No abstract

Superconductors are considered in view of applications to atom chip devices. The main features of magnetic traps based on superconducting wires in the Meissner and mixed states are discussed. The former state may mainly be interesting for improved atom optics, while in the latter, cold atoms may provide a probe of superconductor phenomena. The properties of a magnetic side guide based on a single superconducting strip wire placed in an external magnetic field are calculated analytically and numerically. In the mixed state of type II superconductors, inhomogeneous trapped magnetic flux, relaxation processes and noise caused by vortex motion are posing specific challenges for atom trapping. PACS: 37.10.Gh Atom traps and guides 85.25.Am Superconducting device characterization, design, and modeling Progress towards this goal demands the control and reduction of magnetic noise produced by the metallic components of the atom chip. Randomly fluctuating magnetic fields are generated by thermal current noise in the conducting chip elements and reduce the number of trapped atoms (losses), increase their temperature (heating) and lead to a phase uncertainty in the atom's state (decoherence) -see, for example, [1, 2, 3] and references therein. Theoretical analysis of the magnetic noise generated by a normal metal [4,5,6,7] predicts a fast reduction of the lifetime with the decrease of the distance z t between the trapped atom and the metal surface (trap height); this is in excellent agreement with lifetime measurements [8,9,10]. At a trap height less than 10 ÷ 20 µm, thermal magnetic noise exceeds all other harmful influences on the atom cloud (technical noise due to the current supply instability, residual gas collisions, stray magnetic fields) and provides the dominant limit for the lifetime [1,3]. In the last few years, the application of superconducting materials to atom chips has been widely discussed as a perspective to extend the lifetime of cold atoms [7,11,12,13,14]. A recent theoretical estimate [13] of the magnetic noise caused by a superconductor in the Meissner state showed that the lifetime of atoms trapped above a superconducting layer would be, at least, six orders of magnitude longer than above a normal metal in the same geometry. The analysis presented in [14] predicts an atom lifetime of 5000 s at a trap height of 1 µm. For comparison, at the same height in a normal metal trap the lifetime is less than 0.1 s [9]. At larger heights, the lifetime in a superconducting niobium chip can be much larger (up to 10 11 s at temperature T = 4.2 K).Two first realizations of atom chips with superconducting elements have been reported in Refs. [15] and [16]. In both setups, the trapped atoms were 87 Rb. In the experiment by Nirrengarten et al. [15], the current-carrying wires (in "U" and "Z" shape) were made of niobium and operated at about 4.2 K. The obtained atom spin relaxation time was estimated as 115 s. This value is comparable to the best one achieved for atoms trapped near normal-metal wires [17]. In the second ...

We discuss the contribution of the material type in metal wires to the electromagnetic fluctuations in magnetic microtraps close to the surface of an atom chip. We show that significant reduction of the magnetic noise can be achieved by replacing the pure noble metal wires with their dilute alloys. The alloy composition provides an additional degree of freedom which enables a controlled reduction of both magnetic noise and resistivity if the atom chip is cooled. In addition, we provide a careful re-analysis of the magnetically induced trap loss observed by Yu-Ju Lin et al. [Phys. Rev. Lett. 92, 050404 (2004)] and find good agreement with an improved theory. PACS numbers: 39.25.+k Atom manipulation 72.15.-v Electronic conduction in metals and alloys 07.50.-e Electrical and electronic instruments and components 03.75.-b Matter waves

The use of superconducting films in the Meissner state reduces the level of noise in micro-and nanochips. Here we present a numerical scheme for computing the Meissner transport current distribution in superconducting films of arbitrary shape, including multiply connected films. The scheme is used for simulating a 3D magnetic trap for cold atoms. Our algorithm is easily generalized for computing the Meissner-London distribution of transport current.

We analyze atom-surface magnetic interactions on atom chips where the magnetic trapping potentials are produced by current carrying wires made of electrically anisotropic materials. We discuss a theory for time dependent fluctuations of the magnetic potential, arising from thermal noise originating from the surface. It is shown that using materials with a large electrical anisotropy results in a considerable reduction of heating and decoherence rates of ultra-cold atoms trapped near the surface, of up to several orders of magnitude. The trap loss rate due to spin flips is expected to be significantly reduced upon cooling the surface to low temperatures. In addition, the electrical anisotropy significantly suppresses the amplitude of static spatial potential corrugations due to current scattering within imperfect wires. Also the shape of the corrugation pattern depends on the electrical anisotropy: the preferred angle of the scattered current wave fronts can be varied over a wide range. Materials, fabrication, and experimental issues are discussed, and specific candidate materials are suggested. PACS. 37.10.Gh Atom traps and guides -39.25.+k Atom manipulation -72.15.-v Electronic conduction in metals and alloys -03.75.-b Matter waves