History of Atomic Clocks

Abstract

The history of atomic and molecular standards of time and frequency is traced from the earliest work on molecular and atomic beam resonance techniques to more recent developments that promise improved standards in the future. The various devices currently used as standards are discussed in detail from an historical prospective. The latter part of the article is devoted to a discussion of prospective developments which hold promise for major improvements in accuracy, stability and reproducibility.

Early History of Magnetic Resonance

“…Although I was not asked such questions, I was fascinated by how the ideas developed. 2 The earliest reported search for a dependence of magnetic susceptibility on frequency was that of M. H. Belz in 1922 with different solutions of paramagnetic salts. 3 No frequency dependence was found.…”

“…The resonance frequencies are primarily determined by the interaction of the proton magnetic moment with the external magnetic field, but the states of different m I and m J are displaced relative to each other by the different values of the nuclear spin-spin and spin-rotational magnetic interaction energies. Source: Ramsey, Molecular Beams (ref 2,. 1956, 1985), p. 6.…”

Application of Millisecond Pulsar Timing to the Long-term Stability of Clock Ensembles

“…15 As recognized in the definition, the great majority of the timing data used to compute TAI are from cesium-based frequency standards. 16 Cesium atoms in the (3,0) state are created in an oven and isolated from cesium atoms in other states by their deflection in an inhomogeneous magnetic field. They are then allowed to travel through a magnetically shielded cavity, in which they are exposed to 9 GHz microwaves at each end.…”

Experiments with Separated Oscillatory Fields and Hydrogen Masers (Nobel Lecture)

“…(3) The earlier use of the separated oscillatory field method involved two oscillatory fields separated in space, but it was soon realized that the method with modest modifications could be generalized to a method of successive oscillatory fields with the separation being in time, say by the use of coherent pulses. ['61 (4) If more than two successive oscillatory fields are utilized it is not necessary for the success of the method that they be equally spaced in time;141 the only requirement is that the oscillating fields be coherent-as is the case if they are all derived from a single continuously running oscillator. In particular, the separation of the pulses can even be random,['61 as in the case of the large-box hydrogen maser[171 discussed later.…”