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Demonstration of a microwave trap for cesium atoms
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Physica B 194-196 (1994) 893--894 North-Holland
DEMONSTRATION OF A MICROWAVE TRAP FOR CESIUM ATOMS Lori S. Goldner, a R.J.C. Spreeuw, a C. Gerz, a W.D. Phillips,~M.W. Reynolds, b* S.L. Rolston, ~ Isaac F. Silvera b and C.I. Westbrook at aNational Institute of Standards and Technology, Gaithersburg, MD 20899, USA bLyman Laboratory of Physics, Harvard University, Cambridge, MA 02138, USA We report on the first realization of a microwave trap for neutral atoms, as proposed originally for use with spin-polarized hydrogen [1]. This trap is advantageous for attaining Bose condensation because it can contain strong or weak field seekers, while static magnetic traps contain only the less-stable weak-field seeking states. In this experiment, Cs atoms were confined in the microwave field of a room-temperature spherical resonator (Q~ 5500) whose TE 110 mode is tuned to the blue of the ground state hyperfine splitting (9.2 GHz). Using up to 83 W of microwave power, the trap is shallow (< 100#K), large (~. 1 cm) and cannot, by itself, hold Cs atoms up against gravity. We levitate atoms in the trapping state (F = 4, mF = 4 ground state) with a static magnetic field gradient. The trap is loaded with atoms cooled to 5 ttK in optical molasses. To detect the atoms, we either release them from the trap and detect their fluorescence as they fall through a probe laser beam, or we observe them with a ccd camera as we illuminate them with laser pulses. Plans for applying this technique to spin-polarized hydrogen will be discussed in a separate presentation [2].
Optical atom traps based on the electric dipole interaction were realized in 1986 [3]. In analogy to this, a proposal was put forth for a microwave trap based on the magnetic dipole interaction [1]. Here we report on the first demonstration of such a trap, using Cs atoms. Consider first a two-level atom with the levels coupled by a magnetic dipole transition. A near-resonant microwave field couples and shifts the "bare" atomic states. The direction of the shift for any coupled eigenstate depends on the sign of the detuning of the microwaves from the atomic transition, and its amplitude depends on the detuning and field intensity. In a field gradient, the shift in energy with position provides a trapping force. Since there is no radiative decay, either state can be trapped depending on the detuning of the field. For static magnetic traps, strong-field seeking states, like the ground state, cannot be trapped, since static field maxima are prohibited in free space. The transition we use is between the F = 3, *Current address University of Amsterdam, Valckenierstraat 65, NL-1018 XE Amsterdam, The Netherlands. tCurrent address Institut d'Optique, Centre Scientifique d'Orsay, B.P. 147 91403 France. Elsevier Science B.V. S S D I 0921-4526(93)E1027-J
m F ----- 3 and F = 4, m F = 4 ground state hyperfine levels of Cs (9.2 GHz). A static magnetic field (~ 70 G) is used to split the magnetic sublevels. The microwave field is provided by the T E 110 mode of a spherical resonator ( Q ~ 5500). At attainable input power levels (< 83 W) and a typical (blue) cavity detuning of 72 MHz, we get a trapping potential for the F = 4 atoms with a depth of 100 /~K at the center and a width of about 1 cm. This trap cannot hold Cs atoms up against gravity (1.6 m K / e m ) , and so we also apply a static levitating field gradient of about 23.4 G / c m along the vertical axis. The atoms are first cooled in optical molasses [4] to about 5 pK at the geometric center of the cavity. (There are holes drilled in the spherical resonator for laser, atomic beam, and observational access.) The molasses light is then turned off, and the atoms optically pumped into the F = 4, m F = 4 trapping state before they have a chance to fall. The static (tuning and levitating) and microwave (trapping) fields are then turned on. Due to the combination of these fields and gravity, the trap center is approximately 3 m m below the molasses. The atoms fall through and then oscillate about the trap center with an
894
Figure 1. Atomic motion in the microwave trap, as seen through a I cm access hole in the side. Frames are separated by 67 ms; move across from left to right starting at the top. Here the input power was 42 W. The increase in brightness between frames 12 and 13 results from a rescaling of the intensities. amplitude dependent on the initial distance from the center and a frequency dependent on the trap depth. The trapping lifetime, typically 1 s, is dependent on the trap depth and vacuum quality. With only a levitating field and no microwaves present, no oscillations were seen and the atoms quickly dispersed.
E ~108 E .> 1 0 4 =-
I
I
I
I
~.~ 6
0.0
I
I
I
I
0.5
1.0
1.5
2.0
2.5
time in trap (s) Figure 2. (top) The arrival time of the atoms at the probe vs. the time spent in the trap. (bottom) The width of the distribution vs. time. The input power was 83 W, the largest power used.
After spending a given time in the trap (Fig. 1), the atoms are illuminated with a 5 ms pulse of laser light. The fluorescence of the atoms is detected on a ccd camera. The illumination heats and therefore destroys the sample, so that each frame represents a separate trap loading. In a second data-taking method, (Fig. 2), atoms are dropped out of the trap by turning off the fields. The falling a t o m s are then detected as they fall through and fluoresce in a probe laser beam located 5.2 cm below the center of the trap. Both the width and the arrival time of the distribution of atoms can be measured in this fashion. The arrival time is observed to oscillate as a function of time spent in the trap, and the width oscillates at roughly twice this frequency. This latter oscillation, expected for an ensemble of a t o m s moving in a harmonic potential, can also be seen as a "breathing" motion in Fig. 1. This work was supported in part by the Office of Naval Research and the D e p a r t m e n t of Energy, grant DE-FG02-85ER45190.
REFERENCES 1. 2. 3. 4.
C.C.Agosta, I.F.Silvera, H.T.C.Stoof and B.J.Verhaar, Phys. Rev. Lett. 62,2361 (1991). I.F. Silvera, et al, this volume. S. Chu, J.E. Bjorkholm, A. Ashkin, and A. Cable, Phys. Rev. Lett. 57, 314 (1986). S. Chu et al., Phys. Rev. Lett. 55, 48 (1985).
Physica B 194-196 (1994) 893--894 North-Holland
DEMONSTRATION OF A MICROWAVE TRAP FOR CESIUM ATOMS Lori S. Goldner, a R.J.C. Spreeuw, a C. Gerz, a W.D. Phillips,~M.W. Reynolds, b* S.L. Rolston, ~ Isaac F. Silvera b and C.I. Westbrook at aNational Institute of Standards and Technology, Gaithersburg, MD 20899, USA bLyman Laboratory of Physics, Harvard University, Cambridge, MA 02138, USA We report on the first realization of a microwave trap for neutral atoms, as proposed originally for use with spin-polarized hydrogen [1]. This trap is advantageous for attaining Bose condensation because it can contain strong or weak field seekers, while static magnetic traps contain only the less-stable weak-field seeking states. In this experiment, Cs atoms were confined in the microwave field of a room-temperature spherical resonator (Q~ 5500) whose TE 110 mode is tuned to the blue of the ground state hyperfine splitting (9.2 GHz). Using up to 83 W of microwave power, the trap is shallow (< 100#K), large (~. 1 cm) and cannot, by itself, hold Cs atoms up against gravity. We levitate atoms in the trapping state (F = 4, mF = 4 ground state) with a static magnetic field gradient. The trap is loaded with atoms cooled to 5 ttK in optical molasses. To detect the atoms, we either release them from the trap and detect their fluorescence as they fall through a probe laser beam, or we observe them with a ccd camera as we illuminate them with laser pulses. Plans for applying this technique to spin-polarized hydrogen will be discussed in a separate presentation [2].
Optical atom traps based on the electric dipole interaction were realized in 1986 [3]. In analogy to this, a proposal was put forth for a microwave trap based on the magnetic dipole interaction [1]. Here we report on the first demonstration of such a trap, using Cs atoms. Consider first a two-level atom with the levels coupled by a magnetic dipole transition. A near-resonant microwave field couples and shifts the "bare" atomic states. The direction of the shift for any coupled eigenstate depends on the sign of the detuning of the microwaves from the atomic transition, and its amplitude depends on the detuning and field intensity. In a field gradient, the shift in energy with position provides a trapping force. Since there is no radiative decay, either state can be trapped depending on the detuning of the field. For static magnetic traps, strong-field seeking states, like the ground state, cannot be trapped, since static field maxima are prohibited in free space. The transition we use is between the F = 3, *Current address University of Amsterdam, Valckenierstraat 65, NL-1018 XE Amsterdam, The Netherlands. tCurrent address Institut d'Optique, Centre Scientifique d'Orsay, B.P. 147 91403 France. Elsevier Science B.V. S S D I 0921-4526(93)E1027-J
m F ----- 3 and F = 4, m F = 4 ground state hyperfine levels of Cs (9.2 GHz). A static magnetic field (~ 70 G) is used to split the magnetic sublevels. The microwave field is provided by the T E 110 mode of a spherical resonator ( Q ~ 5500). At attainable input power levels (< 83 W) and a typical (blue) cavity detuning of 72 MHz, we get a trapping potential for the F = 4 atoms with a depth of 100 /~K at the center and a width of about 1 cm. This trap cannot hold Cs atoms up against gravity (1.6 m K / e m ) , and so we also apply a static levitating field gradient of about 23.4 G / c m along the vertical axis. The atoms are first cooled in optical molasses [4] to about 5 pK at the geometric center of the cavity. (There are holes drilled in the spherical resonator for laser, atomic beam, and observational access.) The molasses light is then turned off, and the atoms optically pumped into the F = 4, m F = 4 trapping state before they have a chance to fall. The static (tuning and levitating) and microwave (trapping) fields are then turned on. Due to the combination of these fields and gravity, the trap center is approximately 3 m m below the molasses. The atoms fall through and then oscillate about the trap center with an
894
Figure 1. Atomic motion in the microwave trap, as seen through a I cm access hole in the side. Frames are separated by 67 ms; move across from left to right starting at the top. Here the input power was 42 W. The increase in brightness between frames 12 and 13 results from a rescaling of the intensities. amplitude dependent on the initial distance from the center and a frequency dependent on the trap depth. The trapping lifetime, typically 1 s, is dependent on the trap depth and vacuum quality. With only a levitating field and no microwaves present, no oscillations were seen and the atoms quickly dispersed.
E ~108 E .> 1 0 4 =-
I
I
I
I
~.~ 6
0.0
I
I
I
I
0.5
1.0
1.5
2.0
2.5
time in trap (s) Figure 2. (top) The arrival time of the atoms at the probe vs. the time spent in the trap. (bottom) The width of the distribution vs. time. The input power was 83 W, the largest power used.
After spending a given time in the trap (Fig. 1), the atoms are illuminated with a 5 ms pulse of laser light. The fluorescence of the atoms is detected on a ccd camera. The illumination heats and therefore destroys the sample, so that each frame represents a separate trap loading. In a second data-taking method, (Fig. 2), atoms are dropped out of the trap by turning off the fields. The falling a t o m s are then detected as they fall through and fluoresce in a probe laser beam located 5.2 cm below the center of the trap. Both the width and the arrival time of the distribution of atoms can be measured in this fashion. The arrival time is observed to oscillate as a function of time spent in the trap, and the width oscillates at roughly twice this frequency. This latter oscillation, expected for an ensemble of a t o m s moving in a harmonic potential, can also be seen as a "breathing" motion in Fig. 1. This work was supported in part by the Office of Naval Research and the D e p a r t m e n t of Energy, grant DE-FG02-85ER45190.
REFERENCES 1. 2. 3. 4.
C.C.Agosta, I.F.Silvera, H.T.C.Stoof and B.J.Verhaar, Phys. Rev. Lett. 62,2361 (1991). I.F. Silvera, et al, this volume. S. Chu, J.E. Bjorkholm, A. Ashkin, and A. Cable, Phys. Rev. Lett. 57, 314 (1986). S. Chu et al., Phys. Rev. Lett. 55, 48 (1985).