Ultracold Temperatures Research Articles

Observation of blue satellite bands and photoassociation at ultracold temperatures

We have observed atomic line self-broadening of Cs near 7P{sub 3/2} and 7P{sub 1/2} atomic lines at ultracold temperatures using a magneto-optical trap and resonant ionization detection. We have observed blue satellite band features at detunings of 560 and 800 MHz, respectively, as well as sharp hyperfine-split photoassociative spectra on the red wings of each line and also on the blue wings. Possible explanations of these features are discussed.

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.

S-wave quantum entanglement in a harmonic trap

We analyze the quantum entanglement between two interacting atoms trapped in a spherical harmonic potential. At ultra-cold temperature, ground state entanglement is generated by the dominated s-wave interaction. Based on a regularized pseudo-potential Hamiltonian, we examine the quantum entanglement by performing the Schmidt decomposition of low-energy eigenfunctions. We indicate how the atoms are paired and quantify the entanglement as a function of a modified s-wave scattering length inside the trap.

Manipulation of ultracold atomic mixtures using microwave techniques

We used microwave radiation to evaporatively cool a mixture of of 133Cs and 87Rb atoms in a magnetic trap. A mixture composed of an equal number (around 104) of Rb and Cs atoms in their doubly polarized states at ultracold temperatures was prepared. We also used microwaves to selectively evaporate atoms in different Zeeman states.

Feshbach resonance cooling of trapped atom pairs

Spectroscopic studies of few-body systems at ultracold temperatures provide valuable information that often cannot be extracted in a hot environment. Considering a pair of atoms, we propose a cooling mechanism that makes use of a scattering Feshbach resonance. Application of a series of time-dependent magnetic field ramps results in either zero, one, or two atoms remaining trapped. If two atoms remain in the trap after the field ramps are completed, then they have been cooled. Application of the proposed cooling mechanism to optical traps or lattices is considered.

Suppression of Angular Forces in Collisions of Non-S-State Transition Metal Atoms

Angular momentum transfer is expected to occur rapidly in collisions of atoms in states of nonzero angular momenta due to the large torque of angular forces. We show that despite the presence of internal angular momenta transition metal atoms interact in collisions with helium effectively as spherical atoms and angular momentum transfer is slow. Thus, magnetic trapping and sympathetic cooling of transition metal atoms to ultracold temperatures should be readily achievable. Our results open up new avenues of research with a broad class of ultracold atoms.

Features of chemical reactions at vanishing kinetic energy: the presence of internally ?hot? reagents

The reactions between vibrationally and rotationally excited H2 and D2 molecules and the F atom are analyzed at ultra-low collision energies using the Coupled Channels quantum approach. The aim of this work is to compare the relative efficiency of the reactive scattering event with that of the vibrational or rotational quenching process in the ultra-cold temperature regime in order to establish general trends, possibly amenable to experiments on this or on more complex systems. We have already compared the rotational de-excitation efficiency with the reactive one in the \(\mathrm{F + D_{2}(\nu = 0}, j = 2)\rightarrow\mathrm{DF + D}\) reaction [1] and we have seen in that case that rotational de-excitation is more efficient than reaction when going down to ultra-low energies. We are investigating here the vibrational excitation case when the internal energy of the molecule becomes large enough to be above the classical barrier, and we are also presenting new results for the rotationally hot H2 partner. We find that, with vibrationally “hot” molecules, the reaction becomes more efficient than the relaxation process, while the relative efficiency of such processes when having rotationally hot molecular partners is much more system-dependent.

Atom interferometry in a vertical optical lattice

AbstractWe have studied the interference of degenerate quantum gases in a vertical optical lattice. The coherence of the atoms leads to an interference pattern when the atoms are released from the lattice. This has been shown for a Bose‐Einstein condensate in early experiments. Here we demonstrate that also for fermions an interference pattern can be observed provided that the momentum distribution is smaller than the recoil momentum of the lattice. Special attention is given to the role of interactions which wash out the interference pattern for a condensate trapped in the lattice but do not affect a spin polarized Fermi gas, where collisions at ultra cold temperatures are forbidden. Comparing the interference of the two quantum gases we find a clear superiority of fermions for trapped atom interferometry.

Chemical reactions in the limit of zero kinetic energy: virtual states and Ramsauer minima in F + H2 → HF + H

The behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H2 and its absence for the reaction with D2 is investigated. Close-coupling calculations of the scattering matrix show that the F–H2 feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F–H2 reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a zero energy resonance and the corresponding zero temperature rate coefficient is 1 × 10−9 cm3 s−1 despite an energy barrier of 300 K. Evidence in support of the virtual state is also provided by the detection of a deep Ramsauer–Townsend minimum in the elastic component of the total cross section for F–H2 which the present calculations predict to occur at low energies.

On the role of van der Waals interaction in chemical reactions at low temperatures.

It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.

Magnetic trapping of rare-earth atoms at millikelvin temperatures.

The ability to create quantum degenerate gases has led to the realization of Bose-Einstein condensation of molecules, atom-atom entanglement and the accurate measurement of the Casimir force in atom-surface interactions. With a few exceptions, the achievement of quantum degeneracy relies on evaporative cooling of magnetically trapped atoms to ultracold temperatures. Magnetic traps confine atoms whose electronic magnetic moments are aligned anti-parallel to the magnetic field. This alignment must be preserved during the collisional thermalization of the atomic cloud. Quantum degeneracy has been reached in spherically symmetric, S-state atoms (atoms with zero internal orbital angular momentum). However, collisional relaxation of the atomic magnetic moments of non-S-state atoms (non-spherical atoms with non-zero internal orbital angular momentum) is thought to proceed rapidly. Here we demonstrate magnetic trapping of non-S-state rare-earth atoms, observing a suppression of the interaction anisotropy in collisions. The atoms behave effectively like S-state atoms because their unpaired electrons are shielded by two outer filled electronic shells that are spherically symmetric. Our results are promising for the creation of quantum degenerate gases with non-S-state atoms, and may facilitate the search for time variation of fundamental constants and the development of a quantum computer with highly magnetic atoms.

Lethal effects of freezing Echinococcus multilocularis eggs at ultralow temperatures.

Methods for killing Echinococcus multilocularis eggs within stool or intestinal samples, without damaging the diagnostic value of the sample, would significantly reduce the risk of animal health providers acquiring alveolar hydatid disease. The first objective of this study was to determine whether E. multilocularis eggs located in fox intestines can survive storage at -70 C for at least 4 days. Results showed that none of 72,000 E. multilocularis eggs remained infectious to defined strains of mice under these conditions, yet, similar eggs recovered from nonfrozen carcasses stored at 4 C for the same time period were viable. The structural identities of adult worms and eggs were not significantly altered by the freezing and thawing processes. These results indicate that ultracold temperatures can be used to kill or inactivate E. multilocularis eggs, making them safe to handle when diagnosing this parasite in definitive hosts. The second objective of this study was to determine whether E. multilocularis eggs could survive freezing to -70 C if commonly used cryopreservation protocols were used. The use of the cryoprotectant solution, 5% dimethyl sulfoxide-35% saline-60% lamb serum, with a -1 C/min freezing rate was unable to prevent the eggs from being killed by freezing to -70 C. Rapid cooling by plunge freezing into liquid nitrogen was also lethal to E. multilocularis eggs. Only a few of the many potential cryopreservation protocols were tested in this study, so it is not yet possible to completely rule out the possibility of preserving these eggs at ultralow temperatures, but it does indicate that temperatures below -70 C are lethal to eggs even under favorable storage conditions.

Deceleration and cooling of radioactive isotope beams—from GeV to μeV

Technical developments on the deceleration and cooling of energetic radioactive isotope (RI) beams (>109eV) to ultracold (<10−6eV) temperatures are discussed. This technique should be a key development for next generation RI-beam facilities, where a wide variety of RI-beams with high-quality, high-intensity and a wide range of beam energy are to be provided.

Ion beam cooling at S-LSR project

A heavy ion storage/cooler ring S-LSR is now under construction by collaboration between National Institute of Radiological Sciences and Institute for Chemical Research, Kyoto University, the circumference and radius of curvature, of which are 22.73 and 1.05m, respectively. Electron beam cooling of laser-produced carbon beam as the feasibility study of its application to the injector for cancer-dedicated pulse synchrotron and laser cooling to ultra-cold temperature of 24Mg+ ions directly injected from the ion source, are major scopes of the project. Superposition of the electric field perpendicular to the magnetic field will enable elimination of momentum dispersion throughout the whole circumference, which is expected to provide an additional tunable knob for realization of three-dimensional crystalline beam.

Longevity of cryogenically stored seeds*1

Though cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual shelf life achieved under ultra-cold temperatures has not been addressed theoretically or empirically. Here, we report measurable changes in germination of dried seeds stored under liquid nitrogen conditions for >10 years. There was considerable variability in the extent of deterioration among species and accessions within a species. Aging time courses for lettuce seeds stored at temperatures between 50 and −196 °C were fit to a form of the Avrami equation to determine rate coefficients and predict half-life of accessions. A reduction in the temperature dependency on aging rate, determined as a break in the Arrhenius plot, occurred at about −15 °C, and this resulted in faster deterioration than anticipated from extrapolation of kinetics measured at higher temperatures. The break in Arrhenius behavior occurred at temperatures in between the glass transition temperature (28 °C) and the Kauzmann temperature (−42 °C) and also coincided with a major triacylglycerol phase change (−40 to −7 °C). In spite of the faster than anticipated deterioration, cryogenic storage clearly prolonged shelf life of lettuce seeds with half-lives projected as ∼500 and ∼3400 years for fresh lettuce seeds stored in the vapor and liquid phases of liquid nitrogen, respectively. The benefit of low temperature storage (−18 or −135 °C) on seed longevity was progressively lost if seeds were first stored at 5 °C. Collectively, these results demonstrate that lowering storage temperature progressively increases longevity of seeds. However, cryogenic temperatures were not sufficient to stop deterioration, especially if initial stages of aging were allowed to progress at higher storage temperatures. This work contributes to reliable assessments of the potential benefit and cost of different genebanking strategies.

A revised formula for 3-body recombination that cannot exceed the unitarity limit

We discuss the recent developments in theory and experiment that bear on our understanding of three-body recombination at ultracold temperatures. Some of these predictions include the fourth-power dependence of the three-body rate on the two-body scattering lenght a, the appearance of quantum mechanical shape resonance and Feshbach resonances, and the existence of destructive interference minima at large positive scattering lenghts. At very large scattering lenghts, however, the a 4 dependence of the recombination rate eventually will exceed the unitarity limit. We propose in this paper a simple way to correct this problem, permitting an approximate extension of previous predictions to slightly higher energies above the three-body fragmentation threshold. A preliminary comparison is carried out with a very recent experiment by Grimm and coworkers, showing reasonable agreement.

Collisions and reactions of ultracold molecules

It is argued that collision dynamics of atoms and molecules at ultracold temperatures (below 1 mK) are not readily predictable from knowledge of collision dynamics above 100 K. In the case of elastic collisions, it is well known that the collision cross section is constant as T → 0 K but mass and symmetry effects are dramatic. The cases of inelastic and reactive collisions are less studied, but a T–1/2 dependence of the cross section as T → 0 K is expected. It seems that extrapolations of high-temperature inelastic and reactive behavior normally greatly underestimate ultracold-temperature rates. The prospects for experimental observation of ultracold collision dynamics are rapidly improving.Key words: ultracold molecules, collisions, reactions, hydrogen, scattering length.

Longevity of cryogenically stored seeds

Though cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual shelf life achieved under ultra-cold temperatures has not been addressed theoretically or empirically. Here, we report measurable changes in germination of dried seeds stored under liquid nitrogen conditions for >10 years. There was considerable variability in the extent of deterioration among species and accessions within a species. Aging time courses for lettuce seeds stored at temperatures between 50 and −196 °C were fit to a form of the Avrami equation to determine rate coefficients and predict half-life of accessions. A reduction in the temperature dependency on aging rate, determined as a break in the Arrhenius plot, occurred at about −15 °C, and this resulted in faster deterioration than anticipated from extrapolation of kinetics measured at higher temperatures. The break in Arrhenius behavior occurred at temperatures in between the glass transition temperature (28 °C) and the Kauzmann temperature (−42 °C) and also coincided with a major triacylglycerol phase change (−40 to −7 °C). In spite of the faster than anticipated deterioration, cryogenic storage clearly prolonged shelf life of lettuce seeds with half-lives projected as ∼500 and ∼3400 years for fresh lettuce seeds stored in the vapor and liquid phases of liquid nitrogen, respectively. The benefit of low temperature storage (−18 or −135 °C) on seed longevity was progressively lost if seeds were first stored at 5 °C. Collectively, these results demonstrate that lowering storage temperature progressively increases longevity of seeds. However, cryogenic temperatures were not sufficient to stop deterioration, especially if initial stages of aging were allowed to progress at higher storage temperatures. This work contributes to reliable assessments of the potential benefit and cost of different genebanking strategies.

Collisional properties of trapped cold chromium atoms

We report on calculations of the elastic cross section and thermalization rate for collision between two maximally spin-polarized chromium atoms in the cold and ultracold regimes, relevant to buffer-gas and magneto-optical cooling of chromium atoms. We calculate ab initio potential energy curves for Cr2 and the van der Waals coefficient C6, and construct interaction potentials between two colliding Cr atoms. We explore the effect of shape resonances on elastic cross section, and find that they dramatically affect the thermalization rate. Our calculated value for the s-wave scattering length is compared in magnitude with a recent measurement at ultracold temperatures.

Disalignment transitions in cold collisions of3Patoms with structureless targets in a magnetic field

A method for quantum-mechanical calculations of cross sections for the Zeeman transitions in collisions of {sup 3}P atoms with structureless targets in a magnetic field is presented and applied to the study of magnetic and electronic relaxation in oxygen-helium and carbon-helium collisions at cold and ultracold temperatures. The rate constants for collisionally induced transitions between Zeeman levels in ground-state oxygen have large magnitudes in a 1 T field. It is shown that magnetic fields induce the forbidden {sup 3}P{sub 1} {yields} {sup 3}P{sub 0} transition in ultracold collisions of carbon with helium. The cross section vanishes at zero energy for field-free collisions, but becomes infinitely large in a finite magnetic field, varying with velocity v and magnetic field B as B{sup 2}/v.