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arXiv:1205.1698 · Europhysics Letters (EPL)

Single vortex fluctuations in a superconducting chip as generating dephasing and spin flips in cold atom traps

Amir Fruchtman, B. Horovitz

Published Sep 1, 2012• 6 citations
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Abstract

We study trapping of a cold atom by a single vortex line in an extreme type-II superconducting chip, allowing for pinning and friction. We evaluate the atom's spin flip rate and its dephasing due to the vortex fluctuations in equilibrium and find that they decay rapidly when the distance to the vortex exceeds the magnetic penetration length. We find that there are special spin orientations, depending on the spin location relative to the vortex, at which spin dephasing is considerably reduced while perpendicular directions have a reduced spin flip rate.

DOI
10.1209/0295-5075/99/53002

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We study trapping of a cold atom by a single vortex line in an extreme type-II superconducting chip, allowing for pinning and friction. We evaluate the atom's spin flip rate and its dephasing due to the vortex fluctuations in equilibrium and find that they decay rapidly when the distance to the vortex exceeds the magnetic penetration length. We find that there are special spin orientations, depending on the spin location relative to the vortex, at which spin dephasing is considerably reduced while perpendicular directions have a reduced spin flip rate.
Atom chips made of superconducting material can generate magnetic traps with significantly reduced noise. Recently, several designs for superconducting chips have been theoretically analyzed and experimentally tested for cases with many vortices considered as an average vortex density. Here we show theoretically how the magnetic field of a single vortex, pinned by a superconducting nanodisk of radius \ensuremath{\sim}100 nm and combined with an external bias field parallel to the disk surface, yields a closed three-dimensional trap for cold atoms. The size of the trap and its height above the superconductor surface are typically tens or hundreds of nanometers. We estimate the average lifetime \ensuremath{\tau} of 87Rb^{87}\mathrm{Rb}87Rb (rubidium) atoms (subject to thermal escape and Majorana spin flips) in the range 0.05--1.0 ms. We model the trap in a quantum adiabatic approximation and apply Fermi's rule to estimate the lifetime of 87Rb^{87}\mathrm{Rb}87Rb atoms in the ground state of this trap. We obtain similar lifetimes \ensuremath{\tau} as in the semiclassical estimate, in the range 0.05--3.5 ms. We find that \ensuremath{\tau} depends on the gradient B0{B}_{0}B0​ of the vortex's magnetic field according to $\ensuremath{\tau}\ensuremath{\sim}{B}_{0}^{\ensuremath{-}2/3}$.
The lifetime of 87Rb^{87}\text{R}\text{b}87Rb atoms trapped in a $\mathbsf{z}$ wire trap generated by a closed-circuit superconductive current on a MgB2{\text{MgB}}_{2}MgB2​ strip is measured as a function of the distance between the atom and the strip. The lifetime is found to be longer than 10 s at a distance of $40\text{ }\ensuremath{\mu}\text{m}$. This value is an order of magnitude longer than the lifetime of a trap generated by a normal current at the same distance. However, it is many orders of magnitude shorter than the theoretical decay rate induced by the spin-flip transition caused by the fluctuation of the current. This shows that for a type-II superconductor the dominant trap loss mechanism is not the spin-flip transition caused by noise as with a normal current atom trap. An analysis of our measurement suggests that magnetic field distortion resulting from flux penetration into the superconductor leads to much faster decay.
We investigate superconducting chip structures for trapping and manipulating atoms. These structures are based on the average magnetic field of vortices induced in a type-II superconducting thin film. This magnetic field is the critical ingredient of the demonstrated vortex-based atom trap, which operate without transport current. We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultra-cold atoms. The experimental realization can be described by the Bean’s critical-state method to model the vortex field through mesoscopic super currents induced in the thin strip. Various vortex patterns can be obtained by programming different loading-field and transport current sequences. Furthermore we will discuss the expected enhanced lifetime of atoms trapped close to a superconducting surface in comparison with a metallic surface.
We investigate superconducting chip structures for trapping and manipulating atoms. These structures are based on the average magnetic field of vortices induced in a type-II superconducting thin film. This magnetic field is the critical ingredient of the demonstrated vortex-based atom trap, which operate without transport current. We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultra-cold atoms. The experimental realization can be described by the Bean's critical-state method to model the vortex field through mesoscopic super currents induced in the thin strip. Various vortex patterns can be obtained by programming different loading-field and transport current sequences. Furthermore we will discuss the expected enhanced lifetime of atoms trapped close to a superconducting surface in comparison with a metallic surface.
In a recent paper [Phys. Rev. Lett. 97, 070401 (2006)] the transition rate of magnetic spin-flip of a neutral two-level atom trapped in the vicinity of a thick superconducting body was studied. In the present paper we will extend these considerations to a situation with an atom at various distances from a dielectric film. Rates for the corresponding electric dipole-flip transition will also be considered. The rates for these atomic flip transitions can be reduced or enhanced, and in some situations they can even be completely suppressed. For a superconducting film or a thin film of a perfect conducting material various analytical expressions are derived that reveals the dependence of the physical parameters at hand.
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.