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Ideas and Stumbling Blocks in Quantum Electronics

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Ideas and Stumbling Blocks in Quantum Electronics 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 that the 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 effects rather small,and presumably for 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 for good 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 who understood 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 were some 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
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