Hydrogen maser, and

N. F. Ramsey (Harvard) invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks. 

Originally it was thought that atomic hyperfine transition frequencies and atomic magnetic moments were too small for atomic hyperfine masers to be possible, but Ramsey and Kleppner pointed out that by storing atoms in a suitably coated bottle [15] coherency could be maintained for several seconds and the hyperfine resonance would be made so narrow that maser oscillations would take place. In 1961 Goldenberg, Kleppner and Ramsey [15] made an atomic hydrogen maser and found it had extremely high stability. 

Hess H F, Kochanski G P, Doyle J M, Greytak T J, and Kleppner D 1986 Spin-polarized hydrogen maser Phys.Rev. A 34 1602-4... 

Also, the laser or maser measures time very precisely. The hydrogen maser, for example, can measure times with a precision of about 10 15. It can also measure short times - as short as 10 15 seconds. So we can see atomic and molecular phenomena occurring on very short time scales. 

Maser amplifiers are used where the requirement for a very low noise amplifier outweighs the technological problems of cooling 10 low temperatures. They have been used in passive and active radiostronomical work, in satellite communications, and us preamplifiers for microwave spectrometry The ammonia and the atomic hydrogen masers have been studied as frequency standards and have heen used in accurate tests of special relativity. 

The cold collision frequency shift is important in precision frequency measurements and has been observed in hydrogen maser [31] and fountain clock [32,33] experiments. From the point of view of precision measurements, the cold collision shift is an obstacle. However, in BEC experiments, the shift provides a helpful diagnostic for the density. 

There is indirect evidence from hydrogen maser studies [221] that the reactions H + H2, HD, D2 (v = 1) show a preference for resonant exchange reactions in the case of H + H2 (v = 1) and H + HD(i> = 1) and for non-resonant exchange for H + D2 (v = 1) in accord with theoretical calculations [222]. With recent experimental developments, particularly UV lasers, it can be expected that spectroscopic methods will be applied to measuring energy disposal for these reactions. 

Hydrogen masers are used as a high-precision frequency standard its frequency of 1,420,405,752 Hz (long-term stability = 1 part in 5 x 1016 in 5 years ) also corresponds to the most intense microwave source in the universe, due to the energy difference between the (S = 1/2,7 = +1/2) and (S = 1/3, 7=1/2) states the energy separation is due to hyperfine coupling in hydrogen. 

In Ramsey and Kleppner s design of the hydrogen maser clock. 

Figure 18.1 Norman Ramsey (center), Daniel Kleppner (right), and another Ramsey graduate student, Stuart B. Crampton, standing beside a version of the hydrogen maser clock.

The microwave field of frequency 1,420 megacycles per second (Me/s) is sustained in the storage bulb by the constant entry of hydrogen atoms into the bulb from the incident beam. A tiny pickup probe is inserted into the storage bulb and an electrical current is induced in this probe at the same microwave frequency. This signal is fed into a series of electronic circuits that convert the frequency into timing pulses, or the ticks of the hydrogen maser clock. 

The hydrogen maser clock is stable to about one second in 300 million years. For many years it has been the most stable clock available. However, there is a drawback to the hydrogen maser clock its accuracy can only be maintained over a period of a few days. After a few days the stability of the clock deteriorates due to the collisions of the hydrogen atoms with the walls of the storage cavity that alter the resonance characteristic of the cavity and the clock s accuracy is compromised. 

Hydrogen maser clocks have been used in many practical applications. For example, they are used to track space probes traveling in the Sun s planetary system and to locate stars or quasars billions of light years from Earth. Ta accomplish this, two widely sepa-... 

Closer to home, hydrogen maser clocks have been used to track the motion of the Earth s tectonic plates. Erom tracking stations separated by 209 miles along the San Andreas fault, it was learned that over an eleven-week period, the distance between the two stations increased by eight inches. An earthquake occurred shortly after this determination. There is a likely correlation between this plate motion and the earthquake. The east-west dimension of the United States is more stable. Tracking stations in Massachusetts and California remained a relatively constant 154,680,381 1 inches apart over a five-year period. 

H. M. Goldenberg, D. Kleppner, and N. F. Ramsey, Atomic Hydrogen Maser, Physical Review Letters 5, 361-362 (1960). 

Trapped ion spectroscopy does enable tests of fundamental symmetries, such as of the equivalence principle, made for example by comparing the frequency of a Be "clock with the frequency of a hydrogen maser [26]. In another comparison of microwave atomic clocks, based on Cs and Mg, Godone et al. [27] were able to set new experimental limits on the time variation of various fundamental constants. 

Ramsey, N.H. Experiments with separated oscillatory fields and hydrogen masers. Rev. Mod. Phys. 1990, 62, 541-554. 

USNO is the official source of time used in the U.S. USNO timekeeping is based on an ensemble of cesium beam and hydrogen maser atomic clocks. USNO diseminates information on time offsets of the world s major time services and radionavigation systems. 

Kleppner D, Berg HC, Crapmton SB, Ramsey NF, Vessot RC, Peters HE, Vanier J. (1965) Hydrogen-maser principles and techniques. Phys. Rev. 138 A972-A983. 

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