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Quantum Electronics and Laser Science Conference

Tightly confining waveguide for cold atom interferometry

Wilbert Rooijakkers, Mukund Vengalattore, Gary Zabow, Mara Goff Prentiss

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. The integration of atom optics components on a small surface may lead to many new experiments exploring the boundaries of quantum mechanics. Coherent transport of atoms is a key ingredient, and for this reason many groups are developing magnetic waveguides for cold atoms. So far the confinement in these guides is rather poor, resulting in the population of many transverse modes. Typically 10 Gauss over 100 /spl mu/m gives a mode spacing of 40 kHz for Cs atoms. We have developed a new magnetic waveguide, based on ferromagnetic material. At saturation the surface-field is 1 Tesla, which can be applied over a 10 /spl mu/m distance corresponding to a mode spacing of 18 MHz. We use a sandwich of mu-metal and copper sheets to generate a magnetic field. An electrical current of up to 5 A is running through the copper, encircling the mu-metal sheet. This induces an upward or downward field in the mu-metal, similar to a solenoid with a ferromagnetic core. Thin kapton sheets are used to provide electrical isolation.

DOI
10.1109/qels.2001.961927

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Physical Review Letters•Feb 14, 2000•152 citations
Summary form only given. The integration of atom optics components on a small surface may lead to many new experiments exploring the boundaries of quantum mechanics. Coherent transport of atoms is a key ingredient, and for this reason many groups are developing magnetic waveguides for cold atoms. So far the confinement in these guides is rather poor, resulting in the population of many transverse modes. Typically 10 Gauss over 100 /spl mu/m gives a mode spacing of 40 kHz for Cs atoms. We have developed a new magnetic waveguide, based on ferromagnetic material. At saturation the surface-field is 1 Tesla, which can be applied over a 10 /spl mu/m distance corresponding to a mode spacing of 18 MHz. We use a sandwich of mu-metal and copper sheets to generate a magnetic field. An electrical current of up to 5 A is running through the copper, encircling the mu-metal sheet. This induces an upward or downward field in the mu-metal, similar to a solenoid with a ferromagnetic core. Thin kapton sheets are used to provide electrical isolation.
This thesis presents the first demonstration of a guide for cold atoms based on a miniature structure of four current-carrying wires. The four wires are embedded within a hollow silica fibre. Atoms are guided along the centre of a fifth hole on the axis of the fibre by the Stern-Gerlach force. A vapour cell Magneto Optical Trap (MOT), formed 1 cm above the mouth of the waveguide is the source of cold "8"5Rb atoms. After cooling the atoms to 25 #mu#K in optical molasses they fall under the influence of gravity through a magnetic funnel into the waveguide. After propagating for 2 cm, the atoms are reflected by the field of a small pinch coil wound around the base of the guide. The atoms then travel back up the fibre and out into the funnel, where they can be imaged either in fluorescence or by recapturing in the MOT. A video sequence of atoms falling into the guide and re-emerging after reflection from the pinch coil graphically illustrates the operation of the guide. The coupling efficiency and transverse temperature of the atoms is measured experimentally and in a Monte-Carlo simulation. We find an optimum coupling efficiency of 12% and we measure the spatial extent of the cloud within the fibre to be of order 100 #mu#m. We find good agreement between experimental data and results from the numerical simulation. We have also been able to observe different thresholds for the reflection of different positive m_F levels. In another experiment we are able to trap the atoms in an elongated Ioffe trap for up to two seconds, increasing the distance over which the atoms are guided. We are able to guide the atoms over distances of 40 cm with a loss rate indistinguishable from the free space loss rate. (author)
Abstract Atom chips provide flexible technologies for implementing modern concepts in quantum optics, quantum measurement, and quantum information processing. Atom chips are miniature devices that confine, control and manipulate cold atoms using electric, magnetic, and light fields. Due to the shrinkage of scale, arbitrary magnetic traps can be generated from current sources outside the chip’s vacuum compartment, in contrast to a traditional setup. This makes it easy to change trap configurations in an experiment that involves rapid prototyping of quantum states and quantum trajectory designs in free space. In this work, we show relevant parameters needed for transferring a cold atom cloud at the recoil limit from a magneto-optical trap (MOT) to an area close to the atom chip. To create a movable magnetic potential for this transfer, we used the MOT coils and an additional pair of coils in an anti-Helmholtz configuration. The properties of the movable potential were obtained by performing the Computer Simulation Tool (CST EM Studio suite®). Furthermore, an appropriate magnetic trap on a chip is developed, based on the simulation from COMSOL Multiphysics. We used a magnetic field gradient of around 20 G/cm to transport the cold atom with a distance over 20 mm with a temperature gain below 100 micro-Kelvin. The simulation results are based on an atom chip with a size of 2×2 cm2 and a copper wire thickness of 2 mm. The atom chip consists of Z, U and I-shaped wires that generate a quadrupole magnetic field. The resulting field minimum can be made at least 7 mm away from the chip surface.
For the last decade it has been possible to cool atoms to microkelvin temperatures (~1 cm s -1 ) using a variety of optical techniques. Light beams provide the very strong frictional forces required to slow atoms from room temperature (~500 m s -1 ). However, once the atoms are cold, the relatively weak conservative forces of static electric and magnetic fields play an important role. In our group we have been studying the interaction of cold rubidium atoms with periodically magnetized data storage media. Here we review the underlying principles of the forces acting on atoms above a suitably magnetized substrate or near current-carrying wires. We also summarize the status of experiments. These structures can be used as smooth or corrugated reflectors for controlling the trajectories of cold atoms. Alternatively, they may be used to confine atoms to a plane, a line, or a dot and in some cases to reach the quantum limit of confinement. Atoms levitated above a magnetized surface can be guided electrostatically by wires deposited on the surface. The flow and interaction of atoms in such a structure may form the basis of a new technology, `integrated atom optics' which might ultimately be capable of realizing a quantum computer.
This project focuses on a novel experimental method for interfacing cold atoms with optical waveguides. This relies on the introduction of cold atoms into microscopic laser-drilled holes that are perpendicular to the propagation axis of the waveguide. Direct interfacing of cold atoms with the guided mode of a waveguide is an attractive mechanism by which to create atom-photon interfaces, as the small mode area increases the interaction rate. Unlike many previous approaches, this technique can be applied in almost any existing waveguide system, including chip-based waveguide arrays and other complex environments. It therefore has great promise as a way of creating hybrid atom-photon quantum devices. Using this method, we demonstrate coupling between cold atoms and the light propagating in the core of an untapered single-mode optical fibre. This was achieved by laser-drilling a cylindrical, transverse hole (30 μm diameter) through the core of the fibre. Ensembles of cold caesium atoms can be tightly confined through an optical dipole trap within the microscopic void. Probe light, resonant with the Cs D2 line, is then coupled into the fibre. By measuring the transmitted optical power through the interface, it was determined that up to 87% of the probe power could be absorbed by the atoms. The corresponding optical depth per unit length of the atom cloud is over 700 cm^(-1), higher than any value reported to date for a comparable system. This will be a key parameter for the miniaturisation of atom-optical systems as well as for enhancing spatial resolution in sensing applications. The dependence of this absorption on several experimental parameters was also characterised and found to be in line with theoretical expectations. The atomic transition is not noticeably broadened by the presence of the fibre. We have also carried out numerical simulations of light transmission across wave-guide junctions of this type, proving that tailored hole geometries can enable enhanced optical transmission. The achievable degree of improvement is such that it is conceivable to place the void within an optical resonator, for example using laser-written Bragg gratings, and to achieve the strong coupling between the atoms and the guided light. Altogether this work demonstrates the potential of this technique to interface atoms with tightly confined light, allowing for integration in otherwise purely photonic circuits. In such environment the interaction between the atomic ensemble and the light can act as a node for the storage and processing of quantum information.
A cloud of laser-cooled 85Rb atoms is coupled through a magnetic funnel into a miniature waveguide formed by four current-carrying wires embedded in a silica fiber. The atom cloud has a approximately 100 &mgr;m radius within the fiber and propagates over cm distances. We study the coupling, propagation, and transverse distribution of atoms in the fiber, and find good agreement with theory. This prototype demonstrates the feasibility of miniature guides as a tool in the new field of integrated atom optics, leading to single-mode propagation of de Broglie waves and the possible preparation of 1D atom clouds.