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Physical Review Letters

Demonstration of neutral atom trapping with microwaves

R. J. C. Spreeuw, Christoph Gerz, Lori S. Goldner, William D. Phillips, S. L. Rolston, Christoph Ira Westbrook, M. W. Reynolds, Isaac F. Silvera

Published May 16, 1994• 77 citations
Publisher

Abstract

We demonstrate trapping of neutral Cs atoms by the magnetic dipole force due to a microwave field. The trap is formed in a spherical microwave cavity tuned near the ground state hyperfine transition (9.193 GHz). With a microwave power of 83 W, the trap is \ensuremath{\approxeq}0.1 mK deep. It is loaded with Cs atoms laser cooled to \ensuremath{\approxeq}4 \ensuremath{\mu}K. We observed oscillatory motion of atoms in the trap at frequencies of 1--3 Hz. This type of trap has certain advantages for achieving the conditions for Bose-Einstein condensation in hydrogen or the alkalis, because it can confine atoms predominantly in the lowest energy spin state.

DOI
10.1103/physrevlett.72.3162

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We demonstrate trapping of neutral Cs atoms by the magnetic dipole force due to a microwave field. The trap is formed in a spherical microwave cavity tuned near the ground state hyperfine transition (9.193 GHz). With a microwave power of 83 W, the trap is \ensuremath{\approxeq}0.1 mK deep. It is loaded with Cs atoms laser cooled to \ensuremath{\approxeq}4 \ensuremath{\mu}K. We observed oscillatory motion of atoms in the trap at frequencies of 1--3 Hz. This type of trap has certain advantages for achieving the conditions for Bose-Einstein condensation in hydrogen or the alkalis, because it can confine atoms predominantly in the lowest energy spin state.
We duscuss a resonant microwave trap for neutral atoms. Because of the long spontaneous radiation time this trap is remarkably different from the optical trap. It also has advantages over static magnetic traps that trap the excited spin state of the lowest electronic level, in that atoms predominantly in the spin ground state can be trapped. We analyze the relaxation-ejection lifetime of atoms in such a trap using the formalism of dressed atomic states. Results are appliedi to atomic hydrogen and the possibility of Bose-Einstein condensation is considered.
Trapping particles is a key starting point for a range of physics experiments. The ability to confine, manipulate and interrogate a trapped species enables an invaluable degree of control and precision. Each trap offers different advantages and possibilities, and emerging experiments often demand different methods of trapping. This thesis describes a new three-dimensional microwave trap for atoms and molecules. Like an optical dipole trap, it can trap ground state polar molecules, but its depth and volume are larger by factors of 10^3 and 10^9 respectively. The experiment consists of a lithium oven, Zeeman slower, magneto-optical trap (MOT), a moving magnetic trap, and a high-quality Fabry-Perot cavity which serves as the microwave trap. Starting from a MOT of 1×10^8 lithium-7 atoms, the atomic cloud is cooled from a temperature of around 1mK to 41(1)μK with the recently developed Raman grey molasses method. Next, the population is optically pumped to a magnetically trappable state, and up to 85(2)% are loaded into a quadrupole magnetic trap. These atoms are then transferred to a moving magnetic trap and translated 600mm to the centre of the microwave cavity. After optimizing the transport to minimize atom heating and loss, we can produce samples of 2×10^7 atoms in the microwave chamber, at a phase space density of 3.58(15)×10^7. Finally, loading of the microwave trap is demonstrated and its basic properties studied. When the input power to the cavity is 610W, the electric field amplitude at the centre reaches roughly 20kV/cm. At this input power, centre-of-mass oscillation frequencies in the microwave trap were measured as 28.55(5)Hz along the axis of the cavity, and 8.81(8)Hz along the orthogonal direction. The observed lifetime was 1.76(12)s, limited by the vacuum quality. The atomic cloud expands and cools to 22(3)μK when loaded into the microwave trap, with a phase space density of 4(1)×10^7. This is consistent with adiabatic loading from the magnetic trap.
Ultracold atom experiments use a gas of neutral atoms with temperatures less than 100 µK above absolute zero and offer unmatched experimental control of quantum states and coherence, which has allowed ultracold atom-based measurements to be some of the most precise to date. While ultracold atom experiments can control almost all atomic degrees of freedom, spin-dependent trapping and spatial manipulation has remained difficult if not inaccessible. We are developing a method of spin-dependent trapping and spatial manipulation for ultracold neutral atoms using the AC Zeeman force produced by a microwave magnetic near-field gradient generated by an atom chip. We measure the AC Zeeman force on ultracold rubidium atoms by observing its effect on the motion of atoms in free-fall and on those confined in a trap. We have studied the force as a function of microwave frequency detuning from a hyperfine transition at 6.8 GHz at several magnetic field strengths and have observed its characteristic bipolar and resonant features predicted by two-level dressed atom theory. We find that the force is several times the strength of gravity in our setup, and that it can be targeted to a specific hyperfine transition while leaving other hyperfine states and transitions relatively unaffected. We find that our measurements are reasonably consistent with parameter-free theoretical predictions.
We report the experimental achievement of 87Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation. Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87Rb atoms to transit from the ground-state hyperfine state |F = 2,mF =2> to |F = 1,mF = 1>. Compared with RF-induced evaporation, 87Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1,mF = 1> state being pumped back into the trapping |F = 2,mF = 1> state.
We report the experimental achievement of 87 Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation.Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87 Rb atoms to transit from the ground-state hyperfine state |F = 2, mF = 2 to |F = 1, mF = 1 .Compared with RF-induced evaporation, 87 Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1, mF = 1 state being pumped back into the trapping |F = 2, mF = 1 state.