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arXiv:quant-ph/0108138 · Physical Review Letters

Storage Ring for Neutral Atoms

J. A. Sauer, M. D. Barrett, Michael Chapman

Published Dec 7, 2001• 144 citations
arXivHTMLPDF

Abstract

We have demonstrated a storage ring for ultracold neutral atoms. Atoms with mean velocities of 1 m/s corresponding to kinetic energies of approximately 100 neV are confined to a 2 cm diameter ring by magnetic forces produced by two current-carrying wires. Up to 10(6) atoms are loaded at a time in the ring, and seven revolutions are clearly observed. Additionally, we have demonstrated multiple loading of the ring and deterministic manipulation of the longitudinal velocity distribution of the atoms using applied laser pulses. Applications of this ring include large area atom interferometers and monochromatic atomic beam generation.

DOI
10.1103/physrevlett.87.270401

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We have demonstrated a storage ring for ultracold neutral atoms. Atoms with mean velocities of 1 m/s corresponding to kinetic energies of approximately 100 neV are confined to a 2 cm diameter ring by magnetic forces produced by two current-carrying wires. Up to 10(6) atoms are loaded at a time in the ring, and seven revolutions are clearly observed. Additionally, we have demonstrated multiple loading of the ring and deterministic manipulation of the longitudinal velocity distribution of the atoms using applied laser pulses. Applications of this ring include large area atom interferometers and monochromatic atomic beam generation.
Several techniques for manipulating neutral atoms (more precisely, ultracold clouds of neutral atoms) in chip-based magnetic traps and atomic waveguides have been demonstrated. Such traps and waveguides are promising components of future quantum sensors that would offer sensitivities much greater than those of conventional sensors. Potential applications include gyroscopy and basic research in physical phenomena that involve gravitational and/or electromagnetic fields. The developed techniques make it possible to control atoms with greater versatility and dexterity than were previously possible and, hence, can be expected to contribute to the value of chip-based magnetic traps and atomic waveguides. The basic principle of these techniques is to control gradient magnetic fields with suitable timing so as to alter a trap to exert position-, velocity-, and/or time-dependent forces on atoms in the trap to obtain desired effects. The trap magnetic fields are generated by controlled electric currents flowing in both macroscopic off-chip electromagnet coils and microscopic wires on the surface of the chip. The methods are best explained in terms of examples. Rather than simply allowing atoms to expand freely into an atomic waveguide, one can give them a controllable push by switching on an externally generated or a chip-based gradient magnetic field. This push can increase the speed of the atoms, typically from about 5 to about 20 cm/s. Applying a non-linear magnetic-field gradient exerts different forces on atoms in different positions a phenomenon that one can exploit by introducing a delay between releasing atoms into the waveguide and turning on the magnetic field.
Devices that exploit the properties of quantum mechanics for their operation can offer unique advantages over their classical counterparts. Interference of matter waves can be used to\ndramatically increase the rotational sensitivity of gyroscopes. Complete control of the quantum evolution of a system could produce a new powerful computational device known as a quantum computer. Research into these technologies offers a deeper\nunderstanding of quantum mechanics as well as exciting new insights into many other areas of science. Currently, a limiting factor in many quantum devices using neutral atoms is accurate motional control over the atoms. This thesis describes two recent advancements in neutral atom motional control using both magnetic and electromagnetic confining fields. Part I reports on the demonstration of the first storage ring for neutral atoms. This storage ring may one day provide the basis for the world's most\nsensitive gyroscope. Part II describes the optical delivery of neutral atoms into the mode of a high-finesse cavity for applications in quantum computing and communication.
Laser-cooled neutral atoms from a low-velocity atomic source are guided via a magnetic field generated between two parallel wires on a glass substrate. The atoms bend around three curves, each with a 15-cm radius of curvature, while traveling along a 10-cm-long track. A maximum flux of $2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0ex}{0ex}}\mathrm{atoms}/\mathrm{sec}$ is achieved with a current density of $3\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0ex}{0ex}}\mathrm{A}/{\mathrm{cm}}^{2}$ in the $100\ifmmode\times\else\texttimes\fi{}100$- $\ensuremath{\mu}\mathrm{m}$-cross-section wires. The kinetic energy of the guided atoms in one transverse dimension is measured to be $42\ensuremath{\mu}\mathrm{K}$.
A thermal atomic Na beam is opposed by a resonantly tuned laser beam. The radiation pressure decelerates and compresses the thermal velocity distribution, bringing the average velocity to zero and reducing the width from ~1000 m/s to a few tens of m/s or less.1 The density of stopped atoms is > 106/cm3 within a bandwidth of a few m/ s. These densities and velocities are suitable for trapping by a number of proposed optical and magnetic traps. Using these stopped atoms, we have demonstrated the first electromagnetic trapping of neutral atoms. The trap, a magnetic quadrupole formed by two separated, coaxial, opposed current loops, confines the atoms by virtue of the force exerted on the Na magnetic dipole moment by the inhomogeneous magnetic field. The time constant for exponential decay of the atomic population in the trap is 0.83(7) s and is limited mainly by collisions with fast background gas atoms. The maximum velocity of atoms contained in the trap is 3.5 m/s, corresponding to a maximum energy of 17 mK.
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.
A superconducting neutral trap has been constructed and preliminary tests of it have begun. The tap consists of superconducting solenoids to permit slowing and axial confinement and a superconducting octopole magnet for radial confinement in the trap. The trap includes several smaller winding for manipulating trapped atoms. The trap itself is about 40 cm long and has a diameter of 7 cm. It is about 1 Kelvin deep. Na atoms from an atomic beam are to be slowed and then stopped in the center of the trap with counterpropagating laser beams. Initial testing of the magnets, cryogenic vacuum apparatus, and atom source has been completed. A program to optically trap atoms has been initiated. This is based on the recent discovery of exceptions to the optical Earnshaw theorem. Several possible traps using only spontaneous forces have been suggested and detailed modelling of these traps is underway. An apparatus to trap atoms based on these new ideas is being conducted.