It is a commonplace for historians to write that physicists came out of their World War II radar experience with microwave engineering superadded to their knowledge of quantum physics. But what exactly was the content of this new amalgam? How fully was it achieved and by what processes? Publications from the 1950s by four American physicists, Charles Townes, Joseph Weber, Robert Dicke, and Israel Senitzky show that the amalgamation of physics and microwave engineering was far from complete, that it was stimulated by new instrumentation, and that part of the process entailed a re-thinking of the concepts of coherence and noise.
It is a commonplace for historians to write that physicists came out of their World War II radar service with microwave engineering superadded to their knowledge of quantum physics. But what exactly was the content of this new amalgam? How fully was it achieved and by what processes? I suggest that one approach to these questions is via a study of noise and coherence in the 1950s. In these years, novel instruments were proposed and/or operated that were of interest for both engineering and physics; among them various forms of the maser. These instruments required the deployment of both engineering and quantum physics concepts. At the same time, they raised problems for both physics and engineering notions of noise and coherence. The attempt to bring physics and engineering knowledge into a cohesive whole, and the attempt to forge adequate concepts of coherence and noise, thus went hand in hand. In the process, the field of quantum noise was born, while ground was laid for the 1960s emergence of quantum coherence.
In the United States, the maser arose from the felt needs of the military on the one hand, and of microwave spectroscopists on the other. During World War II, radar scientists had succeeded in generating electromagnetic waves as short as single centimeters. But by 1950, the military foresaw that millimeter waves might provide lighter weight, more compact equipment. And spectroscopists wanted to generate millimeter waves because many molecules exhibited quantum transitions at those wavelengths (Bromberg 1991, pp. 12–15). In turn, the idea occurred to a number of them of pressing those very transitions into service for generating millimeter waves or for amplifying them: that is, the idea occurred of “quantum electronics.” Thus the first maser used a quantum transition in the ammonia molecule that sent it from a higher energy to a lower energy state. The freed up energy from a large number of such molecules, whose transitions were provoked by an incoming microwave, would add to the microwave energy and thereby amplify it.
But what were the implications for noise when quantum mechanical systems formed the heart of amplifiers and generators? For, in addition to the radiation emitted when molecules were stimulated by an incoming wave, there was the phenomenon of spontaneous emission: systems jumping from higher to lower energy states even though unprovoked. Whether and to what extent this contributed noise to an amplifier, whether there was an irreducible minimum to this “quantum noise” and whether it derived from the uncertainty principle or some other quantum mechanical process were all questions that were up for grabs.
The list of scientists wrestling with these problems is long. Here I consider only four. All were American. All were involved in one way or another with the maser. They are Charles Hard Townes, whose Columbia University group built the first operating maser, Joseph Weber, who independently proposed a quantum amplifier at about the time Townes conceived his, Robert Henry Dicke who thought up a variety of schemes for using quantum systems to move into shorter wavelengths, and Israel R. Senitzky who, as a scientist and a contract monitor at the Signal Corps Engineering Laboratories in New Jersey, was in close contact with these men and dealt with their work in his publications. Although they represent only a sample of the scientists who wrote on noise or coherence in the context of the new quantum amplifiers, and although the physics and engineering that they brought to bear is only a sample of the different approaches, they are evidence enough to show that examining this part of science can give us a purchase on the ways in which engineering and physics knowledge were brought together in the post-war period.1
2. Charles H. Townes
Charles Townes (b. 1915) completed his Ph.D. at the California Institute of Technology in 1939 with a thesis on nuclear physics. At the Bell Telephone Laboratories, where he went next, he worked on a variety of projects on the interface between physics and electrical engineering. These included microwave generators, computers, and bombing systems that incorporated radars. He became even more involved with microwave technology after the war ended. At Bell Labs, and at Columbia University, whose physics department he joined in January 1948, he helped pioneer the new field of microwave spectroscopy (Nebeker 1993).
Townes conceived of the maser in 1951 and his Columbia University group brought it into operation in 1954. The idea, as indicated above, is that ammonia molecules (and more generally, quantum systems) could be stimulated to emit electromagnetic energy when hit by suitably chosen radiation. Assuming, with historian Paul Forman, that “microwave spectroscopists particularly took it as understood, agreed, and accepted that the radiation that atoms and molecules emitted when stimulated by a passing photon was coherent with that stimulating photon,” and that any spontaneous emission would be incoherent, it is nevertheless the case that, until the end of the decade, coherence was a peripheral matter for Townes and one that seems largely unscrutinized.2 On the other hand, the papers he co-authored on the maser from 1955 to 1959 form a narrative of the growing insertion of quantum mechanical ideas into the treatment of microwave noise.
Townes’ concern with noise, and neglect of coherence, may have been dictated by the maser’s applications. Conceived at first as a generator of millimeter electromagnetic waves, the maser, once built, was perceived to be useful rather as a spectrometer, a frequency standard, and a narrow-band amplifier. As a spectrometer, the desiderata were sensitivity, (that is, a high ratio of signal to noise power) and good resolution (that is, a narrow line width). And sensitivity could be achieved because as an emission spectrometer, its signal was much greater than that of the older absorption spectrometers (Gordon 1955, p. 1256).3 As a frequency standard, the desiderata were a narrow line width and stability over time. As an amplifier, it was that it added a minimal amount of noise to the signal it received. The issues, then, were sensitivity, bandwidth, stability and noise.
What kinds of noise did Townes and his coworkers envision? Prior electrical devices, like triodes and klystrons, had depended on the interaction of charged particles with electromagnetic fields. The inevitable consequence was “shot noise:” since the number of charged particles crossing a given area would vary randomly, the charge they transported, and hence the current, would fluctuate. The Columbia University maser ran on the interaction of a stream of neutral ammonia molecules with the microwave fields sent into a cavity, and it looked as though the molecules’ neutrality promised the elimination of shot noise. The Townes group, in fact, early on claimed this as one virtue of their device (Gordon et al. 1955, pp. 1264, 1273–74). But they refined this claim in their next paper, on “Further Aspects of the Theory of the Maser.” There would be fluctuations in the numbers of molecules entering the cavity, even though these fluctuations would not be accompanied by fluctuations in charge. They therefore computed the effect that would cause and concluded that, because of the large number of molecules in the cavity at any one time, it would be negligible (Shimoda et al. 1956, pp. 1317–18).
What would be sizeable, however, was that old bugaboo of microwave electronics, thermal noise, that is, the noise contributed by the erratic, temperature dependent, motion of the electrons in the walls of the cavity and of the wave guides connected to it. Their computation of its magnitude combines microwave engineering and elementary quantum ideas.4 The fields inside the cavity are represented as classical fields, and are analyzed in terms of the power flows between wave guides and cavity, and the lossiness of these elements, as was standard in microwave engineering. But the energy emitted by the excited ammonia molecules is equated to the number of molecules that enter the cavity in a given period multiplied by the energy each releases, that is, Planck’s constant times the frequency of the molecular transition. Thus, photon particles and classical fields participate together in the computation of thermal noise.
This early paper does look briefly at spontaneous emission. It notes that “spontaneous emission of radiation by the molecules also produces random fluctuations [in the cavity fields].” It holds, however, that their nature is “precisely the same as that of fluctuations due to thermal radiation and considerably smaller,” so that spontaneous emission is not a significant noise source in a maser (Shimoda et al. 1956, p.1318).
We can probably read something about Townes’ view of coherence out of these early papers’ treatment of stability. The Columbia University group tested the stability of their ammonia beam maser’s frequency by letting the output of two different masers interfere (Gordon et al. 1955, pp.1266–67). Now the ability to interfere was taken in optics as the test of coherence, and the question of whether or not light beams from independent sources could interfere was at that time a matter of debate (Forrester 1956). On the other hand, in radiofrequency engineering it was taken as a matter of course that independent sources could produce interference. Masers produce microwaves, not light. The fact that the Townes group nowhere raised the question of whether the output of two independent masers can interfere I take as an indication that they were seeing the phenomena through a microwave engineering lens.
The idea of including spontaneous emission noise as a form of thermal noise, however, and of treating it as minor, was challenged by Marcel W. Muller (Muller 1957). Muller, who was just receiving his Ph.D. from Stanford, was working at Varian Associates, which had a Signal Corps contract to study the ammonia beam maser. Rather than assimilating spontaneous emission noise to thermal noise, Muller pointed out that while the latter was produced by systems that were in thermal equilibrium, the quantum systems that formed the heart of maser systems were in non-equilibrium situations. More of them had necessarily to be in higher energy states than in lower ones, which was an unnatural condition at any reasonable temperature. Prudence required different treatments for the two. Muller’s results agreed with the Townes groups’ in showing that spontaneous emission noise was negligible at room temperature. But by this time, maser amplifiers had been proposed for use with radio telescopes and for this application they were expected to be run at liquid helium temperatures. In this case, Muller’s equations represented spontaneous emission as a noise source of considerable magnitude.
Townes was then spending an academic year abroad. In the fall of 1955, he was in Paris visiting the research group of Alfred Kastler. In the spring and summer of 1956, Townes was in Tokyo as a Fulbright Professor. Here he co-authored a paper with his former post-doctoral student, Koichi Shimoda, and Hidetosi Takahasi, whom he later characterized as “a very good applied mathematician” (Townes 1999, p. 84). This paper, “Fluctuations in Amplification of Quanta with Application to Maser Amplifiers,” which acknowledges “a helpful correspondence” with Muller, does take stock of spontaneous emission. The authors point out that their present paper gives results that “differ somewhat from the conclusions of Gordon, Zeiger, and Townes, partly because the earlier work did not take into account spontaneous emission from the molecules. Although this random spontaneous emission is generally small compared with the stimulated emission occurring in the cavity, it is amplified and therefore may become an important source of noise” (Shimoda et al. 1957, pp. 838–39).5
The paper has a curious structure. The creation of additional photons in the process of stimulated emission is analogized to the creation of additional neutrons in a nuclear chain reaction, the creation of additional electrons in a photomultiplier tube, of additional particles in a cosmic ray shower, or, indeed, to “the fluctuations in the number of mutants of some organism” (Shimoda et al. 1957, p. 827). The first five sections continue with this purely particle ontology. In the second part of the paper, however, “Applicability of Results to Maser-type Amplification,” waves make a strong reappearance. The photons travel in wave packets. The input signal that enters the cavity from the input wave guide can be pictured as “a long train of coherent waves” (Shimoda et al. 1957, p. 836). To calculate quantities of interest, the authors equate quantities derived from a classical calculation for the waves with quantities derived from a particle picture. For example, they calculate the ratio of stimulated to spontaneous emission by equating the flow of power from m quanta coursing down a wave guide with the power flow, computed classically, in the wave packet in which the quanta are enveloped (Shimoda et al. 1957, p. 834).6 The word “coherent” is hardly mentioned in this paper. When it is, as in the phrase “a long train of coherent waves,” it means a wave with very small frequency spread, that is to say, coherence is taken to mean monochromaticity.
“Coherence” does appear in the celebrated article in which the maser concept was extended to infra-red and visible light. This was the paper “Infrared and Optical Masers” that Townes co-authored with his brother-in-law, Arthur L. Schawlow (Schawlow and Townes 1958). Coherence entered in terms of potential applications. A maser at these high frequencies, the authors explain, would have a different set of virtues and vices than the microwave maser. It would not have the advantages of the microwave maser’s low noise. Nor did an optical maser promise the tunability that was offered by the newer solid state masers. It could register single photons, but so could other instruments in the universe into which such a maser would be born. What did not exist, at the time, were other generators and amplifiers of coherent radiation at these wavelengths. And so that is what the authors advertise: “the attractive promise of coherent amplification at these high frequencies and of generation of very monochromatic radiation” (Schawlow and Townes 1958, p. 1940). Despite this elevation of the coherence property, there is, in this paper, very little consideration of its physics. The idea of phase does enter. The coherence of stimulated emission lies in the circumstance that it produces radiation of the same phase as the stimulating radiation, while spontaneous emission produces radiation of random phase. But the stress continues to be on monochromaticity and the way in which these random phase radiations threaten it.
It would not be until the following year, when Townes, abandoning semiclassical theory, teamed up with Robert Serber, nuclear physics and high energy particle theorist and Townes’ colleague in the Columbia University department of physics, to give a crisp definition of coherence and spell out a theory of quantum noise (Serber and Townes 1960). Coherent amplification, which the maser permits, “implies reproduction and therefore measurement of phase as well as of intensity of an input wave.” And they go on, “Fluctuations in intensity or numbers of quanta in maser-type amplifiers have already been extensively studied, but less attention has so far been given to fluctuations in phase” (Serber and Townes 1960, p. 233). The paper aims to show, by considering such fluctuations, that quantum mechanics’ uncertainty principle establishes an absolute minimum of noise for any amplifier. There are two proofs. The first treats the field as a quantum mechanical harmonic oscillator. It assumes an uncertainty relation between the coordinate q of this field and a conjugate momentum p, and, by incorporating results from Townes’ prior papers, deduces irreducible fluctuations in the maser’s output. The second proof, presumably due to Serber, uses second quantization and quantum field theory to yield the same result.
We see, therefore, a steady progression in Townes’ treatment of noise. The first Columbia University maser papers focused on thermal noise and shot noise and made heavy use of engineering. By his 1957 stay in Japan, spontaneous emission noise loomed larger. And finally Townes and Serber tackled quantum noise head on with an analysis that included quantum field theory, asserted the existence of an irreducible minimum of quantum noise, and singled out the uncertainty relations as the responsible culprit.
3. Joseph Weber
Joseph Weber (1919–2000) is best known in maser history for his proposal, roughly at the same time as Townes’ and made independently, for a microwave amplifier run on stimulated emission (Weber 1953a). His paper considered gases and crystals that absorb at microwave frequencies. It made use of a 1951 paper by Edward M. Purcell and Robert V. Pound showing that such substances could be put into non-equilibrium conditions in which more of their constituent elements are in excited states than in ground states. Weber pointed out that in that case, the substances could amplify rather than absorb microwaves and, “if a suitable transition (and relaxation time) could be found in either a solid or liquid,” (4) practical power gains could be achieved. But Weber published other papers in the first half of the fifties, and when his amplifier proposal is viewed in their context, another picture emerges. It is that of a young scientist who becomes acquainted with a hitherto unfamiliar branch of knowledge and throws himself headlong both into its application to material in his old specialty and into using his knowledge of his original field to address problems in the new one.
Weber graduated from the US Naval Academy in 1940. He first saw active duty in World War II, and later, in 1943, studied electronics at the Naval Postgraduate School. From 1945 to 1948, Weber served as head of the electronics countermeasures section at the Navy’s Bureau of Ships, and in 1948, he joined the faculty of the College of Engineering at the University of Maryland at College Park. A condition of employment was obtaining a Ph.D. Weber enrolled in the physics department at the Catholic University in Washington DC, and here became acquainted with stimulated and spontaneous emission, with quantum mechanics and with quantum electrodynamics (Bromberg 1991, pp. 19–20; Yodh and Wallis 2001).
Four papers, submitted between October 1953 and June 1954, followed Weber’s molecular amplifier proposal. In the first of these, “Quantum Theory of a Damped Electrical Oscillator and Noise,” (Weber 1953b), he discusses a circuit with inductance (L), capacitance (C), and resistance (that is, with conductance G). Treating the LC circuit in terms of its ambient fields and applying quantum electrodynamics to these fields, he represents the system as one in which the LC part of the circuit exchanges quanta with the resistance part. In this exchange, Weber discerns two terms. One is a quantum mechanical version of the Nyquist noise, that is, the noise caused by Brownian-motion fluctuations of the electrons in the resistance. The other is a contribution which is, Weber writes, “closely analogous to the spontaneous emission which atoms undergo even if the radiation fields are in their lowest states” (1953b, p. 980). Radiation fields in their lowest states are fields devoid of photons; their average energy is zero, but the average of the fluctuations in their energy is not. Thus, this second term is a purely quantum mechanical effect, due to the “zero-point fluctuations” in the fields created by the LC circuit. Weber points out that that fact gives hope that experiments with electrical circuits can be used to observe a phenomenon that is entirely quantum electrodynamic. “When precise noise measurement techniques are developed it should be possible to observe directly the vacuum fluctuations in a low temperature noise experiment” (1953b, p. 982).
In the second paper, (Weber 1954a) Weber extends the analysis to include resonant cavities as well as lumped circuits. In the third, “Vacuum Fluctuation Noise,” (Weber 1954b), a damped oscillator or a damped resonant cavity is allowed to interact with a stream of electrons. Now it is these two systems that exchange quanta. Again there are two processes that are going on, thermal fluctuations in the cavity or circuit, and zero-point fluctuations in their ambient fields. Both processes create fluctuations in the velocity of the electrons. At low temperatures, this noise is mainly due to the vacuum fields. Hence we can again anticipate using electrical apparatus to exhibit phenomena predicted by quantum electrodynamics: “[t]he vacuum fluctuations are directly observable as noise in the electron stream if the circuit is at low temperature” (1954b, p. 217).7
In all these papers, it is spontaneous emission rather than stimulated emission that is the focus. It is devices that use electron streams—such as vacuum tubes, klystrons and magnetrons—rather than quantum amplifiers that are the systems of choice for the experiments that Weber hopes will reveal essential features of quantum electrodynamics. Viewed in the context they provide, the quantum amplifier proposal seems almost a throw-away, however much Weber would later call attention to it.
Weber does turn his attention to quantum amplifiers in a 1957 paper titled “Maser Noise Considerations.” This is where he brings together his previous work on spontaneous emission with the new family of quantum electronic devices. In its second part, he lays out as current opinion “that maser amplifier noise performance will ultimately be limited by spontaneous emission” (Weber 1957, p. 539). He points out, however, that he, as well as Israel Senitzky, had found that “[s]pontaneous emission also contributes noise in conventional vacuum-tube amplifiers” (Weber 1957, p. 540). And, in fact, the calculation he makes of the spontaneous emission noise from a device using a low-density, low-velocity electron stream yields a result that is only half that of a maser. “[I]t is of interest to inquire whether [spontaneous emission] noise is so fundamental in character that it can never be eliminated” (Weber 1957, p. 540). Weber’s answer is no. He sketches out an apparatus that he thinks would do the trick, and concludes: “Such an amplifier would not have spontaneous emission noise. However, it would not be a maser. … It appears that quantum theory does not set a lower limit to the noise temperature theoretically attainable with microwave detectors and amplifiers, at low temperatures” (Weber 1957, p. 541).
It has been pointed out by several authors that, unlike Townes’ early papers, Weber used terms like “coherent microwave radiation” in his 1953 paper on quantum amplifiers.8 But we have no clue from this paper as to what he means by “coherent” amplification. The most plausible interpretation is that he simply intended to give it the same significance it had in microwave engineering at the time, that of the production of a nearly monochromatic wave with the same frequency as that of the incoming signal. Nor did he make a contribution to the coherence concept in the other papers I have described. On the other hand, he clearly concerned himself with noise. He was one of those who read quantum spontaneous emission noise into conventional devices. And his conclusion that quantum mechanics does not impose an unavoidable minimum of noise, a conclusion that, as we saw, Serber and Townes would soon disagree with, shows us that this whole issue was not a settled one in the late 1950s.
4. Robert H. Dicke
Robert H. Dicke (1916–1997) took a career path that was the reverse of Weber’s. He received a 1941 Ph.D. from the University of Rochester in nuclear physics and then “rushed off to do wartime [radar] research at MIT” (Happer et al. 1997, p. 92; see also Partridge 1997). After the war, Dicke joined the Department of Physics at Princeton University.
Dicke may be the quintessential example of a physicist who moved effortlessly between invention and fundamental physics (Bromberg 1991, pp. 27–30). His microwave inventions became standard in radio astronomy and microwave spectroscopy. But he also explored the equivalence of inertial and gravitational mass and co-authored a generalization of Einstein’s general theory of relativity. One example of his ideas for inventions in the field of quantum electronics was his “hot grid cell,” an ammonia gas maser, but one that would separate high energy molecules from low energy ones in a self contained unit, and hence dispense with the problems associated with the continual through-put of a molecular beam (Wittke 1957, pp. 307–8; Dicke US Patent 2,851,652).
Dicke also had a facility for transporting ideas and methods from one subspecialty to another. His obituarists mention his application of nuclear physics methods to the analysis of microwave devices and his analogies—for pedagogic purposes—of radiating gases to high-gain antennas.9 This catholicity of interests and ability to transfer methods is well illustrated in the paper that concerns us here. It was called “Coherence in Spontaneous Radiation Processes” (Dicke 1954), and in it, Dicke challenged the idea that spontaneous emission is necessarily incoherent.
In the paper, Dicke pointed to the new method of “spin echoes,” just then enriching the field of nuclear magnetic resonance. In previous methods, samples had been perturbed by a radio frequency field and the small signal they emitted, because it was superimposed on that of the exciting field, was difficult to detect. In the spin echo method, two very short radio frequency pulses were applied in such a way that the sample only gave out its own signal after the last of the two had passed. Such an aftereffect would be, in essence, spontaneous emission. This was, as we have seen, usually assumed to be incoherent and incoherent radiation was thought to be significantly less intense than coherent radiation. But Dicke thought rather that the spin echo might be a case of excitation followed by “the subsequent emission of spontaneous coherent radiation” (1954, p. 99).
In the spin-echo case, it was the exciting pulse that Dicke thought was coordinating the molecules to emit coherently. But spontaneous radiation might also be coherent in other cases. “In the usual treatment of spontaneous radiation by a gas,” Dicke argued, “the radiation process is calculated as though the separate molecules radiate independently of each other.… This simplified picture overlooks the fact that all the molecules are interacting with a common radiation field and hence cannot be treated as independent” (1954, p. 99). Were they all independent, a gas of n molecules could only radiate with an intensity proportional to n. But the intensity of coherent radiation is proportional to n2. Dicke went on to construct a new formalism for a gas of n molecules, treating the gas as a single system, and defining for it a novel set of quantum numbers modeled on angular momentum quantum numbers. He showed that for certain values of these quantum numbers, his gas can, indeed, make transitions that release energy proportional to n2. These states are also more likely to send their radiation in a particular direction. That is, if a first photon is emitted in a particular direction, there is an increased probability that the photon that follows will have the same direction. Here Dicke drew a connection to the pre-war work of his Princeton colleague Donald R. Hamilton, who had studied the emission of gamma rays by excited nuclei, and the correlation that existed in the direction of the emitted rays (Dicke 1964, p. 36, Dicke 1969).
Dicke gives classical, semiclassical, and quantum electrodynamic models for his gas. The relations he draws among them are different than the ones in Townes’ Japan paper (Shimoda et al 1957). There the authors calculate separate expressions for energy transport from the classical wave and the particle pictures of radiation and set them equal. Dicke instead uses results from one model to suggest results that should be sought for in another. Only in the case of very large numbers of quanta should we expect classical and quantum models to agree.
What then, is the concept of coherence this paper evinces? I believe that Dicke uses coherence differently in his (semi)classical and quantum electrodynamic models. In the classical model, coherence has its classical meaning. Two radiations are coherent if they can interfere constructively, destructively, or in some intermediate way. In 1951, he published a paper with his graduate student George Newell, about an apparatus that made use of this idea (Newell and Dicke 1951). So too, in the 1954 paper, coherence is used in this sense when the subject is classical oscillators (Dicke 1954, p. 105). On the other hand, when Dicke models the process quantum mechanically, coherent radiation is conceptualized as radiation proportional to the square of the number of molecules.
Dicke not only punctured the consensus that spontaneous radiation was necessarily incoherent. It is an interesting sidelight that his work on coherent radiation led him to substitute his own theory for that picture of the maser’s action as one in which excited atoms are simply stimulated to dump their excess energy. Thus in a paper with an undergraduate research student we have: “In … the maser … the radiating gas or molecular beam is first put into a state for which the upper energy level has a greater population. … Transitions occur into energy levels representing cooperative states of the gas. This leads to the production of spontaneous coherent radiation (‘superradiant state’)” (Griffiths and Dicke 1957, p. 647; Dicke 1964, p. 52).
For Townes and Weber, in the mid-1950s, coherence appears, at least from their published papers, to have been an unexamined concept. Dicke attacked coherence head on. But the relation between the classical idea of ability to interfere and the idea of coherence as radiation proportional to n2 is not spelled out. Nor is there here a fully fleshed out quantum electrodynamic concept. Dicke himself was to write in a talk published in 1960: “It is interesting to note that the extension of the concepts of the radio engineer into the optical frequency region where quantum effects are important has yet to be carried out.” And he singled out coherence, pointing out “how confusing are coherence concepts when quantum effects are important” (Dicke 1960, p. 573).
5. Israel R. Senitzky
We can gain some perspective on the views of Townes and Dicke by looking at the writings of Israel R. Senitzky (1920–2012). In an era in which Townes’ maser was creating excitement, but Dicke’s 1954 paper was neglected, Senitzky studied both authors.10 Senitzky, who got his Ph.D. from Columbia University in 1950, was a physicist at the US Army Signal Corps Engineering Laboratories at Fort Monmouth, New Jersey. He was one of the Fort Monmouth monitors for the Columbia Radiation Laboratory from 1952 (Forman 1996, pp. 284–98). Between 1953 and 1956, he published a series of articles that formed the background for his later consideration of the papers of Townes and Dicke.
These papers may be seen as parallel to, but independent of, the Weber papers we have examined. Both are concerned with quantum effects in devices that depend on the interaction of electrons with microwave structures. Whereas Weber’s papers have a thermodynamic slant, and point out that low temperatures, for example in a resistance, make quantum effects visible, Senitzky takes off from Julian Schwinger and his students and explains that quantum effects become prominent as fields increase in frequency (Senitzky 1953, 1954). Again, like Weber, Senitzky discusses the exchange of quanta between a stream of electrons and a cavity electromagnetic field. He gets the result, in parallel with Weber, that taking quantum mechanics into account reveals a quantum mechanical fluctuation in the velocity, “a random phenomenon which will produce noise in an electron beam” (Senitzky 1954, p. 904). Another paper deals with the same electron traveling through the same cavity, but now the focus is on the quantities that characterize the field. And again, the most important difference from the classical case is the appearance of a fluctuation, this time in the field strength: “since, when applied to a microwave device, … the latter produces noise” Senitzky 1955, p. 878).
What we have in these papers, then, is an overt concern with quantum noise. Unlike Weber, Senitzky doesn’t yet bring in spontaneous emission. The problem of how to translate engineering concepts of coherence into quantum mechanical terms hovers in the background but is not central and not fully articulated.11 On the other hand, there is a full-blown and self-conscious application of quantum field theory. Much is ready in preparation for the next generation of devices, those that were being discussed by Townes, Weber and Dicke, and that would use, not electrons, but molecules and atoms and their quantum transitions.
Between 1958 and 1963, Senitzky published another eight papers and gave three conference reports, and these dealt with the new quantum-transition amplifiers and generators, and particularly with the ammonia beam maser. In them, he discussed spontaneous and induced emission, coherence, noise and energy loss. They show him thoroughly conversant with the work of Townes and Dicke.12 What Senitzky tries to do in all these papers is to go beyond current or conventional treatments. What if one extends perturbation theory calculations beyond simply the first order? What if one concedes that calculations ignoring cavity losses are unrealistic? What if one insists on a meaning for coherent molecular radiation that specifies that its phase has to have been set by either the field to which the molecule is coupled or the state of the molecule? These kind of considerations led Senitzky to point out holes in the works he was studying, and thereby lead us to a better understanding of these same works.
We have already seen Dicke deduce that spontaneous emission can be coherent if certain correlations exist among the molecules emitting it. In his initial paper, Senitzky looks first at uncorrelated molecules. As a test for coherence, he uses the absence or presence of fluctuations in the radiant energy—that is, the absence or presence of energy noise. This criterion yields the result that all of induced emission and part of spontaneous emission is coherent while the rest of the spontaneous emission is incoherent. Between Dicke and Senitzky, the simple assumption that Townes and other microwave spectroscopists had been making that stimulated emission is coherent and spontaneous emission is incoherent is now in disarray.
Senitzky goes on to examine emission from Dicke-like correlated molecules. Here he finds cases in which n such molecules do send out radiation that is proportional to n2 but that is nevertheless incoherent. And he chides Dicke: “He calls the radiation from “super-radiant” states “coherent” because the energy is proportional to N2, but does not differentiate between radiation of well-defined phase and that of random phase” (Senitzky 1958, p. 11). That is, a fully quantum mechanical model of coherence is lacking. There is also in this paper a rebuke of the model Shimoda, Takahasi, and Townes (Shimoda et al. 1957) used: “there has not been complete recognition of the coherence properties of the field, since the formalism used describes the field … as a collection of independent photons” (Senitzky 1958, p. 3).13
Senitzky also tells us that the proof of Serber and Townes, showing that the uncertainty relation establishes an unavoidable floor for noise in a quantum amplifier is based on the unrealistic assumption of loss-free cavities (Senitzky 1961b, p. 1533, 1962b, p. 2869). He asserts that, “As soon as higher order perturbation effects significantly alter the state of the molecules, it is no longer possible to separate induced and spontaneous emission” (Senitzky 1961b, p. 1526). And in a 1962 article, he characterizes contemporary analyses of both fundamental quantum noise and coherence as “unsatisfactory.”14
Senitzky’s remedy in that paper (Senitzky 1962b), is to set down new definitions of these concepts. For a simple system with a single coordinate, Senitzky describes coherence in terms of the mean square fluctuations in that coordinate. “For quantum mechanical variables, the above definition of incoherence may be described as quantum mechanical incoherence in the sense of the uncertainty principle” (1962, p. 2865). Noise gets defined in terms of a correlation function, with the results that a pure sinusoidal oscillation is noise-free while “a frequency spread is a necessary condition for the existence of noise” (1962, p. 2866). Various consequences follow. For a single two-state molecule, for example, the spontaneous emission is completely incoherent but noise-free, while for a collection of such molecules, the spontaneous emission is noisy. What is perhaps most interesting, however, is that in 1962 Senitzky felt the field was ripe for fresh definitions of noise and coherence.
In a 1992 article, Dominique Pestre and John Krige wrote: “What happened in the United States between the 1930’s and the 1960’s … was the emergence of a profound symbiosis previously unknown in basic science, a fusion of ‘pure’ science, technology, and engineering” (Pestre and Krige 1992, p. 93). It would seem that this “fusion” was not reached easily, that, indeed, the struggle to reach it may be worthy of historians’ attention. Much of the work I sketched above can, in fact, be seen precisely as an attempt to create such a symbiosis. Certainly we can view Weber’s papers from this angle. This includes both his proposal to use quantum physics to create new amplifiers and his interpretation of some of the phenomena of vacuum tubes and other amplifying and generating devices in terms of spontaneous emission.
Noise became central because masers and other quantum amplifiers were instruments that scientists and engineers hoped to put to use. But the working core of these instruments was a quantum mechanical medium, hence the need for an understanding of quantum mechanics’ import for engineering noise.15
Noise and coherence are intimately related. Whichever one of the several definitions engineers and physicists gave them in the 1950s, both had to do with fluctuations. For noise this might be fluctuations in frequency. For coherence it might be correlations in the fluctuations of a quantity like electromagnetic field strengths at different space-time points. Again, this was a situation in which new instrumentation intervened. The ideation of the laser brought coherence to the fore. As we have seen, its connection with quantum theory was already a problem in the 1950s. The solution to this problem would come in the following decade. When it did, it would forge a still more extensive link between physics and engineering knowledge. As one textbook writer would put it: “with the invention of such remarkable devices as the transistor and the maser … a knowledge of quantum mechanics has become essential to the engineer … More recently, the advent of the laser has focused the attention of the engineer … on a need for understanding certain portions of quantum field theory and quantum statistics” (Louisell 1964, p. ix).
In an absorption spectrometer, the datum that revealed the wavelength of a spectroscopic line in the sample was the frequency at which microwave power illuminating the sample was absorbed. In the maser it was the frequency of the power that was amplified or emitted.
I therefore understand their calculations in just the way Forman interpreted Townes’ 1951 maser proposal in Forman 1992, p. 118: “[H]is picture of the underlying physical processes is a pastiche of elements drawn not only from the microwave domain, but also from the domain of much higher frequency (energy) transitions.”
Acknowledgment on p. 839. Quotation on p. 838. The analogy between stimulated emission of photons and a cosmic ray avalanche may be what Lamb intended to criticize in an article in the early seventies, when he wrote that “there is no need to speak of stimulated emission or of a photon avalanche” when constructing a theory of the maser (Lamb 1973, p. 104).
Some information on the multiple concepts of the photon that were in play in the 1950s is given in Silva and Freire 2013.
The fourth paper, (Weber 1954c) extends the analysis to oscillators that are heavily damped, and examines the relation between the noise they produce and the degree of their damping.
W. Happer et al. 1999. Paul Forman advances a hypothesis that deserves testing. He suggests there was a melding of nuclear physicists’ practices in separating particle events from noise with pre-war fluctuation theories, in the context of the nuclear physicists’ radar work, that Dicke played an important role, and that this melding was significant for both post-war experimental practice and noise theory (Forman 1995, pp. 435–41).
One testimonial to the neglect of Dicke is given in a 1958 article by A. Gamba (1958, p. 601). Dicke’s paper was cited only 29 times to the end of 1960, and six citations were by Dicke and his students and another six by a single Soviet physicist, but by the end of 2012, it had been cited over 3000 times.
Among the authors Senitzky cites is Lloyd P. Smith, whose 1946 article does raise questions about this problem.
A more spirited disagreement on this topic between Senitzky on the one hand and Nicolai Basov, Soviet maser pioneer, and Townes, on the other, is recorded in the proceedings of the second conference on quantum electronics, Advances in Quantum Electronics, J. R. Singer, ed., (Columbia University Press, 1961), pp. 539–40.
“There has been discussion [citing Serber and Townes and also a paper by Louisell, Yariv, and Siegman] of fundamental noise that is based mainly on the uncertainty principle or on quantum fluctuation, and, as will be shown, is more or less unsatisfactory. A related unsatisfactory situation exists with respect to the concept of coherence …because of the various different meanings attached to the word ‘coherent’” (Senitzky 1962b, p. 2864).
Cohen 2005 gives some of the later history of quantum noise. I am not aware of a comprehensive treatment of its early history.
I am grateful to Alexander Blum, Paul Forman, Christian Joas, Christoph Lehner and Chen-Pang Yeang for useful comments on earlier drafts and to Robin Sinn and Susan Vazakas for help with computer research.