IEEE JOURNAL OF QUANTUMELECTRONICS, VOL. QE-20, NO. 6,JUNE 1984 547 Ideas and Stumbling Blocks in Quantum Electronics

CHARLES H. TOWNES, FELLOW, IEEE

Abstract-Quantum electronics, including in particular the maser and sion had been well understood by many individuals is impres- the laser, represents a marriage between quantum physics and electrical sively demonstrated by anumber of earlyrecords-from engineering which was probably longer delayed than it might have been Richard Tolman’s publication in 1924 discussing amplification because the two were not sufficiently acquainted. The mutualdiscovery of one field by the other is discussed, as well as the misunderstandings by an inversion of population, to serious consideration of an andfalse starts. Specific examples are used to makemore real the experiment to demonstrate stimulated radio emission by John thinking of the earlyyears in this field and the struggles with ideas Trischka at Columbia University several years before invention which, as withmost now-understood sciences or technologies,seem of the maser. In all, I know of eight apparently independent much simpler in retrospect. discussions priorto the maser inventionof how stimulated emission andnonequilibrium populations canincrease the T is sometimes said that there is no single component idea intensity of a wave. There may be others. Iinvolved in the construction of masers or lasers which had Why were the many discussions of stimulated emission not not been known for atleast 20 years before the adventof these followed up to produce some actual demonstration or a useful devices. Of course,a discovery which might have occurred device? In some cases such effectsappeared to be only of earlier is not uncommon in science or engineering. Neverthe- theoretical interest, providing a neat and consistent explanation less, the case of quantum electronics is striking enough that it for characteristics of radiationand of absorption. Further- may be useful to review the development of ideas prior to the more,the simple weakening ofabsorption with increasing time this new field became visible, and the stumbling blocks population in an upper state(e.g., for hv > kT) always seemed which may have delayed its creation. I believe whatever un- to me an adequate demonstration of stimulated emission. The necessary delay occurred was in part because quantum elec- few experiments on stimulated emission attempted in the op- tronics lies betweentwo fields,physics and electrical engi- tical region made demonstration of any large effects seem dif- neering.In spite of the closeness of these two fields, the ficult,and were notfollowed up by other experimenters. necessary quantum mechanical ideas were generally not known Probably the failure to couple the idea of feedback with weak orappreciated byelectrical engineers, while physicists who stimulated emission helped make such effects seem inevitably understood well theneeded aspects of quantum mechanics small. In the case of Trischka, and in my own thinking before were often not adequately acquainted with pertinent ideas of a feedback oscillator was envisaged, a demonstration of ampli- electrical engineering. Furthermore, physicists were somewhat fication in the microwave region due to inversion seemed rather diverted by an emphasis in the world of physics on the photon difficult and not important enough to be worth the required properties of light rather than its coherent aspects. It is still time. In the case of inversion of nuclear spins, which was ob- surprising thatthe basic combinationsof ideas required for tained some years before maser action,the low frequencies quantumelectronics were notmore completely envisaged and nuclearmagnetic dipole moments involved made most somewhat earlier thanthey were. Nevertheless, it is under- stimulated emission effectsrather small,and presumablyfor standable that the real growth of this field came shortly after that reason successful inversion of populations did not direct theburst of activity in radio and microwave spectroscopy attention toward useful amplification. immediately after World War I1 since this brought many phys- In my own case, it was primarily a strong desire to obtain icists intothe borderland area betweenquantum mechanics oscillators at shorter wavelengths than those otherwiseavailable and electrical engineering. that induced me to initiate experimental work on the maser. I know that most of my electrical engineering friends, while This is why the first system designed on paper was for l/2 mm well acquaintedwith absorption of radiation by atomsand wavelengths,in the far infrared. My students and I had pre- molecules, were surprised to learn that excitedmolecules could viously tried many techniques-magnetron harmonics, coher- give up energy coherently to an electromagnetic wave, With ent Cerenkov radiation, and others to obtain shortwavelengths, that knowledge in mind,they might at least have imagined and while most of them worked after a fashion, none gave the utilizing such effects for amplification even if they were not promise which masersdid forgood spectroscopic sources at expert with the specific arrangements required. Many physi- short wavelength. Initially, I did not fully realize the maser’s cists knew of stimulated emission, but few connected it with potential as a lownoise amplifier or as a precision clock, useful amplification.That amplification by stimulated emis- though these twoapplications added considerable interest rathersoon after work on a maser was startedwith James Gordon and HerbertZeiger. Manuscript received February 24, 1984. The author is with the Department oflhysics, University ofCalifornia, As indicated above, my belief is that for many of the physi- Berkeley, CA 94720. cists whounderstood stimulated emission,isolating suchef-

0018-9197/84/0600-0547$01.00 0 1984 IEEE 548 JOURNALIEEE OF QUANTUMELECTRONICS, VOL. QE-20, NO. 6, JUNE 1984 fects seemed somewhatdifficult, and the necessary experi- faulty.The experiment was onlydifficult, not erroneously ments were not very important because to these same physicists planned,and Forrester’spublished result later dispelled any stimulated emission was already rather well understood. The doubts. idea of feedback and large numbers of quanta in single modes Perhaps a somewhat more subtle example of physicists’ bent which might have suggested practicalapplications and given at the time toward thinking in terms of individual particles in- additional value tosuch experiments had not occurred. In volved the frequency spread of a maser oscillator. From the addition, there weresome misunderstandings and confusions point of view of stimulated emission produced by an oscillating which played a role in delaying quantum electronics. About field established in a resonant cavity, it is not hard to under- 1945 I had myself written an internal memorandum at theBell stand that the radiation produced by a maser oscillator could Telephone Labs explaining that molecules and atoms could be indeed have a very narrow frequency band, independent of the used to generate short microwaves, but that intensities could width of response of individual excited molecules.Any real only be low because they would be limited by the second law width has to be due to either the small amount of additional of thermodynamics-use of a nonequilibrium distribution and spontaneous emission or to thermal radiation present, as first population inversion had not yet occurred tome. worked out by James Gordon. However, there was the uncer- Emphasis onthe photon aspect of light deflected some tainty principle relating time and energy, abasic law for physi- physicists fromcoherent amplification. It turned out that cists. With the lifetime t of moleculesin the cavity limited beforethe maser was operational,John Von Neumann had (for the beam-type maser) by the time of transit, how could suggested exciting electrons in a solid by neutron bombard- there be a frequency width much smaller than l/t? An electri- mentand thereby obtaining a powerful cascade ofphoton cal engineer accustomed to the almost monochromatic oscil- emission. Coherence was not mentioned, and I believe no one lationproduced by an electrontube with positive feedback ever attemptedsuch an experiment. J.H.D. Jensentold me would perhaps not have given the problem a second thought. that in the 1930’s he had thought about stimulated emission However, before oscillation was achieved I never succeeded in from an inverted population as a cascade of independent pho- convincing two of my ColumbiaUniversity colleagues, even tons, like a cosmic ray shower. He lost any great interest in after long discussion, that the frequency width could be very the idea after an experiment which seemed to produce such narrow. One insisted on betting me a bottle of Scotch that it effects turned outto be explained otherwise. In thinking about would not. After successful oscillation, I remember interesting light itself, rather than microwaves, it may be that many elec- discussions on this point with Niels Bohr and with Von Neu- trical engineers would have not been any more concerned with mann. Each immediately questioned how such a narrow fre- coherence effects than were these physicists.However, both quency could be allowed by the uncertainty principle. I was engineers and physicists were naturally led to consider coher- never sure that Bohr’s immediate acceptance of my explana- ence when dealing with the radio or microwave region, and I tion based on a collectionof molecules rather than a single one believe this is why initial ideas and development of the field was because he was convinced, or was due simply to his kind- were so dependent on those with experience in radio and mi- ness to a young scientist. My discussion with Von Neumann crowave spectroscopy. hada morespecial twist andoccurred at a socialoccasion. Some of theconfusion about coherence seemed alittle When I told him about our maser oscillator, he doubted that strange even in the early days, and will seem remarkable at this the uncertainty principleallowed ourobservation of such a time,but was real. Consider theearly experiment of A. T. narrow frequency width to be real. After disappearing in the Forrester. Shortly after World War 11, he began an experiment crowd for about 15 minutes, he came back to tell me he now to irradiate a photoelectric surface with two Zeeman compo- understood the situation; my argumentwas correct. That Von nents of an optical line in order to mix the two frequencies. Neumanntook even that long tounderstand impressed me. The idea was to have the two frequency components separated He then went on to urge that I try for stimulated emission ef- justenough to produce a varying photoelectriccurrent at a fects in the infrared region by exciting electrons in semicon- microwave frequency low enough to detect by available elec- ductors. I was puzzledby his strong insistence onthis, be- tronics. Forrester’s interest, I understood, was to demonstrate cause at that time the use of semiconductors for stimulated the effect of mixing two discrete optical lines, an effect which emissionamplifiers seemed much moredifficult thanother should have been detectable by the recently developed high- possible methods. It was only much laterI learned that he had frequency electronics. Theexperiment was not easy atthe already independently proposed such a system to produce a time, and apparently a number of physicists believed it to be powerful avalanche of photonsby stimulated emission, and conceptually wrong. There seemed to be confusion over the must have been avoiding the more usual response of saying he spatial extent of the possible coherence, and questions whether had invented theidea before he heard of our work. the independent particles of different frequencies could coop- 1 must note that, although a number ot good physicists did erate in emitting electrons from a surface and thus give a beat. not find the coherent aspects of masers straightforward, for My belief thenand now is thata brightelectrical engineer most of those in the field of radio and microwave spectros- would have figured outthat the experiment would work. copy, they were fairly obvious. That was true, for example, Nevertheless, the basic idea was challenged by a publication of my colleagues I. I. Rabi, Polycarp Kusch, and Willis Lamb, in the Physical Review in 1948 and by enough scientists that and of course Arthur Schawlow with whom I later collaborated after the experiment was under way I was asked by the spon- on the laser. Electrical engineers, while not so knowledgeable soring agency to review itand advise whetherthe ideawas aboutquantum properties, also foundcoherence properties IDEAS AND STUMBLINGTOWNES: AND IDEAS QUANTUMBLOCKS IN ELECTRONICS 549 very natural and seemed to have the right instincts about many 1960, publications onnew varieties of masers became so com- aspects ofquantum electronics from parallels in ordinary mon that, presumably under the assumption that the excitement amplifiers and circuits. As an illustration, I give an example of must be over, the Letters section ofme Physical Review made how an electrical engineer helped me at one point. public a policy not to accept any more letters onnew masers. The frequency stability of a maser oscillator was an impor- The delay of about six years between masers in the micro- tant question, and I had worked out anexpression for it which wave region and lasers, or masers at shorter wavelengths, was showed, I thought, that the cavity pulling would give an error no doubtalso due in partto some conceptual stumbling blocks. proportional to the square of the ratios of quality factorQ for One of these was imperfect recognition of the possibility of the spectralline to that of the maser cavity. On explaining obtaining ahigh Q and of emphasizinga single mode in a this to an electrical engineer at Stanford (whose name unfortu- structure which is very large compared to a wavelength, like nately I cannot recall), heremarked that such aresult was theFabry-Perot, even thoughFabry-Perot resonators were peculiar since frequency pulling of one resonance by another well known. This problem and some others made it difficult in circuits was proportional to the firstpower of this ratio. to recognize ways that masers operating at infrared or optical While denyingany detailed knowledge of the quantum me- wavelengths couldperhaps be as easy or easier, ratherthan chanical properties, he doubted my result was correct and he harder, than those in the microwave region. I shall not here was right. A little further examination when I returned home try to explore the missing conceptual links, but rather turn to showed that I had neglectedreactive terms of the quantum a differentaspect, an apparent lack of appreciation of the mechanical oscillator and cavity, which thengave just theresult potential of optical masers prior to late 1957. A number of he expected. individuals certainly recognized that maser techniques might As of today, the more engineering ideas of coherence, feed- be extended to much shorter wavelengths. I believe it was in back, and nonlinear frequency mixing have become so inter- 1956 that Bill Otting, Head of Physics for the Air Force Office mingled withthe more physicalideas of discrete statesand of Scientific Research, asked me if his office could support me quantum mechanical processes in the minds of both electrical or someone else I might suggest in work towards an infrared engineers and physical scientists that some of the above con- maser oscillator. It is difficult to remember how many other fusions will probably be hard to believe. That is why the few more casual conversions there might have been on the subject specific examples above may help remind us how things were. or to know how many scientists mayhave considered this, but In addition to conceptual stumbling blocks which affected there were apparently no substantial efforts to explore maser the course of quantum electronics, in the early days there was oscillators at wavelengths much shorter than the microwave also a limited appreciation of the potential of this field, and region before 1957. I know why I myself delayed this long-I this too may deserve illustration. Of course, I do not pretend was busy with and excited by microwave applications of the to have foreseen the field’s full potential myself, thoughI was maser and saw only rather brute-force methods of moving to obviously more impressed by it than many others. muchshorter waves beforethat time. Iwas waiting for a Consistent with my usual practice of working with graduate “neater” idea to occur.About others I have nodirect evi- students, development of the maser proceeded at the rateof a dence, but believe a lack of appreciation of the potential of normalgraduate student thesis project, being completed by lasers and a closely connected effect due to the state of de- Jim Gordon approximately three years after the idea was con- velopment of optics and optical oscillators both played a role ceived.Our laboratoriesat Columbia Universitywere com- in thetime-delay between microwave and optical oscillators. pletely open in the usualway of academic institutions, and Optical spectroscopyhad its heyday for physicists inthe many knowledgeable people visited the maser experiment. 1920’sand 1930’s; by 1940 mostphysicists considered it a However, no one seems to have thought it interesting enough mature field of solid importance but from which no remark- to reproduce or to try to compete with us. As far as I know, able breakthroughs were likely to emerge. There was,I believe, there were noconcurrent efforts to obtain amolecular or an attitude of d6ji vu about optics. After World War I1 optics atomic oscillator except the work of Basov and Prokhorov in andoptical spectroscopy did have a substantial renaissance, the Soviet Union, and in this case I am not familiar with just especially in the hands of French physicists, but was still not what was done in these earliest years. an area to which many turned for forefront physics. As I see Completion of themaser oscillator was exciting to some, but it, lasers might well have been invented during this 1925-1940 evoked no more than mild interest on the part of other of my period, although they would have been more difficult for lack friends and did not immediately generate any great flurry of of certain techniques whichwere further developed in later de- work. For some time it appears that the potential of quantum cades. These include a miscellany of things such as good opti- electronics was unappreciated by many of those not already in cal coatings, flash tubes, and improved varieties of infrared ma- the field. In part this result could have been because the first terials and detectors. maser itself may have been judged both limited andspecialized. As the possibility of high quality, single oralmost single- But also, for understandable reasons, scientistsbusy with their mode lasers came into everyone’s view, interest and intensity own research are not necessarily quick to see the potential of ofattention to this fieldincreased sharply. Nevertheless, new events in other fields. Whatever the reason, when theper- much of its now quite evident potential was initially appre- formance of masers as frequency standards and then as ampli- ciated only by enthusiasts and some not even by them. There fiers became more evident and as new varieties such as many was almost immediate interest in the optical maser proposals solid-state systems were proposed, interest in masers grew. By of Schawlow and myself, but the beauty of the device may 550 IEEE JOURNAL OF QUANTUM ELECTRONICS,VOL. QE-20, NO. 6, JUNE 1984 have been more attractive to most scientists than its potential contributed to this field, and the vigor with which industry applications. A favorite quip which many will remember was pursued and developed it. I can resist discussing these impres- “the laser is a solution looking for a problem.” While an en- sive aspects of the field only because I know others will treat thusiast myself, and aware of the potential for high precision them appropriately. measurements, monochromacity, directivity, and thehigh con- centration of energy thatoptical masers wouldprovide, I missed many potent aspects. The area of medical applications Charles H. Townes (SM758-F’62)was born in is one that did not occur to me initially as promising. In retro- Greenville,SC, on July 28, 1915. Hereceived the B.A. and B.S. degrees from Furman Univer- spect, I can imagine recognizingthe beauty of operating directly sity, Greenville, in 1935, the M.A. degree from through the pupil without other insults to the eye, butsince I Duke University, Durham, NC, in 1937, and the had never heard of a detached retina such an idea would have Ph.D.degree from the California Institute of been another“solution looking for a problem.” My own Technology,Pasadena, CA, in 1939. In addi- tion, he has received honorary doctOi-dteS from scientifice interests were primarilyin thedirection of new 21 colleges and universities. forms of spectroscopy and precision measurement, and hence From1937 to 1939 hewas anAssistant in I needed only modest power. While it was evident that optical Physics atthe CaliforniaInstitute of Tech- nology. Forthe next eight years he was a Member of theTechnical masers could be expected to produce powers of at least a num- Staff at Bell Laboratories. In 1948 he joined Columbia University, first ber of watts, I did not initially think of very short pulsed oper- as an Associate Professor of Physics from 1948 to 1950, then as a Pro- ation at a power level of many kilowatts, as was produced by fessor of Physics from 1950 to 1961 and also Executive Director of the ColumbiaRadiation Laboratory during 1950-1952 and Chairman of Maiman’s ruby laser. the Department of Physics during 1952-1955. He waswith theInstitute In looking back over why the field of quantum electronics for Defense Analyses as Vice President and Director of Research during took as long as it did in getting started and why even then the 1959-1961. From 1961 to 1967 hewaswith theMassachusettsInstitute of Technology, as Provost and Professor of Physics during 1961-1966, buildup was initially notmore rapid, I necessarily mention andInstitute Professor during 1966-1967. In 1967 he became Uni- some of the stumbling blocks, misconceptions, and fumbles. versity Professor of Physics at the University of California, Berkeley. The development of any science by humans has itssimilar mis- Dr. Townes was awarded the Nobel Prize in 1964. He was also the recipient of the Niels Bohr International Gold Medal in 1979. In 1983 takesand illogicalities. Recalling these can keep us humble he was enshrined in the Engineering and Science Hall of Fame. He was and make us aware there may be other exciting events not yet on the Board of Editors of the Revzew of Scientific Instruments from visible around the corner. However, focusing on problems of 1950 to 1952;thePhysicalReview,1951-1953;theJournalofMolecular Spectroscopy, 1957-1960; the Columbia UniversityForum,1957-1959; the past omitsor deemphasizes the remarkableinsights and the Journal on Missile Defense Research, 1963-1965 ;and theProceed- inventionsmade by a large numberof colleagues who have ings, NationalAcademy of Sciences, 1978-1984.