David J. Glaze

Measurement of the Unperturbed Hydrogen Hyperfine Transition Frequency

The results of a joint experiment aimed primarily at the determination of the frequency of the H1 hyperfine transition (F = 1, mF = 0) ? (F = 0, mF = 0) is reported. In terms of the frequency of the Cs133 hyperfine transition (F = 4, mF = 0) ?(F = 3, mF = 0), defined as 9192 631 770 Hz, for the unperturbed hydrogen transition frequency the value ?H = 1420 405 751.768 Hz is obtained. This result is the mean of two independent evaluations against the same cesium reference, which differ by 2 × 10-3 Hz. We estimate the one-sigma uncertainty of the value ?H also to be 2 × 10-3 Hz. One evaluation is based on wall-shift experiments at Harvard University; the other is a result of a new wall-shift measurement using many storage bulbs of different sizes at the National Bureau of Standards. The experimental procedures and the applied corrections are described. Results for the wall shift and for the frequency of hydrogen are compared with previously published values, and error limits of the experiments are discussed.

Results on limitations in primary cesium standard operation

We report on the most recent design changes in our two primary cesium standards, their current operational use, results obtained, and limitations. NBS-4, the shorter device with an interaction length of L = 0.52 m, has been extensively used for many months as a clock. After improvements in the magnetic shielding and microwave feed, we have obtained σ<inf>y</inf> (1 week < τ < 2 weeks) = 7 × 10⁻¹⁵ in a 10-Hz bandwidth for its frequency stability. NBS-6, the longer, more accurate device (L = 3.75 m), features a linewidth (<tex>

The new NIST optically pumped cesium frequency standard

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Progress Toward Optically Pumped Cesium Beam Frequency Standard

</tex> Hz), which is believed to be the narrowest linewidth ever reported for a cesium device. NBS-6 has been operated to give a short-term stability σ<inf>y</inf> (1 s) = 7 × 10⁻¹³ in a 10-Hz bandwidth and has capability of easy beam reversal. The current and past rates of the International Atomic Time (TAI) in terms of our primary cesium standards are reported and compared with the results of other laboratories. With NBS-6 we have calibrated the rate of the NBS time scale of an uncertainty of 0.9 × 10⁻¹³.

Error analysis of the NIST optically pumped primary frequency standard

The development, design, and preliminary operation of the optically pumped cesium beam frequency standard are described. The design follows from an analysis of systematic errors found in cesium beam standards. Systematic effects, the atomic beam tube, the laser systems, and the control electronics are discussed. >

Time Domain Velocity Selection Modulation as a Tool to Evaluate Cesium Beam Tubes

The National Bureau of Standards is planning to build a cesium-beam, primary frequency standard based on the application of optical pumping for state selection and Tiom detection. The goal is an accuracy of 10 . Theoretical studies have been able to identify only Rabi pulling as a mechanism for Majoranatransition-induced frequency shifts. Together with considerations of magnetic field uniformity, this has led us to adopt a longitudinal C-field. In turn, this has required a hybrid magnetic shield design with an active component to cancel ambient fields. Elimination of state-selecting magnets together with polarization control of the optical pumping should eliminate effects of Majorana transitions. Optical pumping should also permit simultaneous operation of counter-propagating beams with closer trajectory retrace than is possible with magnetic state selection. Real-time measurements of end-toend cavity phase shift and even servo control are anticipated. Requirements on distributed cavity phase shift have led to consideration of a “race track“ shaped cavity termination in place of the more conventional shorted waveguide. Noise measurements have shown that simple monolithic diode lasers produce too much FM noise to allow one to reach the shot noise limit in atom detection. Techniques for control of diode noise and linewidth are being tried and compared.

NIST-7, the new US primary frequency standard

The major sources of systematic error in the NIST optically pumped primary frequency standard has been evaluated with an uncertainty of a few parts in 10/sup 14/. The cavity end-to-end phase shift is the only item which differed markedly from expectations based on the design and/or measurements made during assembly. The authors think the difference is attributable to the physical dimensions of the cavity and not to its asymmetry. The device will begin to function as the US primary frequency standard, even before planned extensions to the analysis presented and improvement to the laser and servo electronics allow evaluation at its full design accuracy of one part in 10/sup 14/. >

Accuracy Evaluation and Stability of the NBS Primary Frequency Standards

Pulsed excitation of atomic and molecular beam devices with separated Ramsey-type interaction regions allows the observation of signals due to very narrow atomic velocity groups. The theoretical background of this method is discussed. Experimental operation of a near mono-velocity cesium beam tube is demonstrated. The velocity distribution of a commercial cesium beam tube and of the pr imary laboratory standard NBS-5 a r e obtained using the pulse method. The normal Ramsey pat terns are calculated from the velocity distribution and compared with the measured Ramsey pat terns . The pulse method al lows the direct determination of the cavity phase shift and of the second-order Doppler correction in beam devices. Velocity distributicns obtained via the pulse method allow the use of microwave power shift results for accuracy evaluat ions. These aspects as wel l as the effects of modulation and different velocity distribut ions are discussed in detai l . The pulse method thus shows promise for the evaluation of existing laborato ry as wel l as commerc ia l ces ium beam tubes wi th respect to these effects.

The Realization of the Second

NIST-7, an optically pumped, cesium-beam frequency standard, has replaced NBS-6 as the official US primary frequency standard. The present short-term stability of the standard, measured with respect to an active hydrogen maser, is characterized by /spl sigma///sub t/(/spl tau/) /spl ap/88 /spl times/ 10/sup -13/ /spl tau//sup -1/2//. A first evaluation has resulted in an uncertainty of 4 /spl times/ 10/sup -14/. An improved servo-electronic system is being developed which should improve stability and allow more precise evaluation of the various systematic errors.<<ETX>>

An Optically Pumped Primary Frequency Standard

The National Bureau of Standards has two primary standards for frequency and the unit of time. They are both cesium devices and are designated NBS-4 and NBS-5. The design of NBS-5 is discussed in detail, including its relationship to its predecessor NBS-III, and a brief description of NBS-4 is given. NBS-4 and NBS-5 have been used since January 1973 for a total of twelve calibrations of the NBS Atomic Time Scale. The application of pulsed microwave excitation, and the use in the accuracy evaluations of frequency shifts due to known changes in the exciting microwave power are discussed. Measurements of the atomic velocity distributions are reported. A stability of 9 × 10-15 derived from the comparison of NBS-4 and NBS-5 is reported for averaging times of 20 000 s, and data on accuracy are given. Results obtained to date give an evaluated accuracy of 1-2 × 10-13 with indications that this accuracy may be improved in the future. The bias-corrected frequencies of NBS-4 and NBS-5 agree to within (1 ± 10) × 10-13 with the value obtained for NBS-III in 1969?which value is preserved in the rate of the NBS Atomic Time Scale.