Oral History Transcript — Dr. Malcolm Strandberg
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Interview with Dr. Malcolm Strandberg
Malcolm Strandberg; June 15, 1983
ABSTRACT: Particular scientific style; start as an electrical engineer; work on beam frequency standards with H. Richard Johnson; narrowing of the beam, frequency stabilization; work on phase-locking microwave amplifiers; discusses his consulting work at Hughes; search for alternate solution; ideas for increasing gain bandwidth; paper published in Physical Review "Quantum Mechanical Amplifiers";difference from other papers (Bob Pound's and Mohler's) on same subject; colloquim at MIT on paramagnetic resonance; solution to noise problem; start of group; working from scratch to build own magnet; work with Pound.
Bromberg:This is Professor Malcolm Woodrow Strandberg, and we're in his office at MIT on June 15th . [Joseph] Weber was partly in engineering circles. Were you also in these same engineering circles, going to Electron Tube Conferences?
Strandberg:I knew Ernst Weber. I know Joe [Weber] very well. Joe is a very good friend of mine; I love him dearly. He's a very, very talented fellow. But when I read this thing over, his proposal, I did not then know Joe. For some reason I thought it was a proposal of Ernst. I had no reason to think that Ernst had the credentials to propose a viable or useful amplifier. We all realized that emission devices were possible. And I said, "This is for the birds." You know, he's just doing a hand waving thing. In the first place, he's got no useful properties of it, and he's doing it because it can be done. It's sort of like standing in a hammock or walking a tightrope between the World Trade Towers or something else like that. I don't understand it. That's the trouble. I'm absolutely divorced from it. I had the same attitude toward what the Columbia group was doing. I worked under I. I. Rabi in W.W. II. He would visit me at MIT, and look at the microwave spectroscopes and say, "Why don't you use state selection?" I would say, I did not need it — I had sufficient sensitivity and resolution for spectroscopy.
Bromberg:You could say it's a real element of your scientific style, that one of the things you insist on is that whatever you're going to do has some relation to what the best thing to do is, or some specific—
Strandberg:— that I understand what the properties of the thing are and what the possibilities are and so on. I did a lot of engineering, but I think I was able to do it because I never got indoctrinated by the "in" group, so that the things that I did were quite different, because I wasn't trying to put things together in any particular way. I started out as an electrical engineer, but I never was part of that group. As a matter of fact, that was another thing I was going to mention. I had two students who worked with me on the beam part. One was Harry Dreicer who worked on a beam microwave spectroscope, and a second, trying to get high resolution for the frequency standard, was Dick (H. Richard) Johnson. He got his degree. We managed to get some narrowing of the ammonia absorption line. We did some frequency stabiliazation sufficient to be able to study the very narrow lines. We had to do a lot of work, ancillary work, on frequency stabilization and microwave sources in order to handle these kinds of problems that you get into, if, in fact, you have the higher resolution and have narrower lines. What are the sources that you're going to use in order to look at them, and so forth? Those were not very easy problems.
Bromberg:That's interesting. I didn't have any appreciation of that.
There was a paper by Shimoda. Shimoda and Townes did the research [on maser noise, but] Shimoda, in an earlier paper, talking about the fundamental limitations on frequency stability in microwave oscillators, did some calculations pointing out that there would be some limitations — that one could never get a microwave frequency which was as pure as you wanted. There would always be a finite width, noise, simply because of the photon lengths; the microwave photon length was finite, therefore you would always have some limi tations. [The following paragraph was inserted later:] We all faced the same need for technical developments for the realization of something useful. One was the source frequency stability I have just discussed. The second was a need for a high flux molecular beam source. The third was a need for a state selector with an acceptance solid angle that was large. In 1954 the last two items came available.
At MIT, Zacharias devised the "crinkle foil" source. And in Bonn, Paul demon strated the large acceptance solid angle of the quadrupole focusser. When Jim London visited MIT in the fall of 1954, he saw our work and discussed the Zacharias crinkle foil source with us and Zacharias' group. We did not learn of Paul's quadrupole focusser until after the Columbia group published a description of their ammonia maser in 1955. They had been in a position to implement rapidly their maser with both Zacharias' source and Paul's focusser. They also had changed the goal from producing an infrared source by using the vibration levels of ammonia, to a microwave source by using the inversion fine structure on the vibration levels. In the absence of any of these events their ammonia maser would not have been realized. In any case, my group at MIT was not so agile, nor so well informed to have carried out a similarly happy set of actions. [End of insert] Kind of an interesting idea.
There were a lot of false state ments that were made, of that kind, which you had to straighten out. But you know, that is the thing that sort of haunts you — if you're working in this thing, and that catches up on you, supposing you can't really stabilize your microwave source? Supposing you can't really get a frequency as stable as you want? Then you have a prob lem. There's a lot of work done, and we did, as a matter of fact, work [that] terminated in being able to phase-lock microwave oscillators, even the K-band, so they were absoutely pure tone. Over a second or so, they were a sine wave and within the cycle per second. There was no fundamental limitation on the width of the spectrum of a microwave source. That was a fundamental problem. We could think about it fundamentally, and what were the limita tions of it, or you could just do it. I think that the engineering community might have just done it, if they finally got around to it.
Bromberg:At some point, you were really excited about these kind of [stimulated emission] amplifiers.
Strandberg:That was when my student came back —
Bromberg:— This is Johnson —
Strandberg:Johnson, yes, H. Richard Johnson — he now has a company, Watkins-Johnson. He is a tremendously good electronic engineering man. They're making money still. He was at Hughes, and he came back and said, "Hughes has got this thing that, they're being advised by consultants, (I forget the guy's name now) "they can get into the microwave amplifier business." They were always in it anyway. I said, "How?" He said, "By making an ammonia amplifier." I said, "That's ridiculous, because it's the most useless kind of a device you could ever want."
Bromberg:They were excitied about Townes too?
Strandberg:Yes. The [maser] oscillator, and what had happened was that this guy, I can't think of his name now, a freelancing consul tant, was trying to push them into investing in a large research program in microwave amplifiers using ammonia beam devices. Now, if you take it from this point of view, it's fairly obvious, that's not the thing to do. But believe me, there were a lot of people, without too much of an idea about [the noise]. There was no indication as to what the noise figure of that amplifier would be. That was the first thing I said to Dick, "You know, you don't have the vaguest idea what the noise figure of this thing's going to be; in the second place, it's going to have essentially zilch, no gain-bandwidth product for you. So I really don't know what you're ever going to use it for. It'll be an interesting device, but — I don't know — it will be an ostrich that can fly, or something. I don't know what you're going to use it for." And he said, "Well, so what would you do?" I said, "I would take a solid state, a paramagnetic system, something which had a high density, where you don't have to work with a beam which has a very low density [of] particle, so you can get a large bandwidth and so on." He said, "That's interesting. Why don't you come out and talk to the people?" So I did go out and talk to them.
Bromberg:Did you do any preparatory work? Did you set down some calculations at that point?
Strandberg:Well, I had been thinking about it. I had been thinking about it some, and trying to get it clear, what this business was. I had a fairly good idea of what the limitations of gain-bandwidth product were going to be on the thing, because I knew it had to be associated with the passive width of the line, and I needed to get something which was dense in order to get enough power out of the thing. And I think I was still trying to understand what was the noise figure.
Bromberg:When you say you were thinking about it, do you mean you were thinking about it even before Johnson came back from Hughes?
Strandberg:Yes, because I've got colleagues saying all the time, "What are you doing on ammonia devices and so on?" People around me saying, "Hey, have you seen the latest thing that they're doing?" So I'm saying, "Well, that's great, but I don't know what you do with it." Essentially I was trying to understand from my own point of view what I would want to do if I were doing something in this field, I had to have something, some kind of a reasonable device, in the sense of having a reasonable gain-bandwidth product. The bandwidth over which you can get a reasonable amount of gain tells you something about whether you can use it for, say, a television set, or a whole bunch of things. As a matter of fact, there was an interest in getting band width even for thermal detection. For anything else, you need band width.
Bromberg:Was this kind of a controversy here at the laboratory? Some people were saying, "This is an interesting thing to pursue," and other people like you were saying, "Wait, this doesn't look good enough." I don't know if you remember any of this, but I'm just trying to get some sense of what it might have been like on the subject of the maser. Do you remember anybody who was pro-maser, anybody who was con-maser?
Well, I guess the beams people might have been sort of — I don't remember. I'd have to look up, whether maybe Zacharias' lab, with John King and so on — whether they were exploiting it. I don't care who tells you, whether it's Lord Cherwell [I was a scientist with the Royal Air Force in WWII, 1942, 1943] or anybody else, if it doesn't make intellectual sense to me, it doesn't matter. I have differed with people with reputations and names starting from the beginning of my career. And often time and events have justified my stand. So my willingness to resist what appeared to me to be error or wishful thinking has been enhanced with time. My own mistakes have emphasized to me how global one's thinking must be and how detailed the "homework" must be to avoid traps, and how unregimented the mind must be to uncover the extra dimensions of any problem. I can be very tender-skinned, and emotionally involved with people, but I can also be very hard-skinned, if it's a scientific idea, and somebody says, "What are you doing on this? Look at all the stuff that's coming out." And I'm saying to them, "Well, yes, you know — for what? I don't understand it really." The things that were bothering me, one of them, as I told you, was that business of the noise figure. I didn't really understand that.
That, I think, was pivotal. The ammonia maser was not a good frequency standard, and with the cesium device we did not need another standard. The other argument that I was using, was that at least these paramagnetic materials we were working with would be operable at low temperatures. They had a chance. If I knew what the ingredients of the noise figure were, I had a chance of making a good low noise, usable amplifier. This is a devil of a thing. It doesn't sound like a physicist, in a sense, really, because obviously the physicists were doing things, and then finding out what the properties were after they'd done it. But, anyway, I wasn't doing that. Anyway, I went out to Hughes, and I spent time out with them. Some of them were friends. One of them was a friend from the Radia tion Lab, Lester van Atta. I had a high respect for the people that I met. I had a very high respect for my own student, Dick Johnson, who was there. I worked very hard, trying to transmit some of these ideas. But —
Bromberg:— is that a situation where you were actually doing some research and you brought it to — [telephone interruption]
Strandberg:You're asking, on what basis I was doing it?
Bromberg:No, when you go out on a consulting thing like that — a lot of this is for me just to get my idea of what the scene looked like. Is it that you go for a day or two and talk?
Strandberg:— yes —
Bromberg:— or is it that you go there for a week or so, you sit there and you do calculations, and then you — ?
Strandberg:You go there essentially to talk, and they show me the thing that they're doing, the capabilities that they have, and who their people are. I talk to their people and then I'm in some sort of a position to say, "Well, you know, you either have the facilities and the raw materials to do something in this thing, or you don't. I'd advise you to keep out of it, or I would certainly advise you to get into it, for these kinds of reasons." You know, it's a matter just of understanding them well enough, in their structure, the way that they were operating. You can do this as well in two days or in one. I told you they were friends. Van Atta was a friend, and they had experience because these were traveling wave amplifier people who knew what they were doing. These people were open, in the discussion. I could talk with them, you can do this, or you couldn't do that. It was obvious to me that they had all the capabilities for doing it. The other part of it, just solid state physics, that they had to learn, wouldn't be that difficult. Anyway, I lost it.
Bromberg:— I see, you try to interest them in that —
Strandberg:I tried to interest them, and they dropped it. They decided not to. They took their other consultant's position, and I think they invested a lot of research money very unwisely into trying to make use of ammonia amplifiers, some way or other. They might have come out all right in the end, but nobody's talking about those wonderful Hughes ammonia amplifiers that they built! At least, I don't remember that. Sort of interesting. I think at about that time, I finally got a way of convincing myself and the people around me that I knew what the noise was and what the limitations of it were. And that there wasn't anything unexpected or anything else.
Bromberg:In other words, you convinced yourself that the noise was good.
Strandberg:That the noise was good, that it was controllable. Townes had gone to Japan, worked with Shimoda and Takahashi, and they'd done a solution — an incredibly sophisticated mathematical solution to the problem. In the first preprint of that thing, which I didn't see, but which in fact Bloembergen had reported on at Harvard, the result was that the noise increased as the square root of the gain. So that you would be losing on this thing. [ See photocopy of Strandberg notebook, #1121, p. 13 at the end of the interview. ] It raised a big question mark, because it means there was an inherent noise in the system, which would just be amplified — more than just the thermal noise which you get from the system — which was a kind of a shot noise really. Before they published the paper, they found the error.
Strandberg:But at any rate, you see, you're in a field where people, even those who were thinking about it, didn't really understand it. By the time my paper got published, there was also a paper by Pound, and I didn't even know that Bob Pound was working on noise. There was a paper by Pound in the ANNALS OF PHYSICS. I don't know whether it was published near the same time as mine or not.
Bromberg:There was somebody named Muller —
Strandberg:Muller did one, yes.
Bromberg:I think yours and Muller's came out in the same volume of the PHYSICAL REVIEW.
Strandberg:Yes. Even if I'd read either of those papers, I would still have done what I did. What I did was intellectually different, but it didn't add any more to the total knowledge. It was a different presentation. At that point, I started talking about what was on my mind. I gave this colloquium at MIT. Bloembergen (whom I've mentioned) was there, and at the end he said, because I talked about these amplifiers, "Why do you want to make them?" I said, "That's very simple. It will have a low noise temperature. It's going to be working at low temperatures." I didn't give them the theory. It was a general talk on paramagnetic resonance, and I said, "I can show what the noise figure is going to be and so forth. And that in the limit, it's going to be, the order of the amplifier ambient temperature."
Bromberg:Is the talk that you gave the same thing as the paper that you published? It's called "Quantum Mechanical Amplifiers," and it talks about their low noise and so on. Is that the text of the talk?
Strandberg:No. The talk was a talk on microwave spectroscopy and paramagnetic resonance. I ended trying to point out some applications of this. I described an amplifier that one could make, using these paramagnetic materials, a quantum mechanical amplifier, which was like that text. The talk was a lot more, but that was on the end of the talk.
Bromberg:OK, because that paper was, in a sense, a real call to invent these things.
Bromberg:You said, "We're doing it, and not enough people are doing it, and they should." I remember the phrase, "They should make the electrical designer's eyes sparkle."
Strandberg:Yes. As a matter of fact, it's very hard to give an idea away. Nobody had any problem with keeping an idea. I wouldn't have done Townes's infrared oscillator if he'd twisted my arm to the end of time. Because it was just something I wasn't going to do. And even he didn't do it. Nobody's ever done one of those infrared oscillators using ammonia vibrational states which Townes originally proposed. I don't think anybody did. It finally cracked down in frequency to the microwave transitions to where some technology was available. There's no way you could make a finite cavity, with control of modes, the way you have to for that infrared oscillator to work in anything but a microwave [wavelength] and God knows it was difficult enough making that microwave cavity in the first place — even at 1 centimeter, or a quarter centimeter. So even Townes didn't buy his own original idea for an infra-red source.
Bromberg:Once you got the noise squared away, it sounded from that IRE article as if you had established a group here.
Strandberg:Yes, that's right. But then we were starting from scratch. I asked for some help here, and they said, "What do you need?" I said, "I need a good magnet," and they said, "Well, OK, we'll ask the shop to help you build one." Imagine, I had to build my own magnet! I was starting from scratch.
Bromberg:That wasn't a common thing at —
Strandberg:Well, I thought that they would buy me a magnet.
So I had to go back and design the magnet that I wanted. Yes, I did all that. And by the time I'd gotten all the experimental wherewithal, then other people, other things had come out. That's an unfortunate thing, to have a hangup of trying to understand what you're going to do, and have confidence in where you're going to go. If I'd started much earlier, then I might have thrown all of my resources into building an ammonia amplifier, which I didn't. We never really got that deeply involved in the amplifier. The things that we did were that of intellectualizing the problem. The problem was [with] a single-mode cavity, how do you overcome the limitations of the regenerative narrowing that you get in those kinds of things? I beat this back and forth, and Bob Kyhl, who was my colleague, had the solution, saying, "Look, it's narrowed because of negative reactances. If we had positive reactances too, we can cancel them out, and we can overcome the regenerative narrowing and get a very broad bandwidth." We demonstrated such a device.
So we did very little in amplifiers, except on the basis of trying to understand. The literature was full of the results of people who made something. I'd look at it and say, "Jesus, they're lucky, because I don't think they even calculated the dipole matrix elements for those transitions. What is the filling factor and so on?" For example, one device was described in PHYSICAL REVIEW, which employed a salt with a chromium impurity. This had 4 zeeman states, 1 to 4, with the high energy states, 1 & 2, used to amplify at 10 cm, and the levels 1 & 3 as pump levels at 3 cm. Now I saw that the low pump power had to be used on a highly allowed transition, which was 3 to 4. So what really happened was that 3 to 4 was pumped. This heated the lattice phonons, which then saturated level 1 & 3! This is an important deduction since it means that the phonons are excited only near the pump level frequency. Otherwise the amplifier levels, 1 & 2, would have also been saturated. I published the analysis of such a "bottleneck" process in PHYSICAL REVIEW 110 65 (1958). That paper became an important contribution to the physics of the relaxation process. [ See photocopy of Strandberg notebook #1170, p. 145, and #1170, p. 87 at the end of the interview ] Things were deficient in any kind of intellectual basis. But there were a lot of results.