Argon atoms in Stark states at n approximately 25 have been decelerated and accelerated in inhomogeneous electric fields. The acceleration and deceleration behavior can be understood only by considering the adiabatic Landau-Zener dynamics that take place at the avoided crossings between the Stark states and the limited fluorescence lifetimes of approximately 10 micros.

We show that even in three dimensions an antiferromagnetic spin-1 Bose-Einstein condensate, which can, for instance, be created with 23 Na atoms in an optical trap, has not only singular linelike vortex excitations, but also allows for singular pointlike topological excitations, i.e., monopoles similar to the 't Hooft -Polyakov monopoles. We discuss the static and dynamic properties of these monopoles. DOI: 10.1103/PhysRevLett.87.120407 PACS numbers: 03.75.Fi, 14.80.Hv, 32.80.Pj, 67.40. -w Introduction.-Quantum magnetism plays an important role in such diverse areas of physics as high-temperature superconductivity, quantum phase transitions, and the quantum Hall effect. Moreover, it now appears that magnetic properties are also very important in another area, namely Bose-Einstein condensation in trapped atomic gases. This is due to two independent experimental developments. The first development is the realization of an optical trap for 23 Na atoms [1], whose operation no longer requires the gas to be doubly spin polarized and has given rise to the creation of a spin-1 Bose-Einstein antiferromagnet [2]. The second development is the creation of a two-component condensate of 87 Rb atoms [3], which by means of rf fields can be manipulated so as to make the two components essentially equivalent [4]. As a result also a spin-1͞2 Bose-Einstein ferromagnet can now be studied in detail experimentally.The spin structure of these condensates has recently been worked out by a number of authors [5 -8] and also the first studies of the linelike vortex excitations have appeared [5,9,10]. An immediate question that comes to mind, however, is whether the spin degrees of freedom lead also to other topological excitations that do not have an analogy in the case of a single component Bose-Einstein condensate. The answer to this question is in general affirmative. Indeed, we have recently shown that ferromagnetic Bose-Einstein condensates have long-lived Skyrmion excitations, which are nonsingular but topologically nontrivial pointlike spin textures [11]. Moreover, we show here that also spin-1 Bose-Einstein antiferromagnets have pointlike topological excitations. In particular, there exist singular pointlike spin textures, which are analogous to the magnetic monopoles in particle physics discovered by 't Hooft and Polyakov [12]. Having done so, we then turn to the investigation of the precise texture and the dynamics of these monopoles.As indicated above, Skyrmion and monopole excitations have already been studied in the context of nuclear and high-energy physics, respectively. However, in these areas of physics there does not exist a satisfactory microscopic theory for these topological excitations. For example, the Skyrme model gives only a rather rough description of a nucleon. Moreover, magnetic monopoles

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A Rydberg atom mirror has been designed and its operational principle tested experimentally. A supersonic expansion containing H atoms moving with a velocity of 720 m/s initially propagates toward a quadrupolar electrostatic mirror. The H atoms are then photoexcited to n=27 Rydberg states with a positive Stark shift and move in a rapidly increasing electric field. The H atom beam is stopped in 4.8 micros, only 1.9 mm away from the photoexcitation spot, and is then reflected back. The reflection process is monitored by pulsed field ionization and imaging.

Argon atoms in a pulsed supersonic expansion are prepared in selected Stark components of Rydberg states with effective principal quantum number in the range n* = 15–25. When traversing regions of inhomogeneous electric fields, these atoms get accelerated or decelerated depending on whether the Stark states are low- or high-field seeking states. Using a compact electrode design, which enables the application of highly inhomogeneous and time-dependent electric fields, the Rydberg atoms experience kinetic energy changes of up to 1.2 × 10−21 J (i.e. 60 cm−1 in spectroscopic units) in a single acceleration/deceleration stage of 3 mm length. The resulting differences in the velocities of the low- and high-field seeking states are large enough that the corresponding distributions of times of flight to the Rydberg particle detector are fully separated. As a result, efficient spectral searches of the Rydberg states best suited for acceleration/deceleration experiments are possible. Numerical simulations of the particle trajectories are used to analyse the time-of-flight distributions and to optimize the time dependence of the inhomogeneous electric fields. The decay of the Rydberg states by fluorescence, collisions and transitions induced by black-body radiation takes place on a timescale long enough not to interfere significantly with the deceleration during the first ∼5 µs.