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Coherence and Noise in the Era of the Maser

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Coherence and Noise in the Era of the Maser Coherence and Noise in the Era of the Maser Joan Lisa Bromberg The Johns Hopkins University 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. 1. Introduction 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 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. Perspectives on Science 2016, vol. 24, no. 1 ©2016 by The Massachusetts Institute of Technology doi:10.1162/POSC_a_00193 93 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00193 by guest on 27 September 2021 94 Coherence and Noise in Maser Era 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 spec- troscopists 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 jump- ing 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 de- rived 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 con- tract monitor at the Signal Corps Engineering Laboratories in New Jersey, was in close contact with these men and dealt with their work in his publica- tions. Although they represent only a sample of the scientists who wrote on noise or coherence in the context of the new quantum amplifiers, and al- though 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 1. See also Bromberg 1986 and 1991, pp. 42–4. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00193 by guest on 27 September 2021 Perspectives on Science 95 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 micro- wave 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 stimu- lated 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 dic- tated by the maser’s applications. Conceived at firstasageneratorofmilli- meter electromagnetic waves, the maser, once built, was perceived to be useful rather as a spectrometer, a frequency standard, and a narrow-band am- plifier. 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. 2. This contradicts my account (Bromberg 1991, pp. 17–18), which relied on Townes’ own reconstruction. It follows Paul Forman’s more discerning version, based on contempo- rary documents. (Forman, 1992) The quotation given above is from his article, on p. 122. 3. In an absorption spectrometer, the datum that revealed the wavelength of a spectro- scopic 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. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00193 by guest on 27 September 2021 96 Coherence and Noise in Maser Era 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 elim- ination 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 enter- ing 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 magni- tude combines microwave engineering and elementary quantum ideas.4 The fields inside the cavity are represented as classical fields, and are ana- lyzed in terms of the power flows between wave guides and cavity, and the lossiness of these elements, as was standard in microwave engineering.
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