Oral History Transcript — Dr. Yanhua Shih and Dr. Morton Rubin
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Interview with Dr. Yanhua Shih and Dr. Morton Rubin
Yanhua Shih and Morton Rubin; May 14, 2001
ABSTRACT: Topics include: Yanhua Shih's early interest in physics and in Joseph Weber's experiments on gravitational waves; the Cultural Revolution in China delays Shih's studies; Shih's college studies in China and graduate work at the University of Maryland. His use of spontaneous parametric down conversion; the influence of contacts with Soviet physicsits. Shih takes a job at Baltimore County branch of the University of Maryland. Sheh and Morton Rubin elaborate their biphoton concept; their experiments to elucidate the concept; funding sources.
Bromberg:This is Joan Bromberg and itís May 14th and Iím in the office of Professor Yanhua Shih at the University of Maryland in Baltimore County. To start is to ask you about how you got from Beijing to Baltimore. I donít have any information on — did you get your bachelorís degree here in the United States or in China or what?
Shih:No actually, I got my bachelorís degree in China.
Shih:No, in the Northwestern University, in Xiían. So, you know Xiían, the early capital of China for a thousand years.
Bromberg:Is that where all the soldiers are?
Shih:Thatís correct. Thatís correct. Thatís the place.
Bromberg:And then, so did you get a bachelorís in physics or in some other field?
Bromberg:And what year was that?
Shih:Okay. I started my undergraduate from 1978. Actually itís supposed to be Ď77, thatís the first time in China after the Cultural Revolution the universities started to open for the students and gave the national exam. And everybody has to pass the national exam to get into the university. During the Cultural Revolution, in China, the universities were closed so there is no student, later they only allowed that workers, farmers and the soldiers students got into the university, but they donít need to take exams. And then only started in 1977, the Chinese universities going back to a normal situation.
Bromberg:So I assume that your own schooling was probably interrupted by all of that.
Shih:Oh yes. Actually I can tell the story. Before that, I was a farmer for many years in a very poor countryside in China called Yanan. You know Yanan. Thatís the place. The communist party, during the Second World War, the communist party ruled that poor place and started their base. Their base, it started from there, and thatís a really very poor countryside. So I know how to grow potatoes. Very interesting story.
Bromberg:But then how did you become interested in physics?
Shih:Oh, I had my interest in physics since I was a child. And I was in a pretty good high school in Beijing and very good teachers. I remember when I was in high school I started to read scientific journals. And once, I read an article about [the] Weber Bar. Professor [Joseph] Weber and he studied gravitational wave. So I started to get my interest to general relativity and to the gravitational wave theory and experiment. In that time I made my decision and [wished] to come to the United States, join the University of Maryland, with Professor Weber to study gravitational wave. Thatís very, very early.
Bromberg:So that must have been when, in the 1960s?
Shih:Thatís correct, 1960s, yeah. However in 1966, you know that in China Mao Tse-tung started the so-called Cultural Revolution and all the schools closed and all the students were sent to the countryside to do the farm work. So I was sent to Yanan to be a farmer to grow potatoes there. And I remember that in 1995 when Professor Klyshko from Moscow State University visited here I asked him that, ďWell, what do you do in the summertime?Ē He said he has a country house and he goes there and they grow potatoes. So we started talking about growing potatoes. We know how to select the best seed and how to grow potatoes. Thatís really very interesting.
Bromberg:So then the Cultural Revolution came to an end, and you knew you wanted to do physics and so you enrolled to do physics?
Shih:Thatís correct. Thatís correct. I still wanted to go to university to study physics. Thatís in 1977. China started the national exam for the colleges, and I had the exam and passed the exam and was admitted to Northwestern University, physics department to do that. My class is for the theoretical physics. It was a theoretical physics class. So more likely I have to work on theory in the future.
Bromberg:Is that because they didnít have much experimental equipment?
Shih:They did. They also have the major in lasers and optics and nuclear physics, many other majors, but I decided to study theoretical physics because my interest basically was general relativity, the gravitational wave. That was a very difficult exam, because in ten years the university [had been] closed, so everybody was trying to get into the university, and only 1 percent of the students were allowed — I mean, passed.
Bromberg:To be admitted to the school?
Shih:Yeah, to pass the exam to be admitted to the colleges, universities. And I, actually thatís a four-year college, four-year program. I spent three years and graduated early, one year early. And in that time I had relatives in the United States and I told them when they visited us...in that time the door was just opened, China was opened... I told my cousin my dream was to come to College Park campus, University of Maryland to study general relativity and the gravitational wave detector with Professor Weber. So —
Bromberg:Where was your cousin? What part of the United States was he in?
Shih:He lived very close, in that — he was working in Commerce Department and doing some research, I think on the — I think for some kind of building stuff. Iím not sure — probably construction, whatever. And he lived in Rockville [Maryland], and so very close [to the university]. And so he — I think he went to College Park and got application form for me and then he sent the application to me. I tried to fill it out and send it back and to apply to graduate school.
Bromberg:Did you already know English? Did you learn English in college?
Shih:Yes, I had learned English, yes, in college. And I also, when I was a farmer I tried to practice English every day. I could read pretty well, but my spoken English was not very good because there was nobody to teach me how to speak. I just tried to learn by myself.
Bromberg:You just learned it by yourself. Yeah, and of course English is so difficult to pronounce.
Bromberg:So that itís not easy to learn.
Shih:Yeah. So I still have a strong accent. And itís very interesting that one of my other relativeís friend — Professor Chang — is teaching in Yale, the physics department. Yale University. And when he visited in China, he met John Toll. Professor Toll, he was the president of the University of Maryland at that time, and in Hong-Shan [Mountain], is a mountain, a beautiful place, and he told Professor Toll that my dream is to come to the physics department in his university to study physics. He [Toll] was very happy and said, ďOkay,Ē that [I should follow] the normal procedures for application to the graduate school and also if possible you may, when you meet him again, give him a test to see his level. ďIf heís good Iíll ask you to write a recommendation letter.Ē So later this Professor Chang went to Xiían and met me there in a hotel. He did give me a test, and he saw my physics is good. However my English is poor, so he was really very honest to write on his recommendation letter: ďHis spoken English is poor.Ē And he preferred to send me to an English class to practice my spoken English [first]. So, but anyway, I think that that recommendation letter was well received by the physics department. And I was admitted to the graduate school and the physics department.
Bromberg:And when did you — did you have any trouble getting from China to the United States? Did you have to get special funds, or did the Chinese government help you or —?
Shih:No, no. Actually no. No, I got the teaching assistantship here, so from the first day I was a TA.
Bromberg:And what year was that, which you got here?
Shih:Thatís in 1981, the beginning of Ď81. I think January 15th was the day I entered the United States and became a graduate student at the University of Maryland.
Bromberg:Now you went as a theoretician, you said.
Shih:I was a theoretician in China. I was trained in the theoretical physics program. But when I came here I decided to be an experimentalist.
Bromberg:Why did you, how did you decide that?
Shih:Well, I think...I had a dream you know, a long time ago, trying to study the gravitational wave. I think that only doing theory, it doesnít help. You have to experimentally to find some way to detect that, to find out how to detect gravitational wave and how to use that in the future. So thatís my decision to study experimental physics here.
Bromberg:And then who were you studying with? I mean you get here and I suppose the first thing is they want you to take courses.
Shih:Yes, thatís right. I had to take courses, and also the first semester I did follow Professor Changís suggestion took an English class to improve my spoken English. That was very helpful, and those teachers really were very good, and they tried to correct my English speaking and accent, and so I got a lot of help. But I still have a strong accent right now. Very difficult to change.
Shih:And the courses were pretty easy for me. Then after I passed my qualifying exam. Actually when I got there, College Park, I think on the second day I tried to find Professor Webber.
Shih:Yeah. And I did find him. I found him and had a very nice talk with him. I hope he understood me, because at that time my English was very poor. And he gave me papers and some advice and I donít think I fully understood him at that time. I probably could only understand 50 percent of what he said to me. Itís just, basically he told me, ďYou have to take courses and after your qualify you can come back to see me again.Ē However he said he was very happy to show me his lab. So in the second week when I got to College Park I visited his lab to see his beautiful Weber bars and beautiful equipment and talk with other graduate students there.
Bromberg:And then Professor Misner was also at the college?
Shih:Yeah, Professor Misner was also there, and Professor Brill, Dieter Brill, and Professor Hu, and I think I contacted everyone in that theory group — thatís the relativity theory group there — too. And so after I passed my qualifying I went back to talk with Professor Weber again to see if it was possible to do a thesis on gravitational wave detection. And I think Professor Weber is really a very nice person and a very good physicist. He gave me advice. He said, ďI donít recommend you to do a thesis with me on the gravitational wave, because it would really take a long time.Ē And he said, ďOne day you will want to going back to China. You really donít want to stay too long here.Ē And he said, ďI know some Chinese word,Ē talking about how difficult it is for the gravitational wave detection. He said ďlike somebody trying to find a needle in the seaĒ. Okay. Thatís very difficult. He said, ďAnd can you imagine people trying to find a radio wave — before we had equipment to generate radio signals — how difficult it would be for people trying to detect the radio wave!Ē The gravitation wave probably is more difficult and take a long time. He said, ďYou can do that after your graduate thesis, as a hobby for all your life, as a research project for your future, but I suggest you do a thesis that will take a shorter time.Ē So he suggested me to contact, also contact with Professor Alley. Actually I contacted Professor Alley some time ago already, because I have also interest in quantum mechanics. And I know in that time he was, he and his student were doing an experiment of delayed choice that was proposed by Professor Wheeler. So, I knew that project and I also had a strong interest. So I talked to Professor Alley and I decided to do my thesis with him. I have also talked to many other professors there too, and we were discussing general relativity, gravitational wave, also discussing about photons. And Professor Alley was the only one who really told me straightforward, he said, ďI donít know what is a photon.Ē So thatís the reason I decided to do my thesis with him, because if he really said that then he really take it seriously on the research. So, we want to understand the behavior of the quantum world. And the delayed choice experiment proposed by Wheeler, and I think, going back to early thirties, by Einsteinís idea, is really very surprising. Itís amazing.
Bromberg:Who was the other student? You said there was another student working on this?
Shih:Okay. Thatís Oleg Jakubowicz.
Bromberg:And was he from Russia or —?
Shih:No, I think he is a Native American, but his family was from Poland as I guess.
Bromberg:Because his name doesnít appear on the papers.
Shih:Thatís correct. After they did the experiment and they published a paper in a conference, and then he decided to leave for a job. I think they didnít finally put a paper in Physical Review or other journals. Only published it in conference proceedings.
Bromberg:And by that time had you already taken a course with Professor Rubin in quantum optics?
Shih:Not yet. Not yet. After I joined Professor Alleyís group and I had to start to help other students like Oleg Jakubowicz was to do the delayed choice experiment and also other projects like lunar ranging, and try to learn the basic experimental skills from the graduate students and from other engineers and technicians — and also, of course, from Professor Alley too. And thatís in I think, thatís in 19 — probably Ď84 or Ď85. I found that the department is going to offer a course in quantum optics and was teaching by Professor Rubin. So I took that course.
Bromberg:Now was Wheeler around a lot? I mean did you see him a lot?
Shih:Oh yes, yes, yes, thatís correct. Weíd see him a lot, and once he gave a lecture in College Park, it was a continuing series of lectures for many days. And I still have his lecture book, and I really learned a lot from him, and especially the way of thinking and to be a physicist. How to think and how to find a problem, how to think, how to analyze it, how to get the solution of a physics problem or difficulties. And thatís basically...learned a lot, the way to be a good physicist, or the way of thinking as a good physicist, from him. And also I learned how to give a presentation too. You know, Wheeler, when Wheeler gave a lecture, really, he didnít write too much mathematics or formulas. He said that part you can read. He gave many cartoons. He showed, one by one, cartoons — showed transparency. I got several transparencies from him, and he gave me a copy of the —
Bromberg:His smoking dragon?
Shih:His smoking dragon and also some others. Very, very interesting, yeah. And he was really, he was talking about physics, he was talking about philosophy, he was talking about big things behind those formulas. He always asked questions and asked you to ask questions. And so, I learned. I really did learn a lot from him.
Bromberg:And as far as the experimental work, who were the most important people who were teaching you in terms of experimental technique?
Shih:Oh. For example Professor Alley and other graduate students. There were two graduate students I think that were very helpful to me. One is Oleg, Oleg Jakubowicz; another is Steve Bowman. Steve Bowman was doing laser ranging, a different project.
Bromberg:Also under Alley?
Shih:Also under, yeah, he did his thesis with Professor Alley too, and I learned a lot of laser physics and non-linear optics..., how to align a laser system and even make a laser system, and also how to put a piece of crystal to get a second harmonic: you have infrared laser beam sent to a crystal then on the other side you get green. So these kinds of experimental techniques.
Bromberg:And of course you were doing lasers with the delayed choice experiment too.
Shih:Sure. Yes, yes, yeah, also, yeah. Laser physics and non-linear optics and others.
Bromberg:And that was done with spontaneous parametric down conversion, that experiment?
Shih:Yeah. Thatís a very interesting story. I remember in 1983 Professor Braginsky from Moscow State University visited us. And at that time I was worrying about some of our single photon detectors. The quantum efficiency may be low, and then itís not very sensitive to the delayed choice experiment, where we need a single photon detector. So we planned to send them to NIST, actually at that time called the Bureau of Standards, to calibrate the quantum efficiency. You know, thatís a very expensive job. You have to pay them a lot for them to help you calibrate the quantum efficiency. So when Professor Braginsky visited us I mentioned to him about these calibration problems. And he told us, ďActually in Moscow there is a Klyshko technique can be used to calibrate the single photon detectors.Ē So thatís the first time I learned about using the spontaneous parametric down conversion, using photons here.
Bromberg:And that was just as a way to calibrate the detector?
Shih:Thatís right. Thatís the way they were doing, using that to calibrate detectors. Because you have a pair of photons generated, you put one detector with this photon, one detector with photon two, as simple as that, also record the coincidences. And finally the efficiency of detector one or detector two can be calculated from the coincidences and the single photon counting rate. Itís very easy. And they were using the pair [of photons] for this application. I think at that time Professor Klyshko got some kind of award from Moscow for this technology. They have a name called the Klyshko technology, or Klyshko technique, for the single photon detector calibration. And so I was very happy, very excited, and I found a crystal that other people used for second harmonic generation. I used the second harmonic green light to make a down conversion, actually, the spontaneous parametric down conversion. At that time, we learned from Professor Braginsky, they have a different name. They call it spontaneous scattering, spontaneous fluorescence. So actually, in sixties, seventies most of the pioneer workers who worked in this field, they used these names.
Bromberg:So when you first, when Alley, Professor Alley first started the delayed choice experiment he wasnít thinking of using —?
Shih:No, he was not using the spontaneous parametric down conversion for that experiment. They were using — itís a laser light, but reduced intensity. The intensity was reduced finally to a single photon level. They used that for the delayed choice experiment. Later, for the EPR experiment we started to use this down conversion.
Shih:After, I did all the calibration of the single photon detectors in Professor Alleyís lab in one week. So thereís about, I think about six or seven detectors so we were very, very excited. And in that time, I got very interested to this technique. I started to read papers on the spontaneous scattering, the spontaneous fluorescence. I read many of Professor Klyshkoís papers directly.
Bromberg:So was that common that the Soviets were coming over to the United States at that time, or Braginsky was just sort of abnormal or —?
Shih:I think Braginsky had some collaboration with Weber in that time, and both of them were interested in gravitational [wave] detection. Iím not sure. You may ask Professor Alley whatís the program to bring him to the United States.
Bromberg:I will. I will.
Shih:But thatís the first time I learned about the spontaneous scattering or spontaneous fluorescence. So when I read Klyshkoís paper and also later read some other non-linear optics papers about the spontaneous scattering, the spontaneous fluorescence — actually in the States, people also called it spontaneous parametric down conversion — I found that the phase matching condition is really the energy conservation and the momentum conservation. Itís very similar to the atomic cascade decay. That system, people have used for the EPR type of experiment. And so I wondered that maybe we can use this parametric down conversion to do the EPR type experiment. Because in the atomic cascade decay, the collection efficiency — Okay. They do have a momentum conservation, but they are more like — there is a three body. So thereís a recoil of the atoms makes the conservation of the momentum not very well. That means, sometimes you can have 180†degrees, sometimes not. Okay? Because of the recoil of the atom. So they had a very difficult time to collect the pair. The collecting efficiency is pretty low. Thatís considered to be a big loophole.
Bromberg:Just because you donít know quite where itís going to come out.
Shih:Thatís correct. Yeah.
Bromberg:So at that point then you are reading rather deeply into Soviet papers.
Shih:Thatís right. But most of them are Klyshkoís papers. He studied every detail of the spontaneous scattering, the spontaneous fluorescence.
Shih:And he precisely gave the phase matching condition of w1+w2=w pump and that k1+k2=k pump. Also he put a t1=t2. That means the pairs, they are generated at the same time, generation time is the same. So I was looking at, trying to think about the state of the photon pair. If they have this kind of conservation laws, so the state must be the same as the cascade decay Ė except they have a much better conservation, the momentum conservation. So you know where the photon goes exactly. Then you can collect 100 percent of the photon pairs. And that was a considerable advantage to get rid of the collection efficiency loophole problem. I think thatís an advantage. And also itís very easy. You have a laser, you have a crystal, and you can generate a pair immediately. You donít need to spend millions of dollars like our French colleagues [Alain] Aspect, as Aspect did. Itís a huge lab, full of complicated equipment, in order to have the pair generated.
Bromberg:Well why donít we take a minute and get Professor Rubin to come in and start talking about — do you think? — start talking about —
Shih:Maybe, let me finish the story for the down conversion.
Bromberg:Oh. Please do, please do, yes. Iím sorry.
Shih:Then we go back for that. So we started to use — of course I had a discussion with Professor Alley, he agreed. He said, ďYes. Thatís correct. It seems the state looks very similar to the cascade decay, so we should be able to use that.Ē So I made my, we made our decision that we are going to study the EPR experiment by using this spontaneous parametric down conversion for my thesis. So thatís my decision. And we were very happy. So I started to do that. I think thatís Ď83 or Ď84, in that time. I started from type II down conversion. You know, there are two different down conversions.
Bromberg:I know. And I thought that you didnít really get into type II until you came here, but —
Shih:Yes. Let me give you the story. Okay. The type II is, the photon pair have orthogonal polarization. Actually, type II, thatís very — itís ideal for EPR type of measurement. Itís very easy to get a Bell state so you can have (X1Y2+Y1X2). Or X-X, Y-Y, whatever, okay? Very, very straightforward. That experiment attempt, the type II, took me one and a half year without any result.
Shih:We couldnít see any quantum interference or quantum EPR type correlation from that measurement. I was working very hard every day, sometimes even twenty hours, okay, trying to work in the lab, but without any results. It was a very, very difficult time. Unfortunately, at that time I didnít really sit down and try to look at the — try to calculate the —, wave function or something. Later we did, ten years later, and we found out whatís the problem. At that time I didnít. I just tried! I thought probably it was simply the alignment or something else. So after one and a half year still nothing, I almost decided to give up. So that I should really thank Professor Alley. He said, ďNo, we shouldnít,Ē and ďIf the state is the same as cascade decay, we should have that, just think — letís try to think more and try to continue our research.Ē So one day I suddenly thought, ďIf the type II doesnít work, why donít I try type I?Ē So I decided to try type I. Well, it turns out to work immediately. So the basic design was to have the two [members of the] pairs going to different directions, and both of them are x-polarized and we put a r/2 plate to rotate one polarization to y, and then sent those two to meet at a beam-splitter. Of course that is also very difficult, and for some time I didnít get any quantum correlation until I realized that the position of the beam splitter is so important. So we have to really align that to make the pairs have exactly the same optic path. At that time, I put a micrometer under the beam splitter. So what we do is, I have two other people who help me sitting outside the lab because I have to turn the light off, I was sitting inside the lab, tried to slowly moving the micrometer to scan the beam splitter to find first of all, we find the maximum correlation by moving the beam splitter slowly, slowly, slowly. You see the correlation getting higher, higher to maximum here. Okay? Then we drove rotate our analyzer 90 degrees to get the minimum. So thatís the observation to have the ďpeakĒ or ďdipĒ for this kind of measurement. And in that time, we only considered this as an alignment procedure. I didnít think that we should publish a paper on this, because finally we should publish the whole experiment. Thatís the EPR experiment. But this is our alignment procedure. Now finally I found the maximum and the minimum of that position, I mean itís a ďdipĒ or ďpeakĒ that position; we can have a very good EPR correlation. And finally, we get the EPR measurement and also it took us hundreds of hours, tested Bellís inequalities. We got a violation of Bellís inequality, I think probably three to five standard deviations, because at that time we used the pulse laser and the repetition rate was not very high. It really takes several hundred hours to get the measurement done. Thatís the story. We started from type II and that didnít work and tried type I.
Bromberg:Thatís really curious, because I somehow thought it would be the other way around.
Shih:Yeah. I didnít forget type II. I just kept asking myself why the type II doesnít work. I tried later when I got to UMBC and had my own lab and it still doesnít work, so I started to — in that time I was very happy to start my collaboration with Professor Rubin, and I asked him to help me to think about this. And there was still no progress. And later we found out when we use a thinner crystal and a narrow band filter, immediately, we can see the quantum interference. Yes!
Bromberg:Well, what was the problem then with the thicker crystal?
Shih:Okay. Thatís the question to me at that time. So, why the thinner crystal and the narrow band filter can see the interference? Why the thicker one and the wider spectral filter we cannot see the quantum interference? Itís a big question. So we continued to think about that, and in this time Professor Rubin helped us a lot from the theoretical aspect. Then we found that the wave function, in later work, the effective wave function of the type II is very different than type I. Yeah. Because the type II wave function is asymmetric. That means, if we consider the two particles, okay, and we have — type two, the pair generated by spontaneous parametric down conversion. We have a very special wave function. Itís two-dimensional. Itís not one wave packet times another wave packet. This wave function is two-dimensional. So if you look at the t1-t2 direction, then you are going to find out that it starts from zero and going back to one direction and ended like a rectangular. But on one side. So if you have two amplitudes, one is on the right side, another on the left side, you can never get them overlap. If you cannot get overlap, then of course you cannot see any interference. There are two ways to do it. One way is to make that very flat, the wave function very flat. How do you do that? You use a thinner crystal; use a very narrow band filter. So this one gets flat, other one gets flat, at least you have partial overlap. Later we found a better way: we donít need to use thin crystal, we donít need to use the narrow band filter; we need a compensator.
Shih:Yeah. Compensator is, what you do, you physically move this wave packet to the right, the other one to the left. So you know, in the beginning, one is completely on the right, another completely on the left. If you move the right one one-half [width], the left one one-half [width], you can have 100 percent [overlap].
Bromberg:But what is a compensator?
Shih:A compensator is another piece of crystal. You know, the crystal has, it can support two modes of waves. So in the type II we have an orthogonal pair. After they get out of the crystal one of them is delayed. And now we put another crystal in to compensate the delay.
Bromberg:Okay. I get it.
Shih:But to many people, this ďshift backĒ is very surprising. For example if we have a type II crystal that is one millimeter, the second crystal, the compensator, you cannot shift it back with one millimeter. You only need half millimeter. So in the beginning when we published the paper people didnít understand at all. Some of them just said, ďNo, itís impossible. You cannot do that.Ē But from the study of the two-dimensional wave function, itís very reasonable. One-half, one-half. Okay? You move those two wave functions together!
Bromberg:And so itís really a kind of an interplay between the equipment and the study of the wave function.
Shih:Yes, yes, yes. So finally we get a good idea how to do, how to make this kind of type II spontaneous down-conversion work.
Bromberg:And were you also looking at the wave function, or it was mostly Professor Rubin who was looking?
Shih:Oh. We looked at it together. Yeah. And we started to look at the thin crystal and also narrow band filter, and we calculated those wave functions and we found out that how does it behave, and later we look at the thicker crystal and look at broadband filters and what kind of wave function we have. And then we found a way how to do it.
Bromberg:Now, before we leave College Park, were you already — was this already supported by the Office of Naval Research?
Shih:Yeah, at that time I think Professor Alley did get support from ONR, and also from other agencies.
Bromberg:I mean, were you already in contact with Dr. Pilloff at that time?
Shih:Yes, thatís right. Yeah, in that time Dr. Pilloff started to know me. So we had —
Bromberg:So when you were down in College Park.
Shih:Yes. We had a lot of discussions. Heís a very good physicist really. He understood physics deeply, so we had a lot of discussions, and he was very happy that a graduate student knew that much about physics I think. The very interesting story probably you may want to hear that. After we reported this experiment, my thesis experiment, the EPR experiment, in a Japanese conference, Hitachi Conference — thatís back in 1986. I didnít go there, but Professor Alley went there and gave the talk. And at the same time he talked about delayed choice experiment. And thatís Ď86. I think that some other groups also learned that, and I know that Professor Mandelís student tried to repeat our experiment, and also tried to study the same thing too. And in 1987 in the CLEO Conference, Professor Alley and I, we met two of Professor Mandelís students. One is Fredberg. The other is Ghosh. I think the last name is Ghosh. Itís a girl, Indian student.
Bromberg:And the first one was?
Shih:Yeah, Fredberg. The second is Ghosh. In the conference. And they tell us that they have difficulty to get the quantum interference. They are using a similar design as we have in the Japanese conference to do the EPR experiment. They said they couldnít get the quantum interference. So at that time I — okay, I was a student, so I was very open, so I told them this story that itís very important you have to align your beam splitter to exactly in the middle, I mean the same path. You have to move your beam splitter slowly to get the ďdipĒ or ďpeakĒ in that position in order to have the quantum interference. And also told them many other suggestions. I think that, really, that they were very clever. They immediately realized that by moving the beam splitter to get this ďdipĒ is worth a paper. So they published a paper. And now everybody called it Hong-Ou-Mandel ďdipĒ or interferometer.
Bromberg:But in some way that surprises me, because I would think — you know, when I talk to people I sort of tell them my ideas. You would think that science would be a situation where you would tell people all your ideas, but in fact it sounds from you what youíre saying, that you shouldnít quite tell people everything. Is that —?
Shih:I donít know. Actually, I really think they should acknowledge us. Thatís the reason I think Professor Alley was very unhappy. You may ask him. I was a student. I really didnít care too much at that time, right? But he — I think he really, was pretty upset with this. And I think, he was a pretty good friend with Professor Mandel for a long time, and because of this, there has been some problem. But anyway, I donít know the details, but he was very unhappy about this.
Bromberg:So the process really would be itís all right to use it if you said you got this idea from —?
Shih:Yeah, yeah. You have to acknowledge whom you learned from. Thatís the way. After we did the EPR experiment we acknowledged what we learned from Professor Klyshko about the Klyshko technology to get the photon pair. Even though that their idea is different than ours, because we are doing quantum interference. They havenít gotten there yet. The calibration, you donít need to use the interference effect, is not EPR. So we developed that. But we still acknowledged where we learned it.
Bromberg:One of the reasons Iím asking is because one of things people will probably be looking at all these interviews for is what is the social life like within the quantum optics community. What are the rules of behavior? And so I think Iím getting an idea from you, that you can speak, but people ought to acknowledge where they got the ideas.
Bromberg:There was something else I wanted to ask you. Now, during this period from your first contact with Braginsky and then youíre reading of the papers and your contact with the other Soviet people, weíre going through the period where the Soviet Union is coming to an end.
Bromberg:Did that make any difference in all of your contacts, or —?
Shih:I think probably useful for the information exchange, for scientific information exchange.
Bromberg:You think it did help?
Shih:It did help, I think, yes. Itís easy for professors — I think Professor Alley visited Moscow a couple of times.
Bromberg:And that was both before and after?
Shih:Yes, before/after. And I think if the relationship was not improving then itís not easy for those kind of visits.
Bromberg:Those visits. Okay.
Shih:Also, I believe Professor Braginskyís visit is also very — is because of that trend. Otherwise, how can he come to the States to visit us? It did help a lot. And itís important to have these scientific ideas exchanged. I think that we learned the Klyshko technique, the calibration of photon detectors by using the spontaneous parametric down conversion, and but we developed that. We used that as a source for the fundamental quantum mechanical concept study of the EPR paradox. And they didnít have that idea. They didnít have that idea, but we did. And we developed that, and later they learned that and they are trying to do a similar thing. They also have a group in Moscow. Actually I have [had] several visitors working with me in the lab, and they are from the same group, and they gained that back. And also, they developed the technology of the non-linear optics and some others, we learned from them, and this kind of exchange. Itís very helpful really.
Bromberg:And is it Ė did they have rather a different point of view or is it — what Iím really wondering is whether there was a Soviet style doing physics that was a little different from the style you were used to in this country. Or thatís too extreme. Whether it was just like another group that you might have worked with.
Shih:Actually I didnít see too much difference. But Professor Klyshko, I invited him to visit us several times, he worked in the lab with us for several months, and we had many interesting discussions, we had fun together, weíd see some things differently, he always joked about metaphysics. Metaphysics.
Bromberg:And that was a bad word or —?
Shih:I think itís some way he just — itís not a bad word, itís just that itís a different way. He preferred to calculate. Yeah, he wants to calculate and make a calculation and to show it theoretically works or not. Heís a theoretician of course. But I think for us, especially for me, I was a theoretician and I turned to be an experimentalist, I prefer to talk about physics, to try to understand the physics. And then maybe thatís a little bit different.
Bromberg:Is this a time we should go get Professor Rubin?
Shih:Letís go, yeah.
Bromberg:...tell the tape recorder that we are now being joined by Professor Morton Rubin, and we were just — we just wanted to talk about starting the program here. How, first of all, how you came to choose to come to UMBC, what made you decide to choose that and what made you decide to set up a quantum optics program. And one of the things that I am going to need to do is to go over to the administration and get some of their perspective on starting a quantum optics program. So why donít you tell me about how you set it up. If you want to mention any of the people in the administration who were involved in it — if they were involved in it. Maybe they werenít.
Rubin:You want my background here at UMBC? Is that what you want?
Bromberg:Well, no, I really want to start with how you came — I know you went down to College Park and you taught a course in quantum optics. How did you decide here to bring Professor Shih in? How did you decide to move into that line of research? Was it already going on here, that kind of research, or was this a new area that the physics department started to expand into? That kind of thing.
Rubin:Okay. Well, we did have a graduate — well, we had a masterís degree program at the time, but we didnít have a Ph.D. program. We were seriously talking about starting a Ph.D. program. One of the reasons we didnít have a Ph.D. program had to do with, back then there were very stringent conditions on starting programs within the state system. So when our new chair, Dr. Summers, came in, that it with the understanding that there would be some expansion of the department and some attempt to get a doctoral program. I think one of the first searches we did. Our philosophy here has generally been to hire the best person we could find. And we had done a search and we had brought some people in and basically were not terribly enthusiastic about anyone. And Dr. Shih called me and told me he was finishing up and saw that we had a job opening and did I think he should come up and give a talk. And I said, ďBy all means.Ē And so I set up coming up and give a talk. And he gave an absolutely first rate talk, and I think practically unanimously the department thought that this was somebody we would really like to have onboard.
Bromberg:So itís really not a question of choosing a field; you choose a person and take whatever field theyíre —
Rubin:Right. Because I had worked in a number of fields. I started off in particle physics and I worked in the field of thermodynamics and statistical mechanics and scattering theory and so on. And I had offered to teach this course down in College Park simply because I was interested in teaching the graduate courses. As I said, we didnít have a Ph.D. program.
Rubin:And so they had this course available, so I basically kept three pages ahead of them teaching the course. I was not really an expert in quantum optics.
Bromberg:What do you use at that point? That was sort of the late eighties, wasnít it? What kind of textbook was available at that point?
Rubin:We used [Rodney] Loudon. That was I think the only book available at that time. So we used Loudon. When Dr. Shih came up, since I had always been interested in the foundations of quantum mechanics anyway, he came up, we got him startup money to set up a laboratory which is always one of the conditions of bringing in an experimentalist is to make the university give us sufficient startup funds. Were your startup funds given to you all in one year, or was it one of those things that they spread out over two years? I donít remember.
Shih:I think it was one year.
Rubin:Because sometimes when we ask for funds they say, ďYes, weíll give it to youĒ and then it ends up taking, they do it in two years.
Shih:Very limited. Very limited.
Rubin:And so that was basically the start of our collaboration I think. When he came out here I started talking about things and we worked together.
Bromberg:And the administration was pretty liberal with starting a new laboratory?
Rubin:No. The funds were not, I would say, extravagant. Right? They were sort of bare bones.
Bromberg:And then you went to Pilloff on top of that or NSF or —?
Shih:Actually, it started from the second year; I started to receive funding from him. Thatís really a great help in that, because the starting fund, it was very limited and only a laser, and an optic table, thatís it. So Pilloff did give a lot of help to start the research program and also some equipment funding too.
Bromberg:Does that equipment come from any firm thatís of any interest? Is it at all interesting the contact you have with these equipment firms, or is that just like going to Home Depot and buying screws? I mean, itís not really any kind of intellectual input?
Shih:Oh, to buy the equipment?
Bromberg:Yeah. Do you get any ideas from industry?
Shih:First, you have to know what you want. Thatís very important — which direction you want to go. And what kind of laser, there are hundreds of different kinds of lasers, which you want. Especially you have to get the correct one for your research at least in the next ten years. You are not going to get the funding every year!
Bromberg:I see. How much is a laser? I thought they were pretty cheap.
Shih:A laser? My laser is like 100K, 200K, this kind of costing.
Bromberg:I see. Now one of the things that surprises me — and [to Professor Rubin] I donít know if this is true for you also — but when you come here youíre involved in all of these very applied papers that you are working with. I donít know who they are. I want to ask you who they are, but a number of people — Martin-Marietta and —
Bromberg:Then some other company. What were those papers about?
Shih:Well, because we got some help from them. In Maryland we have MIPS program, and thatís, I think, the university has collaboration from companies, local companies. And the company provides some kind of funding and the state provides some kind of funding too. In the beginning itís a very, very difficult time. We try to get all the possible resources, including those funds with the MIPS program. And thatís the reason we have to do some of the very practical research, like studies of crystals and also some phase conjugation, this kind of research. I think I probably have mentioned Dr. [H.Y.] Zhang.
Shih:Yeah. Thatís one of — they are still working with me. Still here. Yeah.
Bromberg:Are they in physics or are they in engineering or —?
Shih:No, they are physicists. They are from China, from Beijing University. And now they are involved with another project. I told you at the beginning my long-term interest, still one part is general relativity. So we are doing some other experiments with Dr. Zhang to study general relativistic effect of the rotating earth. Okay, this kind of study.
Bromberg:I see. So that you are working with them but you are not working these same [applied problems.] I mean, these — I just brought along these [papers] [on applied research].
Shih:Yes, yes, yeah. These papers, yeah.
Bromberg:And does that help you support graduate students?
Shih:Thatís correct. That helps a lot. And mainly we get a lot of equipment from them.
Shih:In the beginning itís very, very difficult. Then Martin-Marrietta gave us some equipment, and for the MIPS program they can simply provide dollars or provide some equipment, and we received both from them.
Rubin:Yes. The Maryland Industrial Partnership.
Shih:Something like that, yeah.
Rubin:And itís a state program.
Shih:Yeah. And the company gives for example we received 50K and the state matched another 50K, so we used that 50K in part for the equipment, in part to support students and the Maryland State matching money for graduate students or some visitors.
Bromberg:And I assume that thatís going to be really something in experimental [work] that they want to do that, from your point of view, you wouldnít be engaged in that kind of thing.
Rubin:We were part of the MIPS program.
Shih:He [Rubin] had to help us to solve some theoretical problems, and he solves them both I think.
Bromberg:Okay. My question was whether there was any payoff from these MIPS projects and the answer was that there is a synergy between the technological things that you learn on these projects and the fundamental research. Because itís quite a contrast between what is the nature of a photon and some of these [applied] papers, which seem extremely focused on one or another [particular problem] — So itís interesting that —
Shih:The development of technology on the non-linear optics part is a tool — like a tool. And weíre using that for the future fundamental study. But in order to have a good study, you need experimentally, a very — you need a very good tool. You have to prepare that. So many of those studies — we did publish a lot of papers in that orientation, you can consider from the fundamental physics point of view, itís a preparation; itís a technology preparation.
Rubin:I donít think a lot of people realize how much practical knowledge experimentalists have to have. Theorists have to know a lot of mathematics, but I still remember my very first advisor as an undergraduate, youíd go in the lab and youíd see him reading advertisements in technical journals.
Rubin:And basically the idea was to know what was available out there. I mean, even such mundane things as what kind of epoxies are available and things like that — things that theorists never really think about unless they are repairing something at home. But experimentalists have this incredible range of knowledge that they have to have in order to not only carry out the experiments, but to know whatís really available and whatís possible to do. And so these sort of technological type things are of great value. I donít think they are marginal value; I really think they are of great value.
Bromberg:So in other words, itís not something one does with reluctance because thereís pressure on you from above, these kinds of cooperations.
Rubin:No. I think the state encourages them, but I wouldnít say that — I have never noticed that — there is any pressure to do them.
Shih:We did this research, and itís useful for the company, but we never lose our direction. We still keep our direction. We know that. We learn a lot of new technologies from those laser physics studies, non-linear optics studies, and also the detectors. And we studied also those detectors, and so — and thatís very helpful for us. You know, we need new technology.
Bromberg:So does the company come to, say, the chairman of the physics department and says — or the company comes to you, or —?
Shih:Well, I think it was very helpful for us in getting the Ph.D. program. In the beginning we didnít have the Ph.D. program, as Mort has just mentioned, and later we needed support from industry to show that this department does need, or [it] is helpful for the state to have a Ph.D. program. And I think Martin-Marrietta did write a very strong supporting letter.
Bromberg:I see. And is there anybody — is there —? What do they call them, these technology transfer offices here or —? Is that involved in any way, or is that something entirely different?
Rubin:I think there is one, but there wasnít one back then. This is a fairly recent innovation. This university has grown very rapidly, and itís grown especially during the time when a lot of places were contracting. Of course, we started off so small and there was a state commitment to make this a research university with a technological base. And so even though our funding is still very small on an absolute scale, we have grown quite rapidly over that period because of this commitment made by the university and to the university.
Bromberg:Let me ask you first: the biphoton idea, does that come out of all the studying you are doing with parametric down conversion type II, is that going to be whatís going on? Because I think one of things we ought to talk about is this concept that you begin to form that you really need to talk about a biphoton wave function, and I wonder what the genealogy of that is and whether it comes out of this experimental-theoretical interaction or what.
Rubin:Well, the term biphoton comes from Klyshko. Thatís been there for some time.
Bromberg:All right. So thatís even before you started to —?
Shih:Actually I would say that for this kind of concept, we have to go back to 1935. Thatís Einstein-Podolsky-Rosen, their paper suggested the first entangled state. We know that this kind of entangled two particle state is different — is very, very different. And Schrödinger defined it as an entangled state, in that this kind of state is, you do have two particles, but since those two particles, you can never factorize [their state] like a state I times state II, those two are always working together in some sense.
Bromberg:Except that they werenít looking at it that way. I mean this is a different way of conceiving it.
Shih:They didnít. Yeah, sure.
Rubin:Schrödinger did actually. Schrödinger in his early paper in response to the EPR paper had quite a bit of insight into all of this. And I think basically a lot of it was forgotten for a long time. There was really no progress made in that direction, and at least when I was a student, I was told not to worry about those fundamental issues because no progress was being made and none was going to be made, and you would end up in a dead end.
Rubin:There was, I would say in the sixties and the seventies there was a general feeling that there was a lot of physics to be done, a lot of things to be understood, but the foundations of quantum mechanics was the ones that people had worried about and made no progress in. And I think if you look back at that time youíll find a very small number of people actually working in that field.
Bromberg:Because already by the seventies youíve got Bell and you had the Bohm stuff in the 1950s.
Rubin:Yeah, but those were really a very small number of people, and Bell had a lot of trouble getting people to pay attention, even after his first papers. I think itís fair to say that it was a very small number of people who reacted to it.
Shih:You know, he could not publish his paper in the ordinary physical journals, in the Physical Review. He published a paper in a journal called Physics. That journal has only one issue. After that issue it was dead. But this journal got so famous because of his paper. Physics, okay.
Bromberg:Well, show me how you came to your understanding of how to think about entangled photons.
Shih:Okay. Actually I think in the early days that — you know there are two sides. You know that, right? There is an argument first between Einstein and Bohr.
Shih:And Einstein raised the question, Bohr gave answers, later they still — Itís really about the physical reality — You have a two-particle system and for example that if you look at particle 1 it does not have any polarization for example, Particle 2 does not have any polarization too. But if you measure that particle 1 is polarized in a certain direction, the other one must be opposite, or in a certain direction. You know that immediately. So the question was asked, ďDo you think that the photon before the measurement, during the course of the propagation, does each of the photons have a definite polarization?Ē Okay, so for Einstein there must be, but for quantum mechanics the answer is no. You donít know until you measure it. And I think that Wheeler gave a very good summary that, ďNo quantum phenomenon is a phenomenon until it is recorded or registered [phenomena].Ē
Rubin:I think that actually goes back to Bohr.
Shih:Yeah, yeah. He said on his lecture: ďBohr in todayís words. Bohr, in todayís wordsĒ. Yeah.
Rubin:But itís interesting, because Schrödinger already, in his famous cat problem paper, already understood what the problem was, and if you read the cat [paper], he ends up by pointing out that the final result is ridiculous. And there is no way, there doesnít seem to be any clear way out of it. That youíve got this problem of these correlations turning up, but each single particle is not in a definite state. And this is of course what disturbed Einstein so much; the notion that the particle was not, each of these two particles which were very far apart, were in no definite state in there. When you made a measurement — bang — they went into a definite state.
Bromberg:So then at what point did you start struggling with that whole problem here.
Shih:From the beginning, when I did the thesis I studied a lot of historical papers and those things. I felt that on one side people said, ďWell, the quantum mechanics definitely works.Ē Okay? You do have this kind of entangled system, two-particle entangled state. You do have this kind of system. And on other side, everybody was talking about this particle and about that particle. So actually I think, probably I think that finally we realized itís really one system. That two is not one plus one in this entangled state. Itís not one particle against another particle. What the entangled state tells us is only the correlation. There is no individual description for individual particles.
Rubin:I think itís fair to say that the way it started up here was, in your thesis work you had done this type II experiment.
Rubin:And the type II you had not seen could not see any interference.
Shih:Yeah, thatís correct.
Rubin:And so he came and we talked about the problem, and when I sat down and calculated it and started looking at it from the space-time point of view we came to the realization that the problem that they were having was that it wasnít that the state was okay; it was that they just hadnít overlapped them in a way to see the interference. And once we had that picture I think you and Sasha Sergienko came up with a clever way to overlap the wave packets and to show that you indeed had this interference. And so that was really the start here I think, and that was I think the breakthrough experiment here.
Shih:I think itís really — if Iím not mistaken, itís from our study, we were first trying to see that, you just cannot think this entangled two-particle system is one particle against another particle. You cannot think there are two individual particles. And you have to think, ďWhatís the correlation between those two?Ē So when you think of the physics and Mort calculated the effective wave function, when you look at that, you donít have two-wave packets; you have only one two-dimensional wave packet. Thatís very important. Immediately you see that, ďWell, this is one unit, not two.Ē
Bromberg:Now how did — what kind of reactions are you getting to that? Because itís fairly recent, so I suppose you are getting reactions.
Shih:Well, itís very interesting.
Shih:[laughs] When we first tell other people, nobody believed us. Okay? They said, ďYou guys are crazy.Ē
Bromberg:For example when you give it at a conference?
Shih:Yeah, we gave a talk in conference. Even our first paper was almost rejected. Right? And when they looked after this, very carefully, read our paper, they said, ďNo, this is well known. Everybody knows that.Ē
Rubin:Yes. It was one of those peculiar things. You know, ďyouíre wrong,Ē ďyouíre wrong,Ē ďyouíre wrong,Ē ďitís obvious.Ē
Rubin:I think that the funniest incident with that was Toddís delayed choice.
Shih:Yeah. Todd. Yeah. That paper. Yeah. We sent a paper to Physical Review Letters that, ďCan Two-Photon Interference Be Considered Interference of Two Photons?Ē
Shih:Okay. Basically that paper, and for instance they say, ďYouíre wrong,Ē ďyouíre wrong.Ē Then they started to say, ďWell, okay, thatís trivial.Ē Okay? ďEverybody knows that already.Ē So then Todd sent them back a letter to find out about a hundred —
Rubin:No, it wasnít that many.
Shih:Many, many — many, many statements.
Bromberg:When you say they are saying ďyouíre wrong,Ē ďyouíre wrong,Ē is that the referees who were saying this?
Shih:The referees. Yeah, the referees said, first they say, ďYou are wrong,Ē ďyou are wrong,Ē ďyou are wrong.Ē Then when they understood what you said, itís, ďwell, itís trivial.Ē Then they said, ďEverybody has this. We understand already.Ē Okay? You know, so Todd found out many other peopleís papers —
Shih:Including ours — an early paper. So, ďwe caught this photon and that photon and blah-blah-blah.Ē We are trying to think about them as individuals. So he sent back to the —
Rubin:It was a list of all the leading people in the field.
Rubin:And as soon as he sent that, the referee gave up.
Shih:After. Okay. So they accept the paper after that.
Rubin:See, the issue —
Shih:Thatís the situation.
Rubin:Yeah. The issue in that particular paper was a lot of people were thinking that to see interference, the two photons had to get at the beam splitter at the same time.
Shih:Has to meet, yeah, in the beam splitter.
Rubin:And we never believed that. And I think it was Todd who came up, Todd Pittman who came up with a way of actually doing an experiment in which you could — if you believed that these were really two separate wave packets; if you believed that — they would arrive at the beam splitter at different times. But yet after the beam splitter, you could reconstruct the interference. Okay, so that was the paper that went in and that the referee sent back and said, you know, ďThis is obvious, and everybody knows this.Ē One referee said that, and so thatís when Todd drew up this list. Actually it was amusing. I wonít mention a name, but probably one of the most famous people in the field in a conversation with us saying, ďNo, no. Youíre wrong.Ē ďItís obvious.Ē
Bromberg:I do want to ask. This is the wrong question to ask at this particular moment, but this is the moment I am going to ask it. In terms of groups internationally with whom you are interacting, I am assuming that one group is the Berkeley group and one would be the Rochester group. Is that a correct assumption?
Shih:You mean working on the same problem?
Bromberg:Working in the same areas in the nineties.
Bromberg:And what other groups?
Shih:In Europe there are also DiMattini.
Shih:Italy, in Italy DiMattini. And also —
Shih:And also Zeilinger.
Rubin:At that time he was in —
Shih:Innsbruck. Actually, to work on the spontaneous parametric down conversion, those two groups they learned from us, from here. Yeah. And we sent people, even sent our non-linear crystal to them and then they could learn the technology from us.
Bromberg:So the universe in which you are operating and interacting with people — Iíve got Berkeley, Rochester, the Italian group. Where are they?
Shih:University of Rome.
Bromberg:And Zeilinger is in —?
Shih:University of Innsbruck in that time.
Rubin:At that time. There was also an Englishman.
Bromberg:That guy Rarity and Tapster?
Shih:Okay. Rarity and Tapster. Yes, thatís correct. Rarity and Tapster.
Bromberg:They are at one of the government labs, arenít they?
Shih:Yes. Government. When — I think since back in Ď94, Ď95, and we started to — at least I started to — give talks in the conference, trying to sell the idea two is not one plus one. You know, after that paper, ďCan Two-Photon Interference Be Considered Interference of Two Photons?Ē that Science had a short article [on it]. They wrote ďone plus one is not twoĒ, to introduce our experiment. So actually, I called the editor and said, ďWell, I think that youíd better change that to Ďtwo is not one plus oneí.Ē He said, ďWhatís the difference?Ē Actually this is different. [laughs] What we are saying is the two-particle system is not one system against another individual system. Okay? We are trying to say that in that way the physics may better and so on. But in the beginning a lot of people said that, ďYou guys are crazy. What you are saying doesnít make any sense.Ē But I think recently more and more people start to accept start to see, what is the difference.
Rubin:Yeah. I think once the entanglement idea started to spread around and the people started looking at quantum information theory, I think the idea that the states were not separable gained much more acceptance in the field. I think that the information theory peoplesí way of looking at it, I think it enlightened a lot of people.
Bromberg:Are there other schools of interpretation that are sort of rival schools to you? Are there other people putting up important —?
Rubin:Thereís a group — well, I guess in Spain is Santosí group, and people who work with him, they have tried to interpret the quantum state that comes out of down conversion in a classical way. And they periodically write papers. In fact Iíve got one from some people who have worked with that group recently. That tried to give a more or less realistic classical interpretation of down conversion. And they publish some of their works. I mean some of their works, although they are controversial, they still get published. But I think by and large they have not convinced many people. So there is a group like that. But I think right now the mainstream view is the same view we have.
Shih:I am still not 100 percent sure. Because if you look at the papers, other peopleís papers, they still keep the same way of thinking — [there is] one particle and another particle. And especially during the conference and you are talking about this two-photon wave function or two is not one plus one, you get people that look at you like you are doing something wrong or something that really just they are pretty surprised to learn about. But I think the physics, we had a physical idea: ďThis entangled two-particle system is very different.Ē Okay? You cannot separate them into two. All of measurements we did was really the correlation between the two particles. We are not measuring one; we are not studying one particle. We are talking about the correlation. Those kinds of measurements. And not until Mort Rubin provided a very clear mathematical picture. He calculated it. So we can see very clearly what kind of two-dimensional wave packet we have. And I think thatís only, in the physical understanding, after he did the calculation we have a beautiful two-dimensional wave packet to show people. Now I think that will give us something to convince people, really, they can see the picture and then they can have a better understanding.
Bromberg:So which —? I donít know if I have that paper with me, so which paper was that, which you think was a crucial paper?
Shih:Just that theory paper.
Bromberg:Uh-huh [affirmative]. So thatís this December 1994 paper.
Shih:Yeah. Thatís right. Yeah. And here we have a very detailed calculation to show people. You can see the wave packet, and of course the wave packet —
Bromberg:Was that important also for your being able to make the notion more concrete, this calculation?
Rubin:Oh, I think itís fair to say that it certainly helped in formulating further experiments. It certainly helped clarify what sorts of things had to be done in order to see the effects.
Bromberg:And thatís the paper Iím just telling the tape recorder, ďTheory of Two-Photon Entanglement in Type II Optical Parametric Down Conversion. Ē Itís a Physical Review paper in December Ď94. Okay. Now I donít know how much more time you have, and Iíll tell you some of the things I want to ask and you tell me how much time you have. And I wanted to ask about all these optical experiments with ghost images and ghost interferences. I wanted to ask about some of this recent applied work. Maybe Iíll turn this off for just a moment. [tape turned off, then back on...] But also recently I mean youíve gotten involved with stuff like quantum lithography and teleportation. So thatís one of the things I wanted to ask about. And I donít know how much it would be worth pursuing the ONR conferences on quantum optics that I understand you organized from here. And as I say, I donít know what kind of time you have. Some of these things we could really, you know, maybe schedule another hour in a couple of months or something like that if you thought it made more sense. So tell me what you think.
Shih:Actually to study this two-particle entangled state and try to understand the physics of that, especially the non-local problem — non-locality, this kind of problem — that we found there are two approaches in trying to probe the foundations of the physics. One is following the Bellís inequalities. Another is really from the understanding of the uncertainty principle, the complementarity. From that and in another way, a different approach, a different way to see that two is not one plus one is to understand that very peculiar, very surprising behavior of a two-particle system. For example, in a two-particle system you can do an experiment, if you try to interpret that experiment from a single particle point of view, two individual systems, you have a violation of uncertainty. But if you really treat that two-particle as a [single] system, you are not going to have any violation. Thatís the big difference. So weíre trying — this is; I think, only our group is trying to do the experiment and study in that direction, trying to understand the foundation of this concept. The first experiment I will say that we did is [Karl] Popper experiment.
Rubin:The first one was the image transfer.
Shih:Oh. Iím sorry. Yeah. Okay, okay. The image. The ghost interference, the ghost image, thatís where we were trying to study the non-local behavior of the two-particle system.
Shih:Thatís very successful.
Rubin:Yeah. When you say ďapplied,Ē it depends on what you mean by applied. I mean a lot of these things are things that might be ultimately of interest in quantum computing and quantum cryptography. For example the original image transfer experiment in which UMBC was Ė youíve seen that picture — that was done, I think we were sort of semi-motivated by cryptography. But it turns out that there was interest, and that was when NIST got interested, right? And Alan [Migdal] got interested, and wasnít that — or was that the NRL? That was NRL that got interested in it.
Rubin:Yeah. They were interested in that.
Bromberg:In the ghost?
Rubin:Well, the original was the image transfer of that.
Rubin:Yeah, that UMBC image that we transferred. And that — people got interested in it and started working on, from a real practical point of view I think. And so that was the first of the imaging experiments.
Shih:Yeah, if it was non-local behavior we can study from these Bellís inequality type of measurement or EPR type of measurements.
Shih:We got this ghost image and ghost interference. Itís very — you can see the non-local behavior very clearly and immediately.
Bromberg:Now Iím confused, because you did the experiment first and then these other people got interested, or —?
Shih:Yes, yes, yeah, yeah.
Bromberg:So they were — Okay. It wasnít that you were inspired by cryptography to do the experiment.
Shih:No, no, no, no, no, no, no. We did this with another purpose. Our purpose, main purpose, is trying to study the non-local behavior, the fundamental concept.
Shih:Yeah. And after we did the experiment, the Navy found this very useful. Even we received a name from them. They call it the ďcrypto faxĒ machine.
Bromberg:And all these new — I mean, if you look at your papers youíve got ONR funding, ONR funding —
Shih:Thatís right, thatís right, yeah.
Bromberg:And then at the very end new agencies appear in your acknowledgments.
Shih:Yeah, thatís right. Yeah.
Bromberg:One of them I guess is this ARDA, this —
Shih:National Security Agency Institute.
Bromberg:Is ARDA part of the National Security Agency?
Shih:Thatís correct. Thatís one of their programs.
Rubin:Iím not sure exactly. ARDA is sort of an Army research —
Shih:No, no. ARDA is Advanced Research, something like that — Research and Development. Itís under the National Security Agency. Itís under them, but more open, independent — open program to the public, you see.
Rubin:Yeah. They got interested. Their interest started off with quantum computing. And they knew that the group here was one of the leaders in working with these tools. And so they were interested in knowing what we were doing and ended up supporting our research.
Bromberg:And Strekalov was he a graduate student or was he a —?
Shih:He was a graduate student. He did his thesis with me. And now heís in JPL.
Shih:Jet Propulsion Laboratory.
Bromberg:So thatís one of the things I really want to know. Where does, where do your students go? Pittman went to Applied Physics Lab [to work with Franson] and Strekalov went to Jet Propulsion Laboratory.
Rubin:Heís there now, but before thatĖ
Shih:He is there. Before he was post doctorate at New York.
Shih:Yeah, NYU first and then later —
Shih:Yeah. And then Rice the second year. After that he joined JPL. A current student, Yoon-Ho Kim, he did the teleportation experiment. Actually, he just got an award, the Wigner fellowship.
Rubin:Oh, they gave him —?
Shih:Yeah. Wigner. Yeah.
Bromberg:Where is that? Where do you take the —?
Shih:Oak Ridge. Yeah, yeah. Heís going to join Oak Ridge National Lab, and he has turned down two offers from MIT. [laughs] The reason he decided to take Oak Ridge, because basically theyíre going to give him an independent lab.
Bromberg:Does Oak Ridge do quantum optics?
Rubin:Theyíre getting into the business.
Shih:It started. Yeah. Thatís their new program, Quantum Optics Laboratory, and Yoon-Ho basically is going to be the person in charge. Yeah. Iím not sure — probably next year they are going to give him the title of director or something.
Bromberg:So your students really pretty much go into government laboratories and into academics. They donít —
Rubin:Well, the theory students donít.
Rubin:Kim Keller was a part-time student there. He works for Martin-Marietta. And so he got his degree and is still at Martin-Marietta.
Bromberg:Okay. And does Martin-Marietta do anything connected with these?
Rubin:No, not directly. He has tried to convince them, in some of their advanced research stuff to do that, but industry doesnít look that far down the road as these projects. At least people think they are fairly far down the road. But you know industry tends to have a five-year horizon.
Shih:Even if they do they are supposed not to — they cannot tell us.
Rubin:Iím sure there are things going on that are classified.
Shih:Classified or a commercial secret.
Bromberg:I mean even these things like the lithography thing that youíre doing, that the Jet Propulsion Laboratory people are doing there, industry is not yet at a point where they are having any interest in these things?
Shih:I know that some of the companies have contact with JPL, with Jon Dowling, but Iím not sure if they want to use that directly or are just trying to learn.
Rubin:Yeah. I think companies do tend to keep their ear to the ground about these sorts of things, but I think they are basically happy to let the government sponsor the work and to see if there is any application they can get into.
Bromberg:Well what about all these very interesting new experimental arrangements with these filters and with detectors and with that kind of thing — interferometers, which I guess are really very important industrially. Do companies come around and —?
Bromberg:When you say NIST, the National Institute of —
Rubin:Standards and Technology.
Bromberg:Which is, again, is a government laboratory. But I guess itís more tightly connected with industry.
Rubin:Yeah. And I think some of the work theyíre doing has some metrical applications and some standards applications. So thatís their thrust. But I think itís fair to say that Alan started his lab after the collaboration with us.
Shih:Alan Migdal. Alan Migdal. M-i-g-d-
Shih:-a-l. Yeah. And basically we did a technology transfer to his lab too, so he set up everything there and is doing some research, itís applied, like trying to calibrate detectors and also calibrate the intensity of IR field.
Bromberg:Thereís no little startup company thatís coming in and saying, ďMy God, what a beautiful interferometer. Weíre going to manufacture thisĒ? Thatís not happening yet?
Shih:[laughs] Not yet. No. Not yet.
Bromberg:I did ask Professor Franson, Dr. Franson, and it sounded as if some people are actually interested in his interferometer. Thatís if I got a true impression.
Rubin:That may well be. I think the most useful version of his interferometer is the one that was developed here. See, in his interferometer the way he originally did it — and we had actually a lot of fun during that time, because he would come up and discuss it with us. It was a lot of fun actually. But his original interferometer really is a space-time interferometer and depends on timing. And in fact basically half of the amplitudes have to be turned away. They have a fairly narrow time limit. The interferometer version of it that was developed here exploited the polarization in the type II down conversion and ended up so that basically we didnít have to waste half the amplitude.
Shih:Todd did it.
Rubin:Todd. So thatís the — my opinion is that here is the best and most useful version of the Franson interferometer — without taking anything away from Jimís [Jim Fransonís] contribution.
Bromberg:Well I noticed that that was one of the earliest pieces of work you did here, work on that interferometer.
Shih:Actually, the Navy considered using this ghost image to do some crypto type of text-FAX type of a machine. Probably they did. And some of the companies I think also got interested, but I donít know finally whatís the result.
Rubin:That research I think was classified.
Bromberg:Okay. So I wonít try to ask about that.
Rubin:Well, it canít hurt to ask about it. We wonít tell you, but —
Shih:Weíre supposed not to know anyway. I started talking about this other approach to understand this fundamental difference between a two-particle system, an entangled two particle system and two individual particles is from the uncertainty principle. Probably you know Karl Popper. Popper in 1934, he proposed some idea, the Popper experiment, and he said that in this kind of entangled system if you follow the analysis of ordinary thinking you may get a violation of uncertainty principle. So we did the experiment and we found what he said is true — really that the diffraction pattern we measured from the two-photon correlation measurement is getting narrow. Narrower than the diffraction pattern of a single light quantum.
Bromberg:Where did you come across this 1934 paper? How did it come to your attention?
Shih:Oh. Popper kept talking about this in the field. And I learned the Popperís experiment, I think first from Augusto Garuccio, a visitor from Italy. His thesis advisor was Popperís friend. And so we had some discussion about Popperís experiment and I learned from him and —
Bromberg:He actually I think co-authored a paper with Popper at one time.
Shih:I think yeah, probably, yeah. So I got several papers of their early publications, including Popperís paper, and I tried to study that. I thought, ďWell, there must be a way to do it.Ē And in original proposal of Popper, he proposed to generate, to create that two-particle state from a point source. And people criticized that. ďIt cannot be done,Ē because a point source can never get exactly the momentum conservation. But actually we found that itís not necessary to use the point light source; we can use the ghost image. So we have a slit there we can image, so we can have the same experimented conditions required for Popperís experiment. Itís the position entanglement.
Bromberg:So when was it that Garuccio... When did he begin to talk to you about this?
Shih:I think back in 1995-1996?
Rubin:He comes here quite often.
Bromberg:So you were already working with these ghost images at that point.
Shih:We finished already.
Shih:Yeah. And then I realized immediately we could use this ghost image to do Popperís experiment. So we did the Popper experiment and we found, yes, itís true! The diffraction pattern had gotten narrowed down. So we sent to Physical Review Letters, and it was rejected. Three referees. All of them said, ďThis is a violation of the uncertainty principle. It cannot be published.Ē They said, ďThis is wrong. Basically itís wrong.Ē
Bromberg:You sent one of these papers to the Foundations of Physics.
Shih:Okay. Finally it was published there.
Bromberg:Oh, that was that paper?
Shih:Yeah. Thatís the paper. So finally —
Bromberg:How did you decide to send it, what made you decide to send it there?
Shih:Well, first we decided to fight with Physical Review Letters. And also I think, after, Yoon-Ho Kim also sent it to Nature. Nature also rejected it and said, ďWell,Ē basically the referees said they just donít like Popperís idea at all. They donít care what these formulas do; they just donít like it. Yeah. It was rejected.
Rubin:Popper was basically, wrong. I donít think, he is given much credibility in the physics community, but his school of philosophers and scientists kept this thing alive.
Bromberg:Well I think of him as connected with Vigier and people like that.
Rubin:Yeah. There was — again, itís a small group, but I think the general feeling was that his physics wasnít right and I donít think that many people paid much attention to him. To that particular issue.
Shih:Yeah. Probably his physics concept is wrong and also his philosophy may be wrong, but what he said precisely about the prediction of this experiment was correct. And what he said, that you are going to have a narrowed diffraction pattern. We did. We measured that. And of course we found that itís very important: you have to interpret that correctly. So if you think about this as one particle another particle individually, you base on that concept to give interpretation, you get a violation of the uncertainty principle. But if you think is that two is not one plus one, you donít have any violation.
Bromberg:Well, what kind of reaction did you get? Because I should think that would be a whole bunch of, a different bunch of readers for Foundations of Physics. I mean, a lot of philosophers publish there?
Shih:Well, yes, thatís correct. After we published that, and I can tell from giving a talk in conferences, I have been giving talks on Popperís experiment at least for three conferences, most of the people donít believe it. They donít believe me?!
Bromberg:That the experimental results are?
Shih:Okay. They said, ďOkay, we see this. You have an experiment. Okay. We trust you because you guys, you have your reputation and we know how careful your experiments are. We trust your experimental result, but we think there is something wrong. It cannot be correct.Ē And [addressing Rubin] you remember when we had a conference here, I gave a talk. Basically everyone, the leaders in this field, was trying to argue with me. They said, ďThere must be something wrong there.Ē Okay? But I think that thatís the reason we got rejected from Physical Review Letters, from Nature. Yoon-Ho was trying to fight and the Physical Review Letters just finally said, ďNo. We are not going to publish it.Ē Thatís okay. He was trying to fight with Nature. But in that time I met another physicist in a conference. He said, ďWell, you guys got a difficult-to-publish paper. Donít worry. Iíll do it. I can publish it.Ē [laughs] So Yoon-Ho was very nervous and said, ďOkay. Letís send to Foundations of Physics.Ē
Bromberg:And that was this man who said ďI can publishĒ? He was the one who was with Foundations of Physics?
Shih:No, no, no. Not that one. Thatís an experimentalist. I donít want to mention his name. But anyway, Yoon-Ho got scared that other people were going to do it, publish it, so he decided to send it to the Foundations of Physics. We decided to send it there.
Bromberg:Is that a magazine you read pretty regularly?
Shih:No. Not —
Rubin:Not very regularly. Iím called to referee papers in there from time to time.
Shih:Yeah. I was a referee too. So thatís the reason we thought, ďOkay. This magazine usually publishes the very fundamental type of research and itís a radical experiment so they may consider it.Ē We sent it to them, and in the beginning they also had a lot of problems. And finally we know who is the referee, and mainly was arguing about the point source, so we convinced this person to believe us that what we did is different than Popperís original idea, so it was accepted and published. I personally think that is one of the most important experimental papers we did in this group. Thatís a very different point of view for trying to understand that two is not one-plus-one. Very important paper. And based on the Popperís experiment, we did another with the title: ďQuantum Lithography.Ē
Bromberg:And that grows out of the Popper?
Shih:Yeah. Thatís basically very similar to Popperís experiment. Now the quantum lithography paper, we had no problem. They accepted it at Physical Review Letters. Itís going to be appearing in the next couple of issues, I believe. Now this time nobody argues with us this is a violation of uncertainty principle.
Rubin:Yes. But I think that the lithography stuff was floating around, had some other groups interested in it and some other people had had it. And I think that basically the lithography, I would say probably draws its line of development more directly from the entanglement stuff that came out of quantum information theory. As one of its roots. And I think by the time you get to that point more people understood what was going on.
Shih:I think this is because some other people published their theory paper and they got attention from the field.
Shih:And it was a pretty hot topic. And this argument has been done between the theory paper and the other people who read it, the people who have seen it.
Rubin:Yeah, theory. Thereís a lot of theoretical stuff thatís being produced. You know, there are a lot of ideas floating around, but I think they get published too easily in some sense because whether you can actually do the experiments or not is often overlooked, and so people publish all kinds of ideas with very idealized systems. Iím not sure that they should be published.
Bromberg:Itís interesting; because what you were telling me just now is that itís not so easy to get something published.
Rubin:No. Itís not easy to get something published that is genuinely new. Thatís really — that really shows something different.
Bromberg:But these other papers are just not new enough to get a lot of serious obstruction?
Rubin:No. Since basically the quantum information stuff has started to appear more broadly in the literature. A lot more — if you look at Physical Review Letters you can see that there is a period of growth in this sort of speculative paper, and you know people are publishing all kinds of speculations, how to do this, how to do that. We tend to be a little more conservative; we tend to publish papers that there is some chance of actually doing the experiments. Before the Millennium comes or something.
Shih:I think what Mort was saying, is that if you are doing research, you are thinking in a way thatís very different than most of the others. Itís sometimes very difficult. I can tell you a story. Actually there has been a theoretical paper published on lithography by Jon Dowling at JPL. And Dowling got very hot, very popular, very famous for that paper. He had difficulty publishing his paper. And I was the final referee. The editor sent it to me. Okay. Basically there were two referees against publishing his paper. But another one said, ďIt seems itís okay.Ē It doesnít say that itís useful. Itís probably not useful at all, but he saw that for a theory paper maybe it should be published. There were two against one. But I think the editor probably felt it was a very interesting paper. He sent it to me. Of course I said, ďThis is a very important paper.Ē It is more likely that somebody considered it a violation on the uncertainty principle, but actually it is not, because basically the measurements are on a two-particle system and I said to the editor, ďHow many people think it is useful in the beginning when Faraday was playing with his wires?Ē ďHow many people think itís very useful?Ē And nowadays everybody knows itís important. I think that sentence may [have played] a very important role: after that [the] paper was accepted.
Bromberg:Is that the one with Boto and so on?
Shih:Yes, yes, thatís the one. Yeah. I was the final referee that made that paper published. And of course after that paper has been typed out on the press, whatever, okay, everybody knows about quantum lithography, our paper is easy to be accepted. And if you look at the paper, basically there is not too much difference between the designs, especially see this. [Refers to Figure 2 of Yoon-Ho Kim and Yanhua Shih, ďExperimental Realization of Popperís Experiment....Ē.] These are Popperís... these are Popperís experiment. Sam Braunstein was very clever. I saw him at a conference and he said, ďAs soon as you look at the Figure 2 of Popperís experiment, you saw that!Ē You know that we have such a difficulty to publish the Popperís experiment but after the other paper was published, it was very easy.
Bromberg:Now here you say that everything depends on getting a film thatís sensitive only to two-photon light. Is that a practical thing you can do?
Shih:Yes, yes, yes. According to the chemists itís not difficult.
Bromberg:What chemists do you talk to when you —?
Shih:Okay. In JPL they have a huge department that is doing that. The two-photon resistance.
Shih:And they are doing that. Yes. Actually when we started this experiment my student, former student, Strekolov was here. He brought a sample of that material into our lab.
Shih:And we had a deal with them if that sample worked we can have the image on the film and their name is going to be here for this paper. If it doesnít work, they are not going to be on it. We were trying to push them to work harder. But unfortunately, it didnít work, so we didnít put their name here. [laughs]
Rubin:There are substances that are two-photon-sensitive. I mean there is a whole field of two-photon spectroscopy, but thatís a field that depends on intense beams. Whatís hard about this particular problem is that you are really operating on a photon level. In some sense if youíve got to put two photons on the same atom or the same molecule, the way they do it in two photon spectroscopy is they use very intense beams. So that the statistics of actually getting two photons to hit a system becomes high simply because you have huge numbers of photons. This is a little bit more complicated; in fact, itís a lot more complicated. One has to do this for two-photon objects. You go into the two photon level. If you want to try to actually make this into a real system thatís useful where you want more than two photons, thatís going to become very tricky — both the materials aspect and the physics aspect of it. You asked before about what you refer to as the more practical stuff we are doing now. Thatís really kind of ďpracticalĒ in quotes. The idea is to try to find these more intense sources of quantum states that one can then exploit to do some of these more exotic type things — which may have some technological application or may not. It depends on how they stack up against standard competitors. But —
Bromberg:And weíre still very much in the realm of fundamental physics with all of this stuff. Weíre not yet really talking about technology.
Rubin:Yeah. Though, you know quantum cryptography is already being very seriously considered as an application. I was always brought up on the fact that there was something like a 20-year lag between what was done in the laboratory and its applications in industry. And that was often explained as the time it took to [develop] students in a laboratory — students ending as professionals in industry. I think that the time scale is probably shortened some on one hand, but itís probably not going to change on the other hand because the technology is so difficult. On the one hand I think things which are doable, like quantum cryptography, are probably going to get used much more rapidly than a 20-year time span. But a lot of these things, like lithography and so on, are not — unless there are real major breakthroughs, probably do have such a time scale. And I think a lot of people have come to the conclusion that certainly quantum computing is down the road.
Bromberg:Well, I think itís interesting. There are two things that I really wanted to ask in that connection. One is, youíve got all these new sponsors now like ARDA. Are they pushing on you?
Bromberg:They are like Pilloff was? He do just said, ďJust something interestingĒ?
Rubin:No. Pilloff is I think a very farsighted and unique person in the sense that I think he sawĖ You see, he saw a lot farther than I think a lot of people did. Itís always very impressive that there are people who are in government who are very capable people and they have this incredible foresight and then they are kind of willing to sell their bosses on supporting these kinds of things. I think when programs begin to get a certain dollar size, however, they undergo more scrutiny. And I think thatís happening now with the quantum computing stuff. I think there are some issues now that are beginning to come down from above to the people who have been kind of nurturing the field from the government side, telling them that, ďWhen are we going to start seeing practical results? When are we going to start seeing useful things?Ē And I think a lot of these government people in these levels who we deal with fight the good fight. They try to convince their bosses that this stuff needs to be done and that it does have a long time horizon. But this is a constant battle. And I think there is — if you look at the way research dollars are going, they are now trying to say — Well, okay. They are putting more toward solid-state in hopes that those devices may be useful. But —
Bromberg:But anyway, you are not feeling any particular push to spend more or to do more in one direction or another from these contracts? Not really.
Shih:I guess we didnít. We are pretty free to do whatever we want.
Bromberg:But then on the other hand itís kind of interesting that in your own heads you make these connections then, because you go from the study of uncertainty principle and the nature of entanglement to quantum lithography, which means that in some way within you, you can see both the technology and the fundamental physics.
Shih:Actually, I remember Mort mentioned that the first talk I gave here, that of my interview. In that interview I draw a conclusion that the more fundamental, you have more application. I think thatís still true. Okay? If you are really doing a very fundamental new research, that you can find a lot of useful applications. If you look at for example this lithography. Look at this interference pattern. Now the diffraction pattern is normal. Itís a single particle, single photon, and you get a minimum width. When you use two [photon] you can narrow it down. That means, for some people, well, you can violate the uncertainty principle by a factor of two. That is basically, if we sent this paper before that — I mean before the Popperís experiment — we may have trouble. You see, we argue that this, following Popperís argument, if you interpret it as a one [particle], you get a violation of the uncertainty principle, you may have a big problem! Now this is the lithography — something easier for them to see. But for us we see the fundamentals! Itís very, very important here. Now here we specially mention that this is a quantum mechanical two photon phenomena but not a violation of the uncertainty principle.
Bromberg:Well, I would suggest that we stop and get this — [tape turned off, then back on...] Professor Shih was just saying that he was planning to go back to Shanghai, to the Institute of Optics, where Alley, [Carroll] Alley had gotten you a job, and you would have had an unlimited budget to set up a laboratory, but it was 1989 and the Tiananmen Square incident had happened... So you needed a job in the United States. And phoned Professor Rubin — and took the job.
Shih:Yeah. And also Professor Alley helped too. He didnít want me to go too far from College Park.
Bromberg:Right. That was an important part of it. And you were quite willing to obey.
Yes, yes. There was a very interesting story that Professor Wheeler wrote a recommendation letter for me. Actually he dictated it, a recommendation letter. He said to the chair that the secretary should type his recommendation letter, itís a long letter, and he said, his recommendation letter was, ďHire him. — John Wheeler.Ē
 See, for example, figure 5 in Y.H. Shih, D.V. Strekalov, and T.D. Pittman, ďWhy Two-Thoton but Not Two Photons?Ē in G. Hunter et al eds., Causality and Locality in Modern Physics (Kluwer Academic Publishers, 1998), p. 416
 Morton H. Rubin, David N. Klysho, Y.H. Shih and a.V. Sergienko, ďTheory of Two-Photon Entanglement in Type II Optical Parametric Down-Conversion,Ē Physical Review A Vol. 50, Number 6, December 1994, 5122-5133
 Y.H. Kim and Y.H. Shih, ďExperimental Demonstration of Popperís Experiment: Violation of the Uncertainty Principle?Ē Foundation of Physics 29 1849 (1999)
 Yoon-Ho Kim and Yan Hua Shih, ďExperimental Realization of Popperís Experiment: Violation of the Uncertainty Principle?Ē Foundations of Physics Vol. 29, No. 12 (1999), 1849-1861
 Agedi N. Boto et. Al., ďQuantum Interferometric Optical Lithography: Exploiting Entanglement to Beat the Diffraction Limin,Ē Physical Review Letters 85 #13 (25 Sept. 2000) 2733-2736
 Refers to Milena DíAngelo, Maria V. Chekhova, and Yanhua Shih, ďTwo photon Diffraction and Quantum Lithography,Ē Physical Review Letters, forthcoming (2001). See figure 4, and concluding paragraph