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Journal of Physics Conference Series

Atomic spin relaxation and spatial decoherence near metallic and superconducting surfaces

Stefan Scheel, R. Fermani, Per Kristian Rekdal, P. L. Knight, E. A. Hinds

Published Apr 1, 2006• 1 citation
Publisher

Abstract

We derive atomic spin relaxation rates near metallic and superconducting surfaces. Our results are based on a quantum-theoretical treatment of electromagnetic radiation near absorbing bodies. We show that there exists an atom-surface distance for which the expected relaxation rate becomes maximal and we discuss its dependence on the skin depth of the substrate material. In view of this effect we examine the possible use of superconducting materials. Furthermore, we discuss the influence of absorbing materials on spatial coherences in optical lattices.

DOI
10.1088/1742-6596/36/1/028

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We derive atomic spin relaxation rates near metallic and superconducting surfaces. Our results are based on a quantum-theoretical treatment of electromagnetic radiation near absorbing bodies. We show that there exists an atom-surface distance for which the expected relaxation rate becomes maximal and we discuss its dependence on the skin depth of the substrate material. In view of this effect we examine the possible use of superconducting materials. Furthermore, we discuss the influence of absorbing materials on spatial coherences in optical lattices.
We derive scaling laws for the spin decoherence of neutral atoms trapped near conducting and superconducting plane surfaces. A result for thin films sheds light on the measurement of Y. J. Lin, I. Teper, C. Chin, and V. Vuleti\ifmmode \acute{c}\else \'{c}\fi{} [Phys. Rev. Lett. 92, 050404 (2004)]. Our calculation is based on a quantum-theoretical treatment of electromagnetic radiation near metallic bodies [P. K. Rekdal, S. Scheel, P. L. Knight, and E. A. Hinds, Phys. Rev. A 70, 013811 (2004)]. We show that there is a critical atom-surface distance that maximizes the spin relaxation rate and we show how this depends on the skin depth and thickness of the metal surface. In the light of this impedance-matching effect we discuss the spin relaxation to be expected above a thin superconducting niobium layer.
We present a first-principles derivation of spatial atomic-sublevel decoherence near dielectric and metallic surfaces. The theory is based on the electromagnetic-field quantization in absorbing dielectric media. We derive an expression for the time variation of the off-diagonal matrix element of the atomic density matrix for arbitrarily shaped substrates. For planar multilayered substrates we find that for small lateral separations of the atom's possible positions the spatial coherence decreases quadratically with the separation and inversely to the squared atom-surface distance.
The aim of this thesis is to investigate the interaction of neutral atoms with conducting and superconducting surfaces. Experimental advances in the magnetic confinement of ultracold atoms have shown that they can act as a powerful tool in a wide range of phenomena such as electric and magnetic field imaging and matter wave interferometry. Coherent manipulation of atoms and ever smaller magnetic traps are essential elements in the implementation of integrated quantum devices for fundamental research, quantum information processing and precision measurements. This thesis considers main influences on atoms placed within three different environments which are useful in achieving miniaturization and efficient control in atomic magnetic traps: carbon nanotubes, dielectric surfaces and superconducting thin films. The possibility of holding atoms near the outside of a carbon nanotubes will be addressed. In order to give a qualitative analysis of the atom-nanotube interaction, thermally induced spin-flips and the Casimir-Polder potential have been considered. The comparison between these two effects is presented in this thesis. It indicates that the Casimir-Polder force is the dominant loss mechanism and an estimation of the minimum trapping distance is given based on its effect. Secondly, a first-principles derivation of spatial atomic-sublevel decoherence near dielectric and metallic surfaces will be presented. The rate obtained for the decay of spatial coherence has dual implications, on one hand, it can be considered as a measure of the coherence length of the fluctuations of the electromagnetic field arising from a given substrate. On the other hand, it turns out to be relevant for quantum information encoding in double well potentials. Finally, the known spin-flip transition rate will be linked to the flux noise spectrum in superconducting thin films showing the feasibility of using cold atomic clouds in the investigation of vortex dynamics.
We investigate theoretically the magnetic spin-flip transitions of neutral atoms trapped near a superconducting slab. Our calculations are based on a quantum-theoretical treatment of electromagnetic radiation near dielectric and metallic bodies. Specific results are given for rubidium atoms near a niobium superconductor. At the low frequencies typical of atomic transitions, we find that BCS theory greatly overestimates coherence effects, which are much less pronounced when quasiparticle lifetime effects are included through Eliashberg theory. At 4.2K4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}4.2K, the typical atomic spin lifetime is found to be larger than 1000s1000\phantom{\rule{0.3em}{0ex}}\mathrm{s}1000s, even for atom-superconductor distances of one $1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$. This constitutes a large enhancement in comparison with normal metals.
The spontaneous relaxation rates for an atom in free space and close to an absorbing surface are calculated to various orders of the electromagnetic multipole expansion. The spontaneous decay rates for dipole, quadrupole, and octupole transitions are calculated in terms of their respective primitive electric multipole moments and the magnetic relaxation rate is calculated for the dipole and quadrupole transitions in terms of their respective primitive magnetic multipole moments. The theory of electromagnetic-field quantization in magnetoelectric materials is used to derive general expressions for the decay rates in terms of the dyadic Green's function. We focus on the decay rates in free space and near an infinite half space. For the decay of atoms near to an absorbing dielectric surface, we find a hierarchy of scaling laws depending on the atom-surface distance zzz.
Using a consistent quantum-mechanical treatment for the electromagnetic radiation, we theoretically investigate the magnetic spin-flip scatterings of a neutral two-level atom trapped in the vicinity of a superconducting body. We derive a simple scaling law for the corresponding spin-flip lifetime for such an atom trapped near a superconducting thick slab. For temperatures below the superconducting transition temperature Tc, the lifetime is found to be enhanced by several orders of magnitude in comparison to the case of a normal conducting slab. At zero temperature the spin-flip lifetime is given by the unbounded free-space value.