IEEE TRANSACTIONS ON INSTRUTMENTATION AND MEASUREMENT, VOL. IM-21, NO. 2, MAY 1972

History of Atomic and Molecular Standards ofFrequency an Time

NORMAN F. RAMSEY

Abstract-The history of atomic and molecular frequency controlwill be traced from the earliest molecular beam magnetic resonanceexperiments in 1938 through the present.

INTRODUCTION

I N DISCUSSING the history of atomic and molec-ular frequency control, two alternative approachesare available. One is to treat all devices in parallel

on a year-by-year basis. The other is to discuss eachalternative device in succession. It seems clear that thelatter approach is the best in the present case so weshall adopt it but we shall make frequent cross refer-ences to other devices. In following this procedure, it isclear that the first method discussed should be themolecular and atomic beam magnetic resonance method,not only because historically it was the first importantmethod but also because it stimulated the invention ofthe other methods and still remains one of the mosteffective techniques of frequency control. Since theatomic beam devices and the hydrogen masers at presentprovide the most precise frequency and time standards,the greatest attention will be paid to them, but the his-tory of the other atomic and molecular frequency con-trol devices will be given as well.

EARLY HISTORY OF THE MOLECULAR BEAMRESONANCE M\ETHOD

The molecular beam magnetic resonance method arosefrom a succession of ideas, the earliest of which can betraced back to 1927, although it was rather remote fromthe idea of resonance. In 1927 the physicist Sir CharlesDarwin [1]-the grandson of the great evolutionist-discussed theoretically the nonadiabatic transitions thatmake it possible for an atom's angular momentum com-ponents along the direction of a magnetic field to beintegral multiples of h both before and after the directionof the field is changed an arbitrary amount. Inspired byDarwin's theoretical discussion, Phipps and Stern [2] in1931 performed the first experiments on paramagneticatoms passing through weak magnetic fields whose direc-tions varied rapidly in space. Guttinger [3] and Majorana[4] developed further the theory of such experiments.Frisch and Segre [5] continued atomic beam experimentswith adiabatic and nonadiabatic transitions of para-magnetic atoms and found in agreement with Guttinger's

Manuscript received June 19, 1971; revised November 15, 1971.This paper was presented at the 25th Annual Symposium onFrequency Control, Atlantic City, N.J., April 26-28, 1971.The author is with Harvard University, Cambridge, Mass.

and Majorana's theories, that transitions took place wlhenthe rate of chanige of the direction of the fieldl was largerthan or compairable to the Larmor frequet iCY

CIO0 = yijHo, (1)which is the classical frequency of precession of a clas-sical magnetized top with the same ratio yj of magneticmoment to angular momentum. Transitions did not takeplace when the rate of change of the direction of H wassmall compared to the Larmor frequency. However,some of the results of Frisch and Segre were not con-sistent with theoretical expectations. Rabi [6] pointedout that these discrepancies arose from the effects of thenuclear magnetic moments since some of the transitionswere performed in such weak fields that strong or inter-mediate coupling between the nuclei and the electronsprevailed. The transitions in such circumstances werequite different from those for which the effects of thenuclear spins could be neglected. Rabi showed that theresults of Frisch and Segre were consistent with expecta-tions if the effects of the nuclei were included. Rabi alsopointed out that such nonadiabatic transitions could beused to identify the states and hence to determine thesigns of the nuclear magnetic moments. Motz and Rose[7], Rabi [8], and Schwinger [9] in 1937 calculated thetransition probability for molecules that passed througha region in which the direction of the field varied rap-idly.

In all of the above experiments, however, the direc-tion of the field varied in space and the only timevariation arose as the atoms in the atomic beam passedthrough the region. Since the atoms possessed a Maxwel-lian velocity distribution, the atomic velocities variedand the apparent frequencies of the changing field weredifferent for different velocities. Furthermore, the chaingein field direction ordinarily went through only a portionof a full cycle. For both of these reasons no sharpresonance effects could be expected. No suggestion wasmade to use an oscillatory magnetic field, i.e., a fieldthat varied in time instead of space so that the ap-parent frequency would be the same to all the atoms,independent of their velocities. It is surprising that thispossibility was not immediately recognized after Rabi'sbrilliant theoretical paper [8] in 1937. To simplify thetheoretical analysis, Rabi assumed in 1937 that the fieldwas actually oscillatory in time. As a consequence theresults are all applicable to the resonance case with oscil-latory magnetic fields even though the possibility ofactually using fields oscillatory in time was not then

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RAMSEY: HISTORY OF ATOMIC AND MOLECULAR STANDARDS

recognized. Consequently this paper, without alteration,still provides the fundanmental theory for molecular beammagnetic resonance experiments with oscillatory fields,even though the oscillatory field method was invented byRabi [10], [11] only a year or so after the fundamentaltheoretical paper was written. Additional amusing char-acteristics of this brilliant theoretical paper by Rabi[8] are the absence of a verb in the opening sentenceand the repeated appearance of v2 + v2 in the equationswhen the intended expression was v2 + r2.

Gorter [12] in 1936 had suggested that nuclear transi-tions in solids be induced by an oscillatory field froma radio-frequency oscillator. He proposed to detect thetransitions by the absorption of the radio-frequencyradiation and by the rise in temperature of solids sub-ject to such oscillatory fields. Although Purcell et at.

[13] atnd Bloch et at. [14] in 1946 successfully detectedthe absorption of such transitions by the reaction of theradiation on the radio-frequency circuits, Gorter's ex-periments [121 were unsuiecessful in 1936.

Following a visit by Gorter to Columbia Universityin September 1937 in which he described his unsuccess-

ful experiments, Rabi [10], [11] proposed the use ofan oscillatior-driven magnetic field as the transition-in-ducing field in a molecular beam resonance experiment.Two successful molecular beamm devices using this methodwere soon constructed by Rabi [10], [11], Zacharias[10], [11], Rusch [10], Kellogg [11] and Ramsey [11]. Aschematic view of these [10] is shown in Fig. 1. In theseexperiments the atoms and inolecules were deflected bya first inhomogeneous magnetic field and refocused by a

second one. If a resonance transition were induced in theregion between the two inhomogeneous fields, its occur-

rence could easily be recognized by the reduction ofintensity associated witl the accompanying failuire of

refocusing. For transitions induced by the radio-fre-quency field, the apparent frequency was almost thesame for all molecules independent of molecular velocity.As a result, sharp resonances were obtained whenever(1) was satisfied.Rabi et at. [11] soon extended the method to the

molecule H2, for which the resonance frequencies de-pended not only on (1) but also on internal interactionswithin the molecule. The transitions in this case occurredwhenever the oscillatory field was at a Bohr frequencyfor an allowed transition

hp = E,- E2. (2)

For the first time they began speaking of their resultsas "raldio-frequency spectroscopy."

MOLECULAR BEAM MAGNETIC RESONANCE EXPERIMENTS

By 1939 the new molecular beam magnetic resonance

method had demonstrated its usefulness sufficiently wellthat it appeared to Rabi, Kellogg, Ramsey, and Zachariasto be of possible value for the definition of standard mag-

91

netic fields and for use as a time and frequency stan-idard.In 1939 they discussed these possibilities with a scielntistat the Bulreau of Standards-whose name is fortunatelyno longer reinembered-and found little initerest in theuse of subtle molecular beam technique for such prac-tical purposes as standards of magnetic field, time, orfrequency.

In mnost respects the molecular beam technique in 1939was more suitable as a standard of m-lagnetic field thanof frequeney or time since the observed resonances atthat tinme were largely dependent on the externally ap-plied magnetic field. From the point of view of fre-quency control, it was consequently a great step forwardwhen in 1940 Kusch, AMillman, and Rabi [15] [161 firstextended the method to paramagnetic atoms and in l)ar-ticular to AF = +1 transitions of atoms where the rela-tive orientation of the nuclear and electronic magneticmoments were clhanged, in which case the resonance fre-quencies were determined dominantly by fixed internalproperties of the atom rather than by interactions withan externally applied inagnetic field. The first resonancemeasurernents of the Cs hyperfine separation, whi(h hasbeen so extensivelv used in frequency control, were re-ported [16] in 1940.

In 1941 the research with the atomic beam magneticresonance method was mostly interrupted by World WarII and did not resume before 1946. In 1949 Kusch andTaub [17] pointe(d ouit the possibilityv of observing thehyperfine resonances at magnetic fields such that theresonance frequiency was an extremum, in which case thefrequency to first or(ler was independent of the strengthof the magnetic field.

In 1949 Ramsey [18], [19] invented the separatedoscillatory field method for molecular beam resonancefor which tl-he oscillatory field, instead of being dis-tribtuted uniformly throughout the tranisition region, wasconcentrated in two coherently driven oscillatory fieldsin slhort regions at the beginning anti enid of the traisitiorregion. The tlheoretical shape of a resonance curve withthis apparatus is shown in Fig. 2. Ramsey pointed outthat this mnethod has the following advantages: 1) theresonances are 40 percent narrower than even the mostfavorable Rabi resonances with the samne length of ap-paratus; 2) the resonances are not broadened by fieldinhomogeneities; 3) the length of the transition regioncan be muncl longer than the wavelength of the radia-tion, provided that the two oscillatory regions are short,whereas there are difficulties with the Rabi method dueto phase shifts when the length of the oscillatory regionis comparable to the wavelength; 4) the first-order Dop-pler shift cani mnostly be eliminated-l whlen sufficientlyshort oscillatory field regions are used; 5) the sensi-tivitv of resoniance measurements can be increased bythe deliberate use of appropriate relativre phase shiftsbetween the two oscillatory fields. All of these character-istics are of great value for the use of atomic beamresonance devices used as precisioni frequiency and time

IEEB TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, MAY 1972

1~~~LJldHidz

I 1 1 F rI A nmagnet 11 C I I B magnet I

Fig. 1. Schematic diagram [10] showing the principle of the first molecular beam resonance apparatus. The two solidcurves indicate two paths of molecules having different orientations that are not changed during passage through theapparatus. The two dashed curves in the region of the B magnet indicate two paths of molecules whose orientation hasbeen changed in the C region so the refocusing is lost due to the change in the component of the magnetic moment alongthe direction of the magnetic field.

2bl/cx =0*2007r.

0X8

0X7

05

nA0,

0-3

0-z-

0-1

I/tLl 0bS

(vto-v)L/&(For near resonance)-1 0 1

-1~ -05 0 0-5 1-0(v%-v)C/a(For off resonance)

Fig. 2. Theoretical shape for separated oscillatory field resonancepattern [18].

standards. An early molecular beam apparatus [20]using this method is shown in Fig. 3.

ATOMIC BEAM FREQUENCY STANDARDS

With the above developments, it was apparent to mostmolecular beam researehers by 1949 that atomic beammethods could be highly effective for precision frequencycontrol. However, this was less clear to others who thenbelieved that the advances in crystal frequency controltechniques had been so great that atomic devices couldnot be enough better to justify the extra cost and effort.However, in 1952, Sherwood, Lyons, McCracken, andKusch [21], [22] reported briefly on atomic beam reso-

nance research supported by the National Bureau ofStandards and directed primarily toward the develop-ment of an atomic beam clock. A schematic diagram ofa proposed atomic beam clock at that time is given inFig. 4. The financial support for such work soon dwindleddue to advances in microwave spectroscopy and the viewthen held by those supporting the work that a molecularclock based on the microwave absorption by ammonia

at its inversion frequency would be simpler and morepromising.A few years later Zacharias [23], [24] stimulated re-

newed interest in an atomic beam cesium clock. Hisinitial concern was for an entirely new type of cesiumbeam in which ultrahigh precision would be obtainedby the use of extremely slow molecules moving upwardsin a vertical apparatus at such low velocities that theywould fall back down by the action of gravity. Althoughthis fountain experiment eventually failed due to theunexpected great deficiency of the required ultraslowmolecules emerging from the source, it stimulated Zach-arias to develop and to urge others to develop well-engineered atomic beam frequency standards usingnormal atomic velocities. The fountain experiment ofZacharias illustrates the value to science of even someunsuccessful experiments since the existence of this un-successful effort directly and indirectly stimulated threequite different developments: 1) the use of conventionalbut well-engineered atomic beams for frequency control;2) the development by Kleppner et al. [44] of thestored atom technique, which eventually lead to the hy-drogen maser; and 3) high precision resonance experi-ments with ultraslow neutrons [43]. The first report onan atomic beam frequency standard was that of Zach-arias at the 1955 9th Frequency Control Symposium. (Thefirst report of any kind on atomic or molecular researchwas a paper at the 1950 4th Frequency Control Sym-posium by Dicke [26] on the reduction of Doppler widthsin microwave absorption.) Zacharias claimed a shorttime stability of 1 part in 109 for his atomic cesium fre-quency standard.

In 1955 Essen and Parry [25] of the British NationalPhysical Laboratory successfully operated the first prac-tical laboratory atomic cesium beam apparatus that wasextensively used as an actual frequency standard. Theirconstruction and effective use of this device provided amajor impetus to the subsequent development of atomicbeam cesium frequency standards.

In 1956 the first commercial model of an atomic beamfrequency standard appeared on the market. This was

.

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RAMSEY: HISTORY OF ATOMIC AND MOLECULAR STANDARDS

Fig. 3. Apparatus for which the separated oscillatory field wasfirst proposed [18].

Fig. 4. Schmatic diagram of a proposed atomic beam clock(see [221).

National's "Atomichron" developed [27] by Hollowayand Orenberg in collaboration with Zacharias and furtherstudied by McCoubrey and Daley. This device usedRamsey's separated oscillatory field method for increasedprecision, a special design of cesium oven that could beoperated several years without exhaustion, titaniumpumping to permit permanent sealing off of the evacuatedbeam tube, and many other features generally necessary

for an effective commercial device. The first com-mercial Atomichron is shown in Fig. 5. The develop-ment of the Atomichron was largely supported fi-nancially by the U. S. Signal Corps at Ft. Monmouth,N. J., though some support came from the Air Force.A purchase order by the Signal Corps for a relativelylarge number of Atomichrons made possible the develop-ment of mass production techniques and improved en-

gineering to permit sufficient reliability and reductionsin price to assure commercial success.

The early atomic beam frequency standards were sub-ject to various frequency shifts dependent on the ampli-tude of the radio-frequency power used and on othervariables. To account for these results, Ramsey investi-gated with the aid of a computer analysis supported bythe National Company, the various possible distortionsthat would occur in an atomic beam resonance [28]. Theelimination of radio-frequency phase shifts and other

Fig. 5. First commercial atomic beam frequency standard, Na,tional's Atomichron [27].

sources of distortion made possible the marked increasesin accuracy that have been obtained with the atomicbeam frequency standards.From 1956 on, the atomic beam frequency standards

developed rapidly. Mockler, Beeler, and Barnes [26],[27] developed an atomic cesium frequency standard atthe National Bureau of Standards in Boulder, Colo.Other commercial organizations such as TRG, Bomac,Varian, and Hewlett-Packard became involved. Manylaboratories outside both England and the U. S. eitherconstructed or purchased atomic beam frequency stand-ards including those in Canada, France, Germany, andthe laboratories of Kartaschoff [26] and Bonanomi [26]in Switzerland. Other materials than cesium were testedfor molecular beam clocks; although thallium and vari-ous polyatomic molecules have some significant advan-tages, they have not yet significantly displaced cesium,which is particularly convenient. Reder, Winkler, andothers [26] at Ft. Monmouth and Markowitz at theNaval Observatory sponsored various world-wide studiesof the comparison of atomic clock frequencies and thesynchronization of clocks. Extensive studies were made ofother atoms such as thallium for use in the atomic beamtubes, and various molecular resonances were studiedfor possible use in a molecular beam electric resonanceapparatus for frequency control purposes. A T1205 fre-quency measurement accurate to 2 parts in 1011 was re-ported by Bonanomi [29]. However, atomic cesium re-mains the most widely used substance in molecular oratomic beam frequency control devices. Particularly ef-fective atomic beam cesium clocks were developed andsold by Hewlett-Packard, which also developed a "fly-ing clock" particularly suitable for the intercomparisonof atomic clocks in different laboratories. A modern beam

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9EER TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, MAY 1972

Fig. 6. Beam tube for an atomic cesium standard manufacturedby Varian Associates for Hewlett-Packard [27].

Fig. 7. A typical microwave absorption experiment using aradio-frequency bridge and heterodyne detection.

tube for an atomic cesium frequency standard is shownin Fig. 63. Accuracies as high as 5 parts in 1013 have beenclaimed in the past for some laboratory cesium stan-dards [26].

In 1967, the 13th General Conference of Weights andMeasures resolved that the unit of time in the Interna-tional System of Units should be the second defined as

follows: "The second is the duration of 9 192 631 770periods of the radiation corresponding to the transitionbetween the two hyperfine levels of the ground state ofthe cesium atom 133."

MICROwAVE ABSORPTION SPECTROSCOPYMicrowave absorption spectroscopy had an early start

in the experiments of Cleeton and Williams [30], [31]in 1934. They observed the absorption of microwaveradiation at the NH3 inversion frequency. However, re-

search on microwave absorption was inhibited at thattime by the lack of suitable microwave oscillators andcircuits so there was no further development of micro-wave absorption spectroscopy until after the develop-ment of microwave oscillators and waveguides for radarcomponents in World War II. Immediately followingWorld War II there was a great burst of activity inmicrowave absorption spectroscopy. Although there were

no publications on experimental microwave spectroscopyin 1945, in the single year of 1946 there were a numberof important publications from many different labora-tories including reports by the following authors [32]:Bleaney, Penrose, Beringer, Townes, Dicke, Strandberg,Dailey, Kyhl, Van Vleck, Wilson, Dakin, Good, Coles,

Fig. 8. National Bureau of Standards ammonia clock [22].

Hershberger, Lamont, Watson, Roberts, Beers, Hill, Mer-ritt, and Walter, and in 1947 there were over 60 publishedpapers on this subject including a number of publicationsby Gordy and Jen as well as those with reports the pre-vious year and by others. A typical microwave absorp-tion experiment at this time is shown schematically inFig. 7.Microwave absorption techniques were quickly recog-

nized to be of potential value for frequency standards.In 1948 a group of workers [22] at the National Bureauof Standards built an ammonia clock that was com-pleted in 1949 and is shown in Fig. 8 and it eventuallyachieved an accuracy of 1 part in 108. Rossel [22] inSwitzerland and Shimoda in Japan devised an improvedammonia absorption clock good to a few parts in 109.The first report pertaining to atomic and molecular fre-

quency standards was that of Dicke [26], [33] at the1951 5th Frequency Control Symposium. In the 7th, 8th,and 9th Symposia he, Carver, Arditi, and others describedthe continuation of this work at both Princeton and RCAwith the financial support of the Signal Corps [26], [33].The microwave absorption studies soon merged with theoptical pumping techniques described in the next section,since the intensities of the resonances were greatly en-hanced by the use of optical pumping.

OPTICAL PUMPINGThe starting point of all research on optical pumping

was a paper by Bitter [34] in 1949, which showed thepossibility of studying nuclear properties in opticallyexcited states. Kastler [35], [36] showed the followingyear that this technique could be effectively combinedwith the double resonance method he and Brossel [35]had developed. Both optical pumping and optical detee-

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RAMSEY: HISTORY OF ATOMIC AND MOLECULAR STANDARDS

Fig. 9. Rubidium frequency standard.

tion techniques serve the purpose of increasing the sig-nal-to-noise ratio of the resonator output signal: theoptical pumping greatly enhances the population of cer-tain states so the signal is not weakened by stimulatedemission nearly cancelling absorption, and the opticaldetection increases the signal-to-noise ratio because ofthe lower noise level of optical detectors over microwavedetectors.The combination of optical pumping techniques with

the buffer gas method for reducing Doppler shift de-veloped by Dicke [27], [33] provided gas cells of realvalue as frequency-control devices. Although many dif-ferent atoms have been used in such gas cells, Rb87 soonbecame the favorite in most such devices. Extensive workin optically pumped gas cells for frequency control hasbeen done at Princeton, RCA, ITT, Space TechnologyLaboratory, the National Bureau of Standards, ClauserTechnology Corporation, Varian Associates, and manyother commercial, university, and government organiza-tions in the U. S. and abroad. Fig. 9 shows a typicaloptically pumped rubidium frequency standard.The optically pumped gas cells have the advantages

of simplicity, relatively low cost, large signal-to-noiseratio, and good spectral purity. Unfortunately the rela-tively large shift in frequency due to numerous buffergas collisions is dependent on purity, pressure, and tem-perature. Changes in the light intensity shift due to vari-ations in the pumping lamp intensity or spectrum mayalso be a problem. As a result, the stability of rubidiumgas cells over a period of several months is ordinarilyno better than a few parts in 1010. These pressure shiftsprevent the optically pumped gas cells from being pri-mary frequency standards but the gas cells are used asfrequency control devices when too much accuracy is notrequired. Research is currently in progress in a numberof laboratories to improve the stability of optically

pumped gas cells; Bouchiat, Brossel [37], and others,for example, have eliminated the buffer gases and as inthe hydrogen maser have used collisions with suitablycoated walls to retain the atoms and reduce the effect ofthe first-order Doppler shift.

MOLECULAR MASERS

In 1951 Pound et al. [38] studied nuclear spin sys-tems with inverted populations and noted that such sys-tems in principle were intrinsic amplifiers rather thanabsorbers. Actually, the first suggestions to use systemswith inverted populations as practical amplifiers andoscillators were made at closely the same time in 1953-1955 and independently by Townes [39], Weber [40],and Basov and Prokhorov [41]. The first such amplifierwas successfully constructed in 1955 by Gordon et al.[39] and called a maser (microwave amplifier by stimu-lated emission of radiation). The device used inhomo-geneous electric fields to focus the higher energy molecularinversion states of ammonia molecules in a molecularbeam. These molecules then emitted coherent stimulatedradiation in passing through a cavity tuned to the24-GHz ammonia inversion transition. A schematicdiagram of the first ammonia maser is shown in Fig. 10.A report by Gordon on the new ammonia maser was amajor attraction at the special meeting on atomic andmolecular resonances sponsored by the Signal Corps En-gineering Laboratory in 1956. In that year Bloembergen[42] proposed the three-level solid-state maser and in1958 Townes and Schowlow [43] pointed out the pos-sibility of masers at the infrared and optical frequen-cies.

Since the announcement of the first successful am-monia maser in 1955 there has been tremendous researchand development activity by scientists and engineers inmany countries. Masers at infrared or optical frequen-

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, MAY 1972

OUTPUT INPUTGUIDE GUIDE

NH3 ,XA LaIst InISOURCE

CAVITYFOCUSER

END VIEW FOCUSEROF SOURCE CROSS SECTION

Fig. 10. Schematic diagram of original ammonia maser [39].

cies (often called lasers) have great potential for fre-quency control even though they have not been used forthis purpose as yet. Further discussion of lasers will bedeferred to a later section of this report. Molecular maser

developments for the purposes of frequency control soon

became intense and went in many directions includingthe search for more suitable mnolecules than ammonia,the development of two cavity muasers analogous to theseparated oscillatory field method [18] for molecularbeams), use of ammonia of different isotopic composi-tion, etc. A value of the Nl'H3 frequency accurate to 5parts in 1011 has been obtained by de Prins and con-

firmed by Barnes [29]. However, after a few years ofintense molecular maser activity, the interest in suchmasers for frequency control waned since the molecularmasers on the one hand lacked the simplicity and lowcost of optically pumped rubidium gas cells, and on theother hand lacked the high precision of either atomiccesium beams or atomic hydrogen masers.

ATOMIC MASERS

In 1957 Ramsey [26] proposed to increase the accu-

racy of the atomic beam magnetic resonance mnethod byretaining the atoms for a mnuch longer time between thetwo separated oscillatory fields, thereby obtaining muchnarrower resonances. His first thought was to confinethe atoms with inhomogeneous magnetic fields in a largering such as the circular tunnel of the 6-GeV Cambridgeelectron accelerator. However, it soon became apparentthat the inhomogeneous confining magnetic fields, whichacted on the atoms for long periods of time, would hope-lessly broaden the resonances. In fact, it became clearthat the frequencies would be much less perturbed by a

confinement force that was present for only a short frac-tion of the time even though the force might be strongerwhen it was applied. The obvious limit of such a devicewas confinement of atoms in a box with suitably coatedwalls. Although many wall bounces would be requiredto achieve marked narrowing of the resonance by longstorage time, the first experiments involved only a fewwall collisions, since most scientists at that time believedthat even atoms in an S state would undergo hyperfinetransitions at even a single wall collision. The first ex-

periments of Kleppner et al. [42] involved only a fewwall collisions and the experiment was appropriatelycalled a "broken atomic beaim resonance experimnent."Cesium atoms and a Teflon coated wall were used inthese first experiments.

Goldenberg et al. [46] then made an atomic beamresonance apparatus that stored atoms of cesium for alonger time and they investigated alternate wall-coat-ing materials. They found that when the storage bulbwas coated with a paraffin-like substance called Para-flint [46], resonances could be observed after as manyas 200 wall collisions. It was recognized that atomichydrogen would probably be a nmore suitable atom thancesiumn because of the low electric polarizability andthe low mass of hydrogen, but cesium could be muchnore efficiently detected than hydrogeni.Kleppner and Ramsey [44], [46], [47] therefore pro-

posed detection of the emitted radiation rather than ofthe atom. In particular, they noted that atoms of hydro-gen in the higher energy hyperfine state could be focusedinto a suitably coated storage bulb by a six-pole inagnetwhile atoms in the lower state would be defocused. Theyshowed that if such a storage bulb were surrounded by amicrowave cavity tuned to the 1420-NIHz hyperfinetransition frequency, then maser oscillation should oc-cur. In 1960, Goldenberg et al. [47] constructed andoperated the first atomic hydrogen maser. This apparatusis shown in Fig. 11. Although the total microwave powerwas small-approximately 10- 12 W the stability was sohigh that the output was concentrated into an extremelynarrow band with a consequently favorable signal-to-noise ratio.Although the first hydrogen masers used wall coat-

ings of Paraflint (a variety of paraffin) or of Dri-Film(dimethyldichloros,ilane [46]), it was soon found that withatomic hydrogen, in contrast to cesium, Teflon-coatedwalls gave longer storage times and smnaller frequencyshifts from wall collisions [48]. Bender [49] soon pointedout that spin exchange collisions of hydrogen atoms couldnot be neglected and might produce a significant fre-quency shift, but Crampton [50] nioted that the normaltuning technique would cancel out such an effect.A commuercial hydrogen maser [51] was developed

by Vessot, Peters, Vanier, MeCoubrey, Levine, and Cut-ler. The work was started at Bomac and successivelytransferred to Varian Associates andl Hewlett-Packardl.It is currently being carried on at the Simithsonian Astro-physical Observatory by Vessot and his associates, atthe Goddard Space Flight Center by Peters and others,and at the Jet Propulsion Laboratory. The H-10 maserdeveloped by Vessot and his associates is shown in Figs.12 and 13. The masers are being built chiefly for longbase-line interferometry in radio astronomy, which bene-fits greatly from the high stability of the hydrogenmaser. Research and developmnent on hydrogen masersis carried out in the laboratories of Kartaschoff [261 inSwitzerland, Audoin [61] and Grivet [26] in France, and

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Fig. 11. Original hydrogen maser [47]. The large coils are tocancel external magnetic fields. In later hydrogen masers, thesewere replaced by two or three concentric cylinders of highpermeability magnetic shielding.

in a number of other countries.l Audoin and his asso-ciates [61] introduced a useful double focusing tech-nique that eliminates undesired molecules from thefocused beam.The chief disadvantage of the hydrogen maser for

time and frequency control has been the existence of asmall frequency shift due to collisions of the atoms withthe Teflon-coated walls of the storage bulb. With a 16-cm diameter bulb this wall shift is about two parts in1011 and can be measured by using bulbs of two differentdiameters. However, until recently the measurementsof the wall shifts have been limited to accuracies of afew percent by variations in different wall coatings.However, Uzgiris and Ramsey [52] at Harvard havereduced the wall shift by a factor of 10 by the use of anatom storage vessel ten times larger in diameter (1.5 m).In the same laboratory, Brenner [53] and Debely [54]have recently developed a technique to measure the wallshift in a single storage bulb: they were able to change thebulb's volume by deforming its shape. Since a single bulbis used in this method, it is free from the uncertaintiesin the nonreproducibility of the wall coatings of differ-ent bulbs. Although this method has so far been usableonly on hydrogen masers with normal size storage bulbs,it should also be applicable to the large storage bulbs.Zitzewitz [55] has also shown that at a temperature ofabout 80°C the wall shift passes through zero; it is thuspossible to operate the hydrogen maser at a tempera-

1 Laboratories that have engaged in hydrogen maser studiesinclude Harvard University, Bomac Laboratories, Varian Asso-ciates, Hewlett-Packard, the National Bureau of Standards,Massachusetts Institute of Technology, the Naval Research Lab-oratory, Goddard Space Flight Center, the Jet Propulsion Labora-tory, the U. S. Army Electronics Command, PTB (Braunschweig,Germany), the National Research Council (Canada), R.R.L.(Tokyo, Japan), LSRH (Neuchatel, Switzerland), and theLebedev Institute (Moscow, USSR).

UPPER SYSTEM 10' mm Hg

LOWER SYSTEM 10-5 mm Hg ORIFICE

FOUR ELEMVENT -tHEXAPOLEVACION PUMP MAGNET

HYDROGEN f SOURCESUPPLY COLLIMATOR

R.F. H, DISSOCIATINGDISCHARGE

Fig. 12. Schematic diagram of commercial hydrogen maser de-veloped by Vessot and his associates [51].

Fig. 13. Commercial hydrogen maser [51].

ture such that the wall shift vanishes and to select thistemperature by the deformable bulb technique. Withthese new methods, it is hoped that an absolute accuracyconsiderably better than 1 part in 1013 can be attained.Although the hydrogen maser is the most stable atomic

maser for long periods of time, Novick, Vanier, and

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, MAY 1972

others [26] hlave developed a high-poNi- er opticallypulmped atomic Rb85 nmaser wlhose relatively high out-put power is useful for short term stability.

OPTICAL MASERS

Townes and Schawlow [43] pointed out that maserscould be produced at infrared and optical frequencies.The first optical maser or laser was successfully madefrom ruby by Maiman [56]. Subsequently there was agreat burst of activity in this field and lasers were madeof a wide variety of materials and at high pulsed power.From the point of view of frequency control the laserusing a helium-neon gas mixture developed by Javan[57] and his associates is of particular interest becauseof its potentially greater stability. As absolute timestandards, most lasers suffer from the most sensitivedetermination of the output frequency being the distancebetween two mirrors while in the atomic hydrogen maser,the primary determination of frequency is the hydrogenatom itself with only a relatively small amounit ofpulling from mistuning of the microwave cavity. How-ever, various methods for determining the laser fre-quency more by the atomic properties have been de-veloped and further improvements are being attempted.An accuracy of 1 part in 1011 has been achieved for a3.39-/L laser stabilized by locking to a saturated absorp-tion signal in methane vapor [58]. Likewise Javan [57]and others are making vigorous efforts by frequencymultiplication to bridge the gap between microwaveradiation and the optical laser frequency. When this isdone the lasers may provide a valuable alternate in fre-quency control. Since this has not been accomplished sofar, lasers have not yet been used for frequency controland they are consequently discussed only briefly in thisreview of frequency control devices.

TRAPPED IONS

Dehmelt [59] in 1959 first used electromagnetic iontraps in radio-frequency resonance studies. By the un-certainty principle, the resonances could be very narrowsince the ions can be retained in the apparatus for verylong times. However, so far the resonances have beensignificantly broadened by the second-order Doppler shiftarising from the ion velocities in the ion trap. Effortshave been made to diminish the broadening by Dehmeltet al. [60], but such trapped-ion devices have not yetprovided as stable frequencies as the best alternative fre-quency standards.

FUTURE PROSPECTS

Although atomic and molecular frequency control hasbeen a reality for a number of years, new developmentsare still occuring at a relatively rapid rate. As a resultit is impossible to forecast reliably the future develop-ments that will lead to the most maj or subsequent ad-vances. For highest stability and reproducibility themost promising prospects now appear to be 1) furtherimprovements in the atomic and molecular beam meth-

ods, 2) hydrogen maser improvements by combinationsof the new deformable bulb technique with either thelarge box maser or operation at a temperature wherethe wall shift vanishes, 3) improved stored ion devices,and 4) use of lasers for frequency control. However,there also may be new ideas and developments that dras-tically improve one of the existing techniques or leadto totally new methods of atomic or molecular frequencycontrol.

This paper provided the basis for an invited talk atthe 25th Annual Symposium on Frequency Control andsince it is the first effort ever made to present in thefollowing References section a coherent historical accountof this subject, omission and errors are inevitable. Theauthor will welcome any letters either correcting or add-ing to this record.

REFERENCES

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Light and Buffer-Gas Frequency Shifts in the

15Rb-Maser Frequency Standard

WILLIAM A. STERN, STUDENT MEMBER, IEEE, AND ROBERT NOVICK, FELLOW, IEEE

Abstract-Recently we reported the operation of an opticallypumped 85Rb maser oscillating at 3.035734 GHz on the field-independent transition 52S112(F = 3, mF = 0 --* F = 2, mF = 0) [1].Apart from cavity pulling, the two major sources of frequency shiftsare those due to the buffer gas and the pumping light. The short-term phase stability of the standard is not appreciably affectedby these shifts. The buffer-gas shift can be used to advantage to

Manuscript received August 5, 1971. This work was supportedin part by the National Aeronautics and Space Administrationunder Grant NGL33-008-009 and in part by the National ScienceFoundation under Grant GP-13479.W. A. Stem is with the General Time Research Center,

Stamford, Conn. 06902.R. Novick is with the Columbia Astrophysics Laboratory,

New York, N.Y. 10027.

select an output frequency that is different from the ground-statehyperfine frequency.

I. THE BUFFER-GAS SHIFT7 HEN AN 55Rb atom collides with a foreign gas

molecule, there is an instantaneous interactionbetween the dipole moments of the two species.

This interaction can be explained in terms of the at-tractive van der Waals forces and the exchange forces[2]. These interactions can be considered to cause eitheran expansion or contraction of the electronic-chargeclouds of the interacting species. This change in elec-tronic-charge-cloud density causes a corresponding in-

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