A Magnetic Waveguide System for Bose-Einstein Condensates

When Bose-Einstein condensation (BEC) was first observed in an ultra cold atomic vapor, it was widely applauded as a remarkable achievement of modern physics. One of the reasons for this is that BEC is an unusually simple and powerful example of massive particles behaving in a wavelike fashion, and thus illustrates a basic principle of quantum mechanics. From the outset however, it has also been expected that condensates will prove useful for practical applications. The number of conceivable applications is large: atom interferometry, quantum computation, nonlinear optics, surface probes, atomic interaction studies, and atomic clocks are some examples. If all this potential can be realized, the impact will be revolutionary. However, a few enabling techniques are yet to be demonstrated. One of these is a method to transmit a condensate along a desired path through space. To make an analogy, the situation is like having a new laser available, but lacking the mirrors and lenses required to deliver the beam to an experiment. It is proposed here to attack this problem through the development of a magnetic waveguide. In particular, the waveguide system will be applied to the construction of a practical atom interferometer.

The primary motivation for pursuing atom interferometry is that it could allow extremely sensitive measurement of a variety of phenomena. For example, the best rotation sensors use the Sagnac effect in an planar loop interferometer . Rotation of the loop gives rise to a phase shift proportional to the rotation rate Ω and to the area enclosed by the loop. The interfering wave can be either an optical field of frequency ω, or a deBroglie wave for a particle of mass m. The sensitivity of the phase dΦ/dΩ is larger in the massive case by a factor of mc²/'ω, which is of order 10¹⁰. While improvements of this large a magnitude may not be practically achievable, substantial gains should be possible, and the example serves well to illustrate the potential benefits of atom interferometry. Other applications include measurements of acceleration, gravitation, electromagnetic fields, atomic interactions, and fundamental constants.

Functioning atom interferometers have been constructed, and the best atom gyroscope has demonstrated a sensitivity about the same as that of the best optical version. Devices to date, however, have been based on thermal atomic sources at relatively high temperatures. In addition to limitations resulting from the incoherent nature of a thermal source, the large atomic velocities involved means that only small angular changes to the motion can be imposed. In a gyroscope this limits the enclosed area and thus the device sensitivity. More generally, it becomes difficult to expose the two arms of the interferometer to different environments, since they are never far separated. These problems point out the need for an atom guiding system of some type. The use of a BEC source facilitates this because the atomic velocities are so low that relatively weak guiding forces can be used. The phase-coherent nature of the condensate also allows for much greater flexibility of design, since it does not require a 'white-light' interference condition.

Several designs for atom guides have already been demonstrated, which attests to their importance. So far, however, no apparatus has combined a guide with a condensate source, which is the critical step for applications. The system proposed here is an integrated BEC/waveguide assembly in which the condensate is produced in the waveguide directly. This will also entail a novel trap design, which may offer significant improvements in itself. The numerous benefits and applications enabled by this technology are a powerful argument for its vigorous pursuit.