Preliminary Evaluation of a Miniature Laser-cooled Cesium Fountain Frequency Standard

The Time and Frequency Division of the National Institute of Standards and Technology (NIST) is developing a miniature laser-cooled Cs-fountain frequency standard. We anticipate that this device will be useful as a transportable reference for comparison of frequency standards at other laboratories. Additionally, it could be useful for measuring the gravitational clock shift at various locations to study models of the geoid. We discuss the objectives of this device, the features of the physics package and preliminary results. INTRODUCTION The miniaturization of a Cs fountain frequency standard is appealing for several reasons. A transportable device similar in concept to the PHARAO (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite) clock [1] allows for the comparison of standards and calibrations at other laboratories and remote sites. Also, such a device could be used to measure the gravitational clock shift at various locations. The gravitational clock shift is a dominant correction applied to primary frequency standards and relies on models of the geoid. The ability to study the models by directly measuring the gravitational shift at various locations is appealing. While a small fountain cannot achieve the sub-Hz linewidths and ultimate accuracy of a primary standard such as NISTF1[2], the compact geometry lends itself to achieving high stability. Considering the t dependence on toss height, a small atomic fountain using a 15 cm toss height still yields relatively long Ramsey times of 0.4 s. The shorter Ramsey time in conjunction with large apertures (1.4 cm) in the microwave cavities allows for a higher flux of Cs atoms through the clock as compared to NIST-F1. Assuming that ≈10 Cs atoms are collected in 300 ms with a temperature of 2 μK, and a 0.3 s Ramsey time, we expect this device to achieve a stability of y ( ) 2 × 10 , provided there is a suitable local oscillator to support it. PHYSICS PACKAGE The physics package for the miniature fountain has been described in detail elsewhere [3], and is shown without the C-field and magnetic shields in Fig. 1. It consists of a cold Cs source region (MOT and optical molasses), state-selection cavity, detection region, and Ramsey microwave cavity. The apparatus fits into a 30 cm×30 cm×1 m package suitable for transportation. Presently, the laser system is on a separate optical table and light is delivered via optical fibers to the physics package. Figure 1. The miniature fountain physics package with the magnetic shields removed. The cold Cs source uses the (0,0,1) beam geometry and operates first as a MOT and then is switched into an optical molasses. Approximately 10 atoms are launched upwards by detuning the two vertical beams. The state-selection cavity is a rectangular cavity operating in the TE104 mode. The apertures have a relatively large diameter of 1.50 cm, thus allowing for a large flux of atoms. Directly above the state-selection cavity is the detection region, which is designed to measure both F=4 and F=3 atom signals in order to generate a normalized transition-probability signal. The Ramsey microwave cavity and toss-tube assembly is constructed from highpurity oxygen-free copper. The cavity is cylindrical, operates in the TE011 mode, and has an unloaded Q of approximately 18,000. As with the state-selection cavity, the apertures in the Ramsey cavity have a large diameter of 1.4 cm. The microwave cavity in NIST-F1 have a diameter of 1 cm and are designed to minimize the distributed cavity phase shift. Here to achieve high stability at the expense of accuracy, the apertures have been made larger. Since the atom flux through the cavity scales with the area of the apertures, a modest increase in the diameter of the apertures has a significant impact on the flux. Microwave radiation is introduced into the cavity with two loop antennas located opposite from each other in the mid-plane of the cavity. The C-field bobbin has a winding pitch that produces a field of 500 nT/mA. The magnetic shield package consists of two layers: one around the Ramsey cavity and toss tube, a second covering the first and extending down to shield the detection region. Figure 2 shows the physics package with the inner (not visible) and outer shields installed. For the preliminary results presented here, only the innermost shield was installed on the system. The electronics and computer which control the miniature fountain easily fit into a standard laboratory rack enclosure. Additionally, we are in the process of designing a compact laser system which can be easily transported along with the physics package and control electronics. Figure 2. The assembled miniature fountain physics package. Ramsey Fringes From Miniature Fountain Frequency 9 192 631 770 (Hz) -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 D et ec te d F =4 , m F =0 A to m S ig n al ( V s ) 0 2 4 6 8 10 12 14 16 18 Figure 3. Ramsey fringes obtained with the miniature fountain. PRELIMINARY RESULTS Presently, the detection system is configured to detect only atoms in the F=4 state. Therefore, the data presented here are un-normalized and contain noise due to shot-to-shot variations in the atom number. Figure 3 is a scan of the detected number of atoms in F=4 versus the frequency of the microwave radiation in the Ramsey cavity, and shows Ramsey fringes and the underlying Rabi pedestal. The atom toss height above the center of the Ramsey cavity was 13.2 cm (40 cm above the source), resulting in a Ramsey time of 0.33 s and a Rabi time of 1.2 ́10 s, which is in good agreement with the measured fringe data. Each point in the scan is the average of two individual measurements. Figure 4 is a narrow scan about the central Ramsey fringes. Here each point is the average of 8 individual measurements, and the solid line is a fit of the data to a sine wave. The central fringe is offset by 2 Hz due to the relatively large C-field ( 7 mT) applied during the measurements. Central Ramsey Fringes From Miniature Fountain Frequency 9 192 631 770 (Hz) -4 -2 0 2 4 D et ec te d F =4 , m F =0 A to m S ig n al ( V s ) 0 2 4 6 8 10 12 14 16 Figure 4. A narrow scan of the central Ramsey fringes. The offsetis due to the relatively large C-field. The solid line is a fit to a sinewave. ACKNOWLEDGMENTSThe authors thank John Vig of the U.S. Army Communications Electronics Command for providing the initial inspirationfor and funding of this work.REFERENCES[1]Abgrall, M., Lemonde, P., Laurent, Ph., Santerelli, G., Petit, P., Clairon, A., The Transportable PHARAO Fountain Clock,Proc. of the 14 European Frequency and Time Forum 2000,pg. 52, Torino, Italy, March 14-16, 2000. [2]Jefferts, S.R., Shirley, J., Parker, T.E., Heavner, T.P., Meekhof, D.M., Nelson, C., Levi, F., Costanzo, G., DeMarchi, A.,Drullinger, R., Hollberg, L., Lee, W.D., Walls, F.L., Accuracy Evaluation of NIST-F1, to appear in Metrologia, 39(5), 2002. [3]Heavner, T., Meekhof, D., Kujundzic, D., Jefferts, S., Development of a Miniature Laser-Cooled Cs Fountain FrequencyStandard at NIST, 2000 IEEE/EIA Int. Freq. Control Symp., pp. 711-713, Kansas City, MO, June 7-9, 2000.