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From the earliest times in the course of human development, a recurring theme has been the inexorable passage of time, bringing with it ever-changing aspects of Nature and the cycle of life and death. Only in the realm of mythology do immortal gods live outside of time in their eternal incorruptible abodes.

A universal property of material objects is their ability to vibrate, whether the vibration results in an audible sound, as in the ringing of a bell, or is subtle and inaudible, as the motion in a quartz crystal. It can be a microscopic oscillation on an atomic scale, or as large as an earthquake. Oscillations in any part of an extended object or medium with undefined boundaries almost always propagate as waves.

As already noted, an oscillatory system will, in the absence of a driving force to maintain the oscillation, eventually come to rest. In order to keep a constant level of oscillation, it is necessary to inject energy into the system, an action most effi- ciently performed by a periodic force at a resonant frequency. It is not necessary that the external source of energy itself be periodic, since the oscillating system can be made to draw energy automatically at its own frequency; it is then called a self-sustained oscillator . In essence, this is accomplished by driving the oscillator from a power source from which the transfer of energy to the oscillator is regulated by the oscillator itself. This amounts to using a power amplifier to drive the oscillator with an amplified version of its own oscillation. To sustain the oscillation, the amplified power must be fed back to the oscillating system in the proper phase to reinforce the oscillation already present. This is called positive feedback and is generally associated with a rapid buildup of energy, which in every practical situation, however, is always limited by the onset of some degradation of the conditions that led to the buildup, or some imposed limit. Self-sustaining motion (not perpetual motion) occurs in many kinds of systems. For example, in a very broad sense a steam engine is a self-sustaining rotator, in the sense that valves controlling the flow of steam into the cylinders to drive the pistons are in fact acting like power amplifiers, whose output, the force on the pistons, in addition to driving the train, also actuates the valves, providing positive feedback.

The use of the stable vibrations of a quartz crystal to control clocks and watches has become so common in recent years that in this age of digital sophistication, we tend to take for granted the revolutionary advance these quartz-controlled time pieces represent. It is true that through the incomparable skill and ingenuity of Swiss watchmakers, the precision achieved in the fabrication and hence performance of mechanical watches has reached truly admirable heights; however, the microelectronic revolution of the 1960s has made it possible to miniaturize the far superior crystal-controlled clock into a wrist watch of greater constancy at a fraction of the cost.

When we speak of oscillations at optical frequencies and their amplification, we are indeed a long way from the world of swinging pendulums and oscillating balance wheels. It is true that classical theory based on Newton's laws of motion and Maxwell's theory of electromagnetic radiation are inappropriate to deal with the interaction of radiation with atoms and molecules; for this we need the quantum theory . However, from a background of classical theory, certain aspects can be sketched in a semiclassical way, in which quantum ideas are superimposed on a classical base. Historically, this characterized the early development of the theory of radiation and the general features of the theory of optical dispersion . In this context “dispersion” refers to the dependence of the refractivity of a medium on the wavelength, which leads to the dispersion of, for example, white light by a glass prism into the colors of the rainbow.

In the evolution of clocks through the ages, there has been a progression from the use of periodic systems on a large scale with relatively slow movement to increasingly smaller, delicately operated devices running at very much higher frequencies. The large, elaborately built water clocks of China and the pendulum clocks of a later age were by the very nature of their mechanical design vastly more susceptible to environmental sources of error than the balance wheel and ultimately the quartz-controlled clock. The next step in this progression is no less revolutionary than the one from a pendulum to the quartz oscillator; it is clocks based on atomic resonators.

All the atomic standards we shall be dealing with are based, in one form or another, on the resonant excitation of atoms or ions, by which they make transitions from one quantum state to another. From the observed resonance spectrum we must arrive at the intrinsic, or proper, frequency of the atoms' response, as it would be observed if they were at rest and free from any outside perturbation. Such perturbations will alter and broaden the resonance spectrum and put a limit on the degree of precision with which the intrinsic atomic frequency can be deduced.

Of the atomic clocks, or more appropriately, frequency/time standards, since their accuracy and sophistication, not to mention their cost, places them far above any ordinary keepers of time, the rubidium clock has the distinction of being the most compact, and therefore the most portable. Rugged versions of the rubidium standard have long been developed for shipboard use as well as for tactical military and missile-borne applications.

We will now take up the type of atomic clock that has been elevated to the status of the primary standard of time, displacing the historical role of astronomical observations in the definition of the unit of time, the second. In 1967 the 13th General Conference on Weights and Measures, attended by delegates from about 40 countries, signatories of the Treaty of the Meter, adopted a new definition of the international unit of time. At that conference there was overwhelming support to the idea that the time had come to replace the existing definition, based on the earth's orbital motion around the sun, by an atomic definition. The wording of the new definition is as follows: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of cesium-133.” The ten-digit number assigned in the definition was chosen to agree with the then existing definition of the second, known as the “ephemeris second,” which had been adopted in 1956. This latter definition was based on the length of the so-called tropical year, that is, the length of time for the earth to complete its orbit around the sun and return to a point where its axis again makes the same angle with respect to the earth-sun direction; it is the repetition period of the seasons. The obvious drawback to this definition is the practical one of not being available except through the intermediary of stable clocks that must be checked after the fact.

The idea of a device using quantum transitions induced in molecules by a radiation field to achieve m icrowave a mplification by s timulated e mission of r adiation, now familiarly known by the acronym maser , was first described by Gordon, Zeiger, and Townes, of Columbia University (Gordon et al ., 1954) and independently proposed by Basov and Prokhorov, of the Lebedev Institute for Physics, in 1954. Townes, Basov, and Prokhorov shared the 1964 Nobel Prize in physics, “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle.”