Helmut Hellwig

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.

The Quantum Beat: The Physical Principles of Atomic Clocks

Celestial and Mechanical Clocks.- Oscillations and Fourier Analysis.- Oscillators.- Quartz Clocks.- The Language of Electrons, Atoms, and Quanta.- Magnetic Resonance.- Corrections to Observed Atomic Resonance.- The Rubidium Clock.- The Classical Cesium Standard.- Atomic and Molecular Oscillators.- The Hydrogen Maser.- The Confinement of Ions.- The NASA Mercury Ion Experiment.- Optical Frequency Oscillators: Lasers.- Laser Cooling of Atoms and Ions.- Application of Lasers to Microwave Standards.- Measurement of Optical Frequency.- Applications: Time-Based Navigation.- Concluding Thoughts.

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>

Time and Frequency

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

</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.

Frequency Stability of Methane‐Stabilized He–Ne Lasers

In 1967 the General Conference on Weights and Measures adopted the cesium resonance frequency for the definition of the second. Universal Coordinated Time (UTC) has used a close approximation to the atomic second since 1972 (1). Time scales which refer to the rotation of the earth such as UTC are generated by inserting or leaving out seconds (leap seconds) at certain specified dates during the year, as necessary. This process is coordinated worldwide by the Bureau International de l’Heure (BIH). UTC is the de-facto basis for civil or legal time in most countries of the world (2). In addition to cesium beam standards, the atomic hydrogen maser has found use as primary frequency reference and clock.

Time, frequency and physical measurement

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.

The importance of measurement in technology-based competition

Free‐running laser stabilities of 1.5×10−11 for the millisecond region and methane‐locked stabilities of 10−13 for 10‐sec averaging time are achieved with a minimum of shock and vibration isolation in an ordinary laboratory environment. Superior stability performance is obtained with dc excitation as compared to rf excitation. The experimental setup is described in some detail.

A Small, Passively Operated Hydrogen Maser

How many basic standards do we need? Standards are necessary to measurement, and for reasons of accuracy and convenience many measurements involve frequency. With atomic and molecular transitions serving as references, measurement precisions near 10−16 are possible. The duration of the second is determined by a resonance in the cesium atom, and international atomic time is the reference for all time and frequency measurements in the world. Furthermore, frequency measurements lead, via the speed of light, to the measurement of wavelengths and, via transducers, to the measurement of many other physical quantities such as temperature and pressure. As a result, time and frequency metrology is at the root of any thinking to revise or improve our system of basic standards of measurement.

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

Challenges in measurement are related to the changes taking place in emerging technologies, quality assessment, and production. Strategies in product genesis and quality are discussed together with the impact of emerging technologies. The differences between traditional and modern approaches in measurement and instrumentation are compared. >