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Lifetime measurements of ultracold clouds near cryogenic surfaces

Brian L. Kasch, H. Hattermann, Daniel Cano, T. E. Judd, Stefan Scheel, Claus Zimmermann, R. Kleiner, Dieter Kölle, József Fortágh

Published Jun 7, 2009• 0 citations
OpenAlex

Abstract

We describe measurements of the trap lifetime of an ultracold atom cloud near niobium and copper surfaces at cryogenic temperatures. Large lifetimes of 35-160 seconds at 20-30 micrometers from superconducting niobium exceed predictions for a high conductivity normal metal with thermally fluctuating currents. This demonstrates clearly for the first time the absence of Johnson noise in the vicinity of the superconductor. Nonetheless we observed a drop in lifetime within 30 micrometers of the niobium wire, a result of the reduced trap depth due to the Meissner effect and corresponding surface evaporation. By comparison, near copper, a further reduction of the lifetime in this range indicates the presence of Johnson noise.

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We describe measurements of the trap lifetime of an ultracold atom cloud near niobium and copper surfaces at cryogenic temperatures. Large lifetimes of 35-160 seconds at 20-30 micrometers from superconducting niobium exceed predictions for a high conductivity normal metal with thermally fluctuating currents. This demonstrates clearly for the first time the absence of Johnson noise in the vicinity of the superconductor. Nonetheless we observed a drop in lifetime within 30 micrometers of the niobium wire, a result of the reduced trap depth due to the Meissner effect and corresponding surface evaporation. By comparison, near copper, a further reduction of the lifetime in this range indicates the presence of Johnson noise.
We report on the measurement of atomic spin coherence near the surface of a superconducting niobium wire. As compared to normal conducting metal surfaces, the atomic spin coherence is maintained for time periods beyond the Johnson noise limit. The result provides experimental evidence that magnetic near-field noise near the superconductor is strongly suppressed. Such long atomic spin coherence times near superconductors open the way towards the development of coherently coupled cold atom/solid state hybrid quantum systems with potential applications in quantum information processing and precision force sensing.
We investigate the stability of magnetically trapped atomic Bose-Einstein condensates and thermal clouds near the transition temperature at small distances $0.5\text{ }\ensuremath{\mu}\mathrm{m}\ensuremath{\le}d\ensuremath{\le}10\text{ }\ensuremath{\mu}\mathrm{m}$ from a microfabricated silicon chip. For a $2\text{ }\ensuremath{\mu}\mathrm{m}$ thick copper film, the trap lifetime is limited by Johnson noise induced currents and falls below 1 s at a distance of $4\text{ }\ensuremath{\mu}\mathrm{m}$. A dielectric surface does not adversely affect the sample until the attractive Casimir-Polder potential significantly reduces the trap depth.
We study theoretically the lifetime of magnetically trapped atoms in the close vicinity of a type-II superconducting surface, in the context of superconducting atom-chips. We account for the magnetic noise created at the cloud position by the vortices present in the superconductor and give specific results for our experiment which uses a niobium film. Our main result is that atom losses are dominated by the presence of vortices. They remain however dramatically smaller than in equivalent room-temperature atom-chip setups using normal metals.
We describe an experiment in which Bose-Einstein condensates and cold atom clouds are held by a microscopic magnetic trap near a room-temperature metal wire 500 microm in diameter. The lifetime for atoms to remain in the microtrap is measured over a range of distances down to 27 microm from the surface of the metal. We observe the loss of atoms from the microtrap due to spin flips. These are induced by radio-frequency thermal fluctuations of the magnetic field near the surface, as predicted but not previously observed.
We have investigated Bose-Einstein condensates and ultracold atoms in the vicinity of a surface of a magnetic microtrap. The atoms are prepared along copper conductors at distances to the surface between $300\ensuremath{\mu}\mathrm{m}$ and $20\ensuremath{\mu}\mathrm{m}.$ In this range, the lifetime decreases from 20 s to 0.7 s showing a linear dependence on the distance to the surface. The atoms manifest a weak thermal coupling to the surface, with measured heating rates remaining below 500 nK/s. In addition, we observe a periodic fragmentation of the condensate and thermal clouds when the surface is approached.
We have measured magnetic trap lifetimes of ultra-cold Rb87 atoms at distances of 5-1000 microns from surfaces of conducting metals with varying resistivity. Good agreement is found with a theoretical model for losses arising from near-field magnetic thermal noise, confirming the complications associated with holding trapped atoms close to conducting surfaces. A dielectric surface (silicon) was found in contrast to be so benign that we are able to evaporatively cool atoms to a Bose-Einstein condensate by using the surface to selectively adsorb higher energy atoms.