Stephen Jarvis

The dependence of frequency upon microwave power of wall‐coated and buffer‐gas‐filled gas cell Rb87 frequency standards

Previous studies of a commerical passive gas cell Rb87 frequency standard showed a strong dependence of the output frequency νRb upon the microwave power Pμλ. A major conclusion of that work was that the dependence of νRb upon Pμλ was due to a line inhomogeneity effect. The line inhomogeneity interpretation suggested that substituting a wall coating for the usual buffer gas would reduce the dependence upon Pμλ. As a part of the present work, a wall coating (a form of paraffin) was used and a reduction of this dependence by a factor of 100 was obtained. The present work has led to a more convincing theoretical demonstration of the line inhomogeneity effect. The paper discusses some of the details of the analytical procedure. There are certain major requirements that a wall coating would have to satisfy if it is to be superior to the usual buffer gas, and these are discussed in the text. The advantages demonstrated by the present work indicate that further studies are warranted to determine if an improved s...

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>

Evaluation and Operation of Atomic Beam Tube Frequency Standards Using Time Domain Velocity Selection Modulation

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Determination of Velocity Distributions in Molecular Beam Frequency Standards from Measured Resonance Curves

</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.

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

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 is obtained using the pulse method. The normal Ramsey pattern of this beam tube is calculated from the velocity distribution and compared with the measured Ramsey pattern. The pulse method allows the direct determination of the cavity phase shift and of the second-order Doppler correction in beam devices. The pulse method thus shows promise for the evaluation of existing laboratory as well as commercial cesium beam tubes with respect to these effects.

Two‐frequency excitation for the Ramsey separated oscillatory field method

It is shown that the Ramsey resonance curves for most atomic beam machines can be conceived as depending on two distributions of velocity, ρ(V) and ξ(V), the second being a correction for beam width. An analysis and computer program are described which permit one to obtain ρ, ξ and the nominal microwave power parameter from three or more measured Ramsey resonance curves at properly spaced power levels whose ratios are known. The determination from the functions (ρ, ξ) of bias errors due to second order Doppler shift, cavity phase difference, and cavity pulling is described. The method may also be used to improve an experimentally obtained velocity distribution (i.e., one obtained through the pulse technique); to provide the proper function ξ; and to provide diagnostic checks of the measurement technique and the validity of the model chosen for the transition probability. The method is applied to the NBS frequency standard. Error estimates indicate that it is feasible by microwave power shift measurements to evaluate the total bias error due to the above sources to within one part in 1013.

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

It is suggested that the systematic frequency shift due to rf phase difference between the two interaction regions in Ramsey’s separated oscillatory field technique may be eliminated by using different frequencies in the two interaction regions. The technique is briefly described, and the advantages are noted, particularly for frequency standards based on atomic beams.

Two-frequency separated oscillating fields technique for atomic and molecular beam spectroscopy

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.

Study of the Dependence of Frequency Upon Microwave Power of Wall-Coated and Buffer-Gas-Filled Passive Rb 87 Frequency Standards

A primary cesium beam frequency standard serves to realize the unit of time, the Second, in accordance with the international definition as formulated at the XIII General Conference of Weights and Measures in 1967. The basic design of a cesium standard is shown in Fig. 1. The cesium beam emerges from an oven into a vacuum, passes a first state selecting magnet, traverses a Ramsey type cavity where it interacts with a microwave signal derived from a slave oscillator. The microwave signal changes the distribution of states in the atomic beam which is then analyzed and detected by means of the second state selector magnet and the atom detector. The detector signal is used in a feedback loop to automatically keep the slave oscillator tuned. The line-Q is determined by the interaction time between the atoms and the microwave cavity. Thus a beam of slow atoms and a long cavity leads to a high line-Q. Commercial devices which for obvious reasons are restricted in total size have line-Q’s of a few 107, whereas high performance laboratory standards with an overall device length of up to 6-m feature line-Q’s of up to 3 × 108.