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Zeitschrift für Naturforschung A

Quantum Reflection of Ultracold Atoms in Magnetic Traps

Rui‐Hua Xie, Paul W. Brumer

Published Apr 1, 1999• 0 citations
Publisher

Abstract

Ultracold neutral atoms can be trapped in spatially inhomogeneous magnetic fields. In this paper, we present a theoretical model and demonstrate by using Landau-Zener tool that if the magnetic resonant transition region is very narrow, "potential barriers" appear and quantum reflection of such ultracold atoms can be observed in this region.

DOI
10.1515/zna-1999-3-401

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Ultracold neutral atoms can be trapped in spatially inhomogeneous magnetic fields. In this paper, we present a theoretical model and demonstrate by using Landau-Zener tool that if the magnetic resonant transition region is very narrow, "potential barriers" appear and quantum reflection of such ultracold atoms can be observed in this region.
Subject of this thesis is the investigation of the quantum dynamics of ultracold atoms in the presence of external inhomogeneous magnetic fields. We discuss the behavior of ground state atoms inside typical magnetic field configurations. Such atoms can be approximately treated as neutral point-like particles which carry a certain spin. In particular we analyze the resonance spectrum and the density of states of both fermionic and bosonic atoms. We point out the existence of so-called quasi-bound states whose wave functions can be approximately calculated analytically. Moreover, we focus on studying quantum mechanical states of ultracold but electronically highly excited atoms - so-called Rydberg atoms - inside magnetic traps. In a first step we explore their internal dynamics by employing a fixed-nucleus approximation. The latter assumes the atomic nucleus to be fixed in the magnetic field minimum. Within this framework we analyze spectral properties as well as properties of the electronic spin and calculate electromagnetic transitions. Finally we present an adiabatic approach which allows for a thorough description of the coupled center of mass and electronic dynamics. We highlight ways to magnetically trap highly excited atoms and present novel quantum states in which the center of mass and the electronic wave function are equal in size. This finding shows that Rydberg atoms even in inhomogeneous fields with moderate gradients cannot be considered point-like.
Magnetic trapping of laser-cooled neutral atoms has been demonstrated at NBS and is progressing in other laboratories. Confinement of any object requires exchanging kinetic for potential energy, and for neutral atoms, this necessarily derives from shifts of internal energy levels. This is implemented through the force experienced by the atomic magnetic moment in a carefully designed, highly inhomogeneous field. Since magnetic fields of convenient strength can shift atomic energy levels by only a few gigahertz (temperature ≌ 0.1 K), neutral atom traps are very shallow and must be loaded with very cold atoms. We present some motivations for using magnetic traps and then discuss the constraints and optimal configurations of various arrangements. For example, no trap can be isotropic. Next we discuss both the classical and quantum mechanical motions of trapped atoms. These motions are important for at least two reasons: First, several schemes under study for further cooling depend on knowing the position and velocity of the atoms, as well as the vector field at each point along the orbit. Second, magnetic traps depend on the atomic moment remaining aligned with the field as the atom orbits in the trap, and this precludes rapid motion through a low-field region.
Ultracold atom experiments use a gas of neutral atoms with temperatures less than 100 µK above absolute zero and offer unmatched experimental control of quantum states and coherence, which has allowed ultracold atom-based measurements to be some of the most precise to date. While ultracold atom experiments can control almost all atomic degrees of freedom, spin-dependent trapping and spatial manipulation has remained difficult if not inaccessible. We are developing a method of spin-dependent trapping and spatial manipulation for ultracold neutral atoms using the AC Zeeman force produced by a microwave magnetic near-field gradient generated by an atom chip. We measure the AC Zeeman force on ultracold rubidium atoms by observing its effect on the motion of atoms in free-fall and on those confined in a trap. We have studied the force as a function of microwave frequency detuning from a hyperfine transition at 6.8 GHz at several magnetic field strengths and have observed its characteristic bipolar and resonant features predicted by two-level dressed atom theory. We find that the force is several times the strength of gravity in our setup, and that it can be targeted to a specific hyperfine transition while leaving other hyperfine states and transitions relatively unaffected. We find that our measurements are reasonably consistent with parameter-free theoretical predictions.
Ultracold atoms trapped in an optical lattice of double-well potential, the so-called optical superlattice, have received much attention in the field of cold atoms. A protocol generalized to three-well optical superlattice is suggested in this paper. The ground-state diagrams of ultracold spin-1 atoms trapped in a symmetric three-well optical superlattice in a weak magnetic field are studied based on the exact diagonalization. It is shown that the ground-state diagrams are remarkably different for the ferromagnetic and antiferromagnetic atoms. There does not exist the type of ground state for the antiferromagnetic interaction atoms, where the magnetic quantum number of the total spin of the system along the external magnetic field are ±2. But for the ferromagnetic interaction atoms, there do exist. In addition, there exist only the fully polarized ground-states for the ferromagnetic atoms in the negative quadratic-Zeeman-energy region. The physicsal origin of the dependence of the ground states on the controllable parameters are analyzed. These quantum spin-states can be controlled easily and exactly by modulating the external magnetic field and the height of the optical barrier, which may be a tool for the study of spin-entanglement.
We report the observation of influence of homogeneous magnetic field on an optical lattice losses of ultracold thulium atoms. The atomic cloud temperature was T = 15 mK. The dependence of trap population on a value of magnetic field has a broad resonance in the low-field region with a center at B = 0.4 G. We also have measured a decrease of optical lattice lifetime in a presence of resonance magnetic field. The observed magnetically-induced trap losses are assumed to be Feshbach resonance which is a dependence of an atomic scattering length on magnetic field.
We show that the magnetic field produced by a chiral ferromagnetic film can be applied to control ultracold atoms. The film will act as a magnetic mirror or a reflection grating for ultracold atoms when it is in the helical phase or the skyrmion crystal phase, respectively. By applying a bias magnetic field and a time-dependent magnetic field, one-dimensional or two-dimensional magnetic lattices including honeycomb, Kagome, and triangular types can be created to trap the ultracold atoms. We have also discussed the trapping height, the potential barrier, the trapping frequency, and the Majorana loss rate for each lattice. Our results suggest that the chiral ferromagnetic film can be a platform to develop artificial quantum systems with ultracold atoms based on modern spintronics technologies.