时考虑量子阱中三类光学声子模(局域类体光学声子、半空间类体光学声子和界面光学声子模)对单电子基态能量的影响,采用变分法讨论GaAs/AlxGa1-xAs量子阱中自由极化子的结合能.得到了压力下三类光学声子模对极化子结合能影响随阱宽的变化关系.结果表明:极化子结合能随外加压力增加.     

有限深量子阱中极化子束缚能

本文用改进的Ｌｅｅ－Ｌｏｗ－Ｐｉｎｅｓ变分法研究了界面光学声子，局域体光学声子以及半无限体光学声子对有限深量子阱中极化子束能的影响，得到了电子束缚态能量和极化子束缚能的解析表达式。     

抛物量子点中极化子性质的研究进展

综述了近年来对抛物量子点中极化子性质方面的部分工作．在第一节中从电子-LO声子系的哈密顿出发首次采用线性组合算符和幺正变换方法，研究抛物量子点中弱耦合极化子的基态能量和结合能的性质．在第二节中研究量子点中与LO声子强耦合极化子的振动频率、基态能量、结合能和光学声子平均数的性质．在第三节中采用改进的线性组合算符方法研究抛物量子点中弱耦合极化子的相互作用能和有效质量的性质．在第四节中研究抛物量子点中与LO声子强耦合极化子的振动频率、相互作用能和有效质量的性质．在第五节中采用Pekar变分方法研究抛物量子点中强耦合极化子基态和激发态的性质．     

极性晶体中三维和二维声学极化子自陷的有效质量特征

运用Huybrechts方法研究了三维和二维声学极化子有效质量作为电子-声子耦合参量的函数的变化特征.对二维声学极化子的研究,采用了我们最近导出的电子-声学声子相互作用哈密顿量.结果表明:电子-声子耦合达到一定强度时声学极化子有效质量发生突变而使极化子自陷,声学极化子在二维系统中比三维条件下更容易自陷.     

抛物线型量子线中束缚极化子基态能变分计算

费曼路径积分的变分方法是计算束缚极化子基态能的最有效方法。文章给出了抛物线型量子线中束缚极化子的哈密顿量 ,运用费曼路径积分的变分方法统一推导出抛物线型量子线约束势中杂质库仑束缚势下极化子的基态能 ,讨论了量子阱的一些特殊情况 :有、无杂质库仑束缚势的量子点和有、无杂质库仑束缚势的量子阱束缚势的量子阱 ,以及自由极化子     

Internal motions of a quasiparticle governing its ultrafast nonlinear response

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fr?hlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties--relevant for transport on nanometre length and ultrashort timescales--are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.     

Attractive and repulsive Fermi polarons in two dimensions

The dynamics of a single impurity in an environment is a fundamental problem in many-body physics. In the solid state, a well known case is an impurity coupled to a bosonic bath (such as lattice vibrations); the impurity and its accompanying lattice distortion form a new entity, a polaron. This quasiparticle plays an important role in the spectral function of high-transition-temperature superconductors, as well as in colossal magnetoresistance in manganites. For impurities in a fermionic bath, studies have considered heavy or immobile impurities which exhibit Anderson's orthogonality catastrophe and the Kondo effect. More recently, mobile impurities have moved into the focus of research, and they have been found to form new quasiparticles known as Fermi polarons. The Fermi polaron problem constitutes the extreme, but conceptually simple, limit of two important quantum many-body problems: the crossover between a molecular Bose-Einstein condensate and a superfluid with BCS (Bardeen-Cooper-Schrieffer) pairing with spin-imbalance for attractive interactions, and Stoner's itinerant ferromagnetism for repulsive interactions. It has been proposed that such quantum phases (and other elusive exotic states) might become realizable in Fermi gases confined to two dimensions. Their stability and observability are intimately related to the theoretically debated properties of the Fermi polaron in a two-dimensional Fermi gas. Here we create and investigate Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas, measuring their spectral function using momentum-resolved photoemission spectroscopy. For attractive interactions, we find evidence for a disputed pairing transition between polarons and tightly bound dimers, which provides insight into the elementary pairing mechanism of imbalanced, strongly coupled two-dimensional Fermi gases. Additionally, for repulsive interactions, we study novel quasiparticles--repulsive polarons--the lifetime of which determines the possibility of stabilizing repulsively interacting Fermi systems.