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We come now to consider what proved to be the culmination of efforts to enhance the spectral resolution of atomic resonance machines: the hydrogen maser, one of the most stable of all present-day atomic frequency standards. Few other microwave quantum devices exceed its overall mid-term frequency stability. Conceptually, the H-maser was a natural outgrowth of the continuing experimental drive to improve the spectral resolution of atomic beam resonance machines by increasing the interaction time between the atoms and the resonant field. Since this long predates the development of techniques for cooling atoms with laser radiation, this was to be achieved by confining the atoms interacting with the field within a space defined by inert walls; however, few would have predicted the degree of inertness exhibited by one fluorocarbon polymer named Teflon and the extraordinary length of perturbation-free interaction time it made possible.

The development of atomic standards based on quantum resonance in neutral atoms confined by diffusion through a buffer gas, or collisions with inert walls, culminated in the hydrogen maser, a standard of astonishingly high stability. However, as explained in the last chapter, for certain applications the hydrogen maser suffers from two deficiencies: First, its lack of portability due to its size and the need for elaborate magnetic shielding, and second, a wall shift that limits its absolute accuracy, and disqualifies it as a primary standard.

The exploitation of ions in confined isolation for the development of a new kind of portable atomic frequency standard was first proposed by Major in 1969; it was seen as an approach that promised extraordinary accuracy in a light, compact device suitable for aerospace applications (Major, 1969). Since then, with the advent of the laser, unimagined new technical frontiers opened up extending the field into the regime of optical frequency standards. The original concept of using resonant optical fluorescence to observe a microwave resonance in ions confined in a Paul trap, an approach of severely limited signal-to-noise ratio using the then available conventional lamps, quickly became an eminently fruitful approach when suitable lasers became available much later.

The first international conference at which papers were presented on the subject of “optical masers,” as they were then called, was at Ann Arbor, Michigan in June 1959. The principal topic of the conference was not lasers, but the optical pumping method of observing magnetic resonance in free atoms, a technique that had recently been introduced by Kastler in Paris. The session devoted to lasers was a “miscellaneous session” in which papers on theoretical aspects of laser oscillation in gas discharges and ruby crystals were presented by Gould, Javan, and Schawlow, among others.

We will now consider specific lasers that typify each of several important classes. Historically, the now ubiquitous helium-neon laser occupies a special place after the ruby laser, as the first gas laser to implement the emerging ideas on how to extend the concept of a maser oscillator to optical frequencies, ideas associated mainly with the names of Townes, Schawlow, and Basov.

As generators of intense, spectrally pure radiation at optical frequencies, lasers serve not only as stabilized sources for frequency standards in that region of the spectrum, but equally important, they have changed the entire embodiment of the microwave cesium standard. They provide a means of slowing down the thermal motion of atomic particles. This is critically important when the resonance frequency of a transition in an atom or ion is used as a reference, since it is essential that the Doppler frequency shifts due to the particle motion be eliminated as far as possible. This can be accomplished by what is now called laser cooling , a truly remarkable technique, which we take up in this chapter. It is a technique that has made it possible to reach particle velocities corresponding to temperatures only a small fraction of a degree above absolute zero, where all thermal motion ceases.

The advent of the laser changed the whole character of atomic frequency/time standards in a number of fundamental respects: from the manipulation of the internal quantum states of the reference particles, to cooling of the center-of-mass motion and prolonged observation of single isolated reference particles. The former application introduced a radical change in the way the cesium clock transition is observed, and the latter made possible, in one revolutionary advance, the realization of the ideal goal that motivated the original pre-laser forays into field suspension of ions, embodied in the Hg^+ ion microwave resonance experiment at NASA. That goal was to completely isolate the reference particle from its environment, and to observe its true resonance, free from Doppler shifts and any uncontrolled random perturbations. In this chapter we will touch on the application of lasers to atomic frequency standards in the microwave region of the spectrum, and take up their role as frequency standards in the optical region in later chapters.

The early development of lasers was marked not only by the explosive proliferation of laser oscillation on different atomic and molecular transitions, but also by efforts to stabilize them and narrow their spectral line width. This was driven by the realization that the very attribute that makes the laser so remarkable is the one that still left room for spectacular improvement: spectral purity. The fundamental quantum limit on spectral purity far exceeds that of any common laser subject to fluctuations in its optical cavity. As we saw in the example given in Chapter 14, the theoretical spectral line width of a 1mW laser with a 1m long cavity is on the order of 3 × 10^−4 Hz, or a fractional line width of 5 × 10^−19!

Among the areas of application made practicable by the advent of atomic clocks we list the following: Space Science: Long-distance tracking and data acquisition from “deep” space probes such as Voyager. Radio Astronomy: Very long baseline interferometry (VLBI), made possible through a common phase reference. Planetary Motion: The dynamics of the Earth as a planet and the variability of the length of the day. Radio Navigation: Perhaps the most useful application. Land-based networks Loran-C and Omega, and the satellite-based systems the TRANSIT system, culminating in the NAVSTAR Global Positioning System (GPS) and GLONASS.

Finally in this chapter we will describe contributions of atomic clocks that go beyond precision time-keeping and its applications, important though these are. Indeed it would be difficult to exaggerate the importance of the impact that such exquisitely stable time standards have already had on civilian and military technology and culture. However, the degree of precision, unimaginable only a few years ago, that has been achieved in the determination of quantum transition frequencies in a variety of atoms and ions, has made it possible to subject to ever greater scrutiny fundamental physical theory, theory that delves deep into the ultimate structure of the universe and the forces that shape its behavior, from the cosmic scale down to the subatomic scale. Tests for extremely subtle violations of symmetry properties that are at the foundation of unified field theories of our universe are now possible using earth-bound and spacecraft experiments based on atomic clocks.