A simple He3-He4 dilution refrigerator

A very simple He³-He⁴ dilution refrigerator is described. The temperature 60 mK was achieved at a circulation of 3×10^–5 mole He³ per second. The cooldown time is one hour.The authors wish to express their gratitude to V. P. Peshkov from the Institute of Physical Problems in Moscow and to K. N. Zinoveva from the same Institute for many helpful hints during the construction of the apparatus.     

A 3He circulating dilution refrigerator with two mixing chambers

A dilution refrigerator with two mixing chambers is described. When compared with a dilution refrigerator with only one mixing chamber the temperature range is extended to lower temperatures. The temperature reduction factor can be up to a factor 2.8. The results of an experiment are reported in which a magnetic temperature of 5.5 mK was obtained in the continuous mode and 3.5 mK in the single cycle mode, using a dilution refrigerator that reaches 13 mK in the continuous mode with one mixing chamber.     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Development of a cryogen-free dilution refrigerator

With thermal fluctuation strongly suppressed, low temperature environment is essential for studies of condensed matter physics and developments of quantum technologies. Ultra-low temperature below 20 mK has demonstrated its importance and significance in physical sciences and information techniques. Dilution refrigeration is by far the best feasible and reliable method to generate and keep lattice temperature in this range. With a potential shortage of helium supply, cryogen-free dilution refrigerator (CFDR), eliminating the necessity of regular helium refill, becomes the main facility for the purpose of creating ultralow temperature environments. Here we describe our successful construction of a CFDR which reached a base temperature of around 10.9 mK for continuous circulation and 8.6 mK for single-shot operation. We describe its operating mechanism and the designs of key components, especially some unique designs including heat switch and alumina thermal link. Possible improvements in the future are also discussed.     

Rovibrational in ultracold quenching of BH 3He collisions

The interaction potential of a He-BH complex is investigated by the coupled-cluster single-double plus perturbative triples （CCSD （T）） method and an augmented correlation consistent polarized valence （aug-cc-pV）5Z basis set extended with a set of （3s3p2dlflg） midbond functions. Using the five two-dimensional model potentials, the first three-dimensional interaction potential energy surface is constructed by interpolating along （r-re） by using a fourth-order polynomial. The cross sections for the rovibrational relaxation of BH in cold and ultracold collisions with 3He atom are calculated based on the three-dimensional potential. The results show that the △v =-1 transition is more efficient than the △v=-2 transition, and that the process of relaxation takes place mainly between rotational energy levels with the same vibration state and the △j=-1 transition is the most efficient. The zero temperature quenching rate coefficient is finite as predicted by Wigner＇s law. The resonance is found to take place around 0.1-1 cm^-1 translational energy, which gives rise to a step in the rate coefficients for temperatures around 0.1-1 K. The final rotational distributions in the state v = 0 resulting from the quenching of state （v = 1,j = 0） at three energies corresponding to the three different regimes are also given.     

Trapping atoms in a gravitational cavity

This paper is devoted to the study of a new atomic cavity consisting of a single horizontal concave mirror placed in the earth gravitational field. Gravity, by bending the atomic trajectories, plays the role of a second mirror closing the cavity. We first discuss the stability criterion for this cavity, assuming that the mirror has a parabolic shape. We then derive the quantum mechanical modes of such a configuration, with particular emphasis on the paraxial (i.e., close to vertical) motion. Finally, we discuss the possibility of populating those modes from an initial cold atomic cloud dropped above the mirror.Laboratoire associé au CNRS et à l'Université Pierre et Marie Curie     

Bright solitons in ultracold atoms

We review old and recent experimental and theoretical results on bright solitons in Bose–Einstein condensates made of alkali-metal atoms and under external optical confinement. First we deduce the three-dimensional Gross–Pitaevskii equation (3D GPE) from the Dirac–Frenkel action of interacting identical bosons within a time-dependent Hartree approximation. Then we discuss the dimensional reduction of the GPE from 3D to 1D, deriving the 1D GPE and also the 1D nonpolynomial Schrödinger equation (1D NPSE). Finally, we analyze the bright solition solutions of both 1D GPE and 1D NPSE and compare these theoretical predictions with the available experimental data.     

Primary populations of metastable antiprotonic (4)He and (3)He atoms

Initial distributions of metastable antiprotonic (4)He and (3)He atoms over principal (n) and angular momentum (l) quantum numbers have been deduced using laser spectroscopy experiments. The regions n = 37-40 and n = 35-38 in the two atoms account for almost all of the observed fractions [(3.0 +/- 0.1)% and (2.4 +/- 0.1)%] of antiprotons captured into metastable states.     

Low-noise detection of ultracold atoms

We have demonstrated a new technique for detecting ultracold atoms. A balanced detection technique was used to reduce laser-induced detection noise in conjunction with modulation-transfer spectroscopy to distinguish cold atoms from a thermal cloud. Using this technique, we have achieved signal-to-noise ratios in excess of 2000:1.     

超冷nS Rydberg原子的态转移

本文从实验上研究了超冷nS Rydberg原子在外电场脉冲作用下的态转移现象. 采用双光子激发超冷基态原子制备超冷Rydberg原子, 通过脉冲场电离法探测超冷Rydberg原子的离子信号, 实验研究了由外加脉冲电场产生的nS能级和多重态的避免交叉, 获得了在外电场脉冲作用下nS态原子的态转移现象, 实验与理论计算的结果相一致. 关键词： Rydberg原子 态转移 能级避免交叉     

Trapping of ultracold polar molecules with a thin-wire electrostatic trap

We describe the realization of a dc electric-field trap for ultracold polar molecules, the thin-wire electrostatic trap (TWIST). The thin wires that form the electrodes of the TWIST allow us to superimpose the trap onto a magneto-optical trap (MOT). In our experiment, ultracold polar NaCs molecules in their electronic ground state are created in the MOT via photoassociation, achieving a continuous accumulation in the TWIST of molecules in low-field seeking states. Initial measurements show that the TWIST trap lifetime is limited only by the background pressure in the chamber.     

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。     

超冷原子向异核四聚物分子A3B的绝热转化

提出了利用Efimov共振辅助的受激拉曼绝热通道(ER-STIRAP) 过程实施超冷原子向异核四聚物分子A₃B转化的理论方案, 得到了转化过程中中间态分别为同核Efimov三聚物A₃和 异核Efimov三聚物A₂B两种途径下系统的暗态解, 证实了ER-STIRAP技术对超冷异核四聚物分子A₃B合成的可行性和有效性. 研究了外场参数, 包括缔合光脉冲的强度、脉宽、磁耦合强度及其失谐量等对A₃B形成的影响. 对两种不同中间态的转化途径进行比较发现, 与中间态为异核Efimov三聚物A₂B的途径相比, 经历中间态为同核Efimov三聚物A₃的途径时系统实现最终四聚物分子A₃B的产率更高. 另外, 还讨论了系统内禀的非线性和中间态的自发辐射损失对异核四聚物分子合成的影响.     

Magnetic trapping of ultracold Rydberg atoms

We investigate the quantum dynamics of ultracold Rydberg atoms being exposed to a magnetic quadrupole field. A Hamiltonian describing the coupled dynamics of the electronic and center of mass motion is derived. Employing an adiabatic approach, the potential energy surfaces for intra-n-manifold mixing are computed. By determining the quantum states of the center of mass motion, we demonstrate that trapped states can be achieved if the total angular momentum of the atom is sufficiently large. This holds even if the extension of the electronic Rydberg state becomes equal to or even exceeds that of the ultracold center of mass motion.