The overlooking of E.C.G. Sudarshan's crucial contribution to quantum theory while choosing this year's winner of the Physics Nobel Prize disappoints the Indian scientific community.

ONE half of this year's Nobel Prize in Physics is for "the contribution to the quantum theory of optical coherence". But the Nobel Committee's decision to name only Roy J. Glauber of Harvard University for this award has surprised many Indian physicists greatly. In doing so, they feel, the Nobel Committee has completely ignored the fundamental contribution of the Indian physicist E.C.G. Sudarshan to the subject on which all subsequent developments in the field of quantum optics have been based. Sudarshan is currently a professor at the University of Texas, Austin, United States.

Indeed, soon after the announcement of the prize, an Indian English-language daily ran a story quoting several Indian physicists who expressed their displeasure with the Nobel selection. But now, with Sudarshan himself raising the issue, the controversy has acquired renewed significance. He had written a letter to the editor of The New York Times (which was not published) to correct the errors in the news report of October 5 on the Nobel Prize. But, more importantly, in a strongly worded letter to the Nobel Committee, he has questioned several statements made in the information document accompanying the award to Glauber.

There is no denying that the credit for introducing the concept of a coherent state to describe light beams and their detection in quantum mechanical terms should go to Glauber. In 1963, he outlined the basic principles underlying the quantum description of light (in terms of the coherent state) in a paper that was published in the American journal Physical Review Letters (PRL), Volume 10, dated February 1, 1963. Very soon, Sudarshan used the Glauber construct of the coherent state to arrive at a far-reaching result, which he published two months later in the same journal (Vol. 10, April 1, 1963).

According to this result, any quantum state of light can be expressed as a mixture or an ensemble of coherent states expressed in a particular (mathematical) expression called the `diagonal representation'. Individual coherent states in the representation had appropriate weight functions that could take both positive and negative values. This `diagonal state representation', Sudarshan argued, established a formal equivalence between classical and quantum descriptions of the all states of the light field, with the weight functions taking the form of (positive valued) classical probability distributions for most optical processes. But the representation had general validity in that it also correctly reproduced purely quantum effects.

In these cases, the weight functions were no longer the simple probability distributions but were of other forms that included `distribution functions' with negative values.

This mathematical equivalence is now known as the `Optical Equivalence Theorem', a fundamental result in quantum optics. The `Diagonal State Representation', which Glauber has referred to as P-Representation in his subsequent work and is known in literature as Glauber-Sudarshan Representation, has formed the starting point for all later developments in quantum optics.

Reacting to the statement "Glauber and others, including E.C.G. Sudarshan,... worked to explain the observations through quantum mechanics" in The New York Times report, Sudarshan said in his letter: "Actually it is my work, establishing for the first time the (correct) `diagonal representation' of the density operator, that is the basis for all subsequent work in this field. With this diagonal representation not only had I shown the form equivalence between classical and quantum theories of light (all kinds), but it is the only representation from which specifically quantum effects... can be derived.

"The theory referred to as `Glauber's theory' in the report or as `Glauber - Sudarshan' representation in scientific papers is really the Sudarshan `diagonal representation', which was, subsequently, adopted by Glauber and renamed as P-Representation. I would like to assert that literally all the subsequent theoretic developments in the field of Quantum Optics make use of Sudarshan's `diagonal representation', Nobel Prize and citation notwithstanding."

In his remarks to the Indian daily, N. Mukunda, a Professor of Theoretical Physics at the Indian Institute of Science (IISc), Bangalore, and a long-time associate of Sudarshan, had said: "Sudarshan claimed unrestricted validity for a result which Glauber said holds under limited conditions. Sudarshan and J.R. Klauder later put the results on an even firmer footing, justifying Sudarshan's original claims."

C.L. Mehta, a former quantum optics physicist from IIT Delhi who had worked with Sudarshan on the detailed aspects of the Equivalence Theorem, had this to say: "If Glauber's work on quantum optics deserves the Nobel Prize, the work of Sudarshan is even more significant. Even in late 1960s we felt very unhappy when people used to refer to [the result as] `Glauber's P representation' rather than as Sudarshan's diagonal coherent state representation. Some people, of course, took a compromised stand by calling it `Glauber-Sudarshan representation'. However, it was Sudarshan who pointed out the general validity of the representation as against Glauber's suggestion of its applicability in special cases."

"It is not really unusual for the Nobel Committee to selectively pick one individual," points out Girish Agarwal, a quantum optics theorist and former director of Physical Research Laboratory (PRL), Ahmedabad, who is currently a Professor of Physics at Oklahoma State University occupying the Nobel Foundation Chair. "It has happened in recent years, for example, in the case of the Russian physicist V.S. Lekhotov's contribution to laser cooling of atoms which got the 1997 Nobel. Sudarshan certainly made a landmark contribution with his 1963 paper," he added.

"Sudarshan's work was certainly path-breaking. But, perhaps his injudicious use of certain phrases in his paper, may have lessened the impact of his result," says R. Simon, a professor at the Institute of Mathematical Sciences, Chennai. "With his language, he perhaps overstated his case. Today, Sudarshan himself would probably disagree with those phrases. A more carefully written paper would perhaps have had greater impact. But Glauber carefully picked those and attacked Sudarshan's claims of "complete equivalence" (of classical and quantum descriptions) and "one-to-one correspondence" (of weight functions and probability distributions) in his detailed papers," says Simon. However, he drew attention to the carefully worded text of the information document that tends to negate Sudarshan's contribution. "It seems to have been done with a lawyer's bent of mind," he said. Of course, Sudarshan has chosen to take the matter directly to the Nobel Committee's door and has requested a revision of the text.

According to some informed sources, the Nobel Committee apparently did debate Sudarshan's case a good deal. But the final decision was apparently dictated by the fact that Glauber's introduction of the coherent state as a fundamental new entity to describe classical optical fields preceded Sudarshan's work and Sudarshan had cited Glauber's work in his paper. Indeed, the Nobel information document appears to have been drafted skilfully to convey specially this aspect as if priority (to proposing the coherent state representation) was the sole criterion for selection.

Consider the following passage: "The mathematical formalism of quantised fields was developed in parallel with Glauber's work on their applications. E.C.G. Sudarshan drew attention to the use of coherent state representations for the approach to classical physics; at this point he refers to Glauber's work. Together with J.R. Klauder, he proceeded to develop the mathematical formalism of Quantum Optics; their approach is presented in their textbook." (emphasis added).

Jan Nilsson, a Swedish physicist and former colleague of Sudarshan who has assisted the Nobel Committee in this year's deliberations, refused to disclose the manner by which the Nobel Committee arrived at the decision as he was bound by an oath of secrecy. When asked whether Sudarshan's communication to the Nobel Committee changed the perception in any manner, Nilsson said that Sudarshan had said nothing new in it that was not known to the Committee and the Committee had taken all facts into consideration.

Nilsson, however, voluntarily stated that to be considered for the award, a person has to be nominated, along with reasons for the nomination and supporting documents. His remark seemed to imply that perhaps Sudarshan was not nominated at all this year. Nilsson, however, refused to confirm or deny this. But it would indeed be strange that if a field of research came up for the award, the Nobel Committee ignored the work of a person, however important, just because he or she had not been nominated. Nilsson would not throw any further light on the manner of the working of the Committee.

Sources point out how it is not unlikely that Sudarshan may not have been nominated at all. Within the U.S. itself, Sudarshan's constituency in the mainstream physics community is almost non-existent, a result of his drifting into fringe areas of science, including such metaphysical pursuits as transcendental meditation popularised by Maharishi Mahesh Yogi from the late 1970s. It is indeed surprising that Sudarshan has not even been elected to the U.S. Academy of Science, considering the pioneering contributions that he made in several fields of physics, quantum optics being important among them.

His other landmark contribution is the formulation of the V - A Theory of Weak Interactions in elementary particle physics in the late 1950s, in association with Robert E. Marshak. This paved the way for a successful programme of unification of weak and electromagnetic interactions by Abdus Salam and Steven Weinberg during the 1970s. Yet another important work relates to the concept of faster-than-light particle called Tachyon. However, with no evidence of its existence so far, the idea remains hypothetical.

But even with regard to the V - A Theory, Sudarshan has not got the due credit as it is generally referred to as the Feynman-Gell-Mann V-A Theory in physics literature. This, as is generally acknowledged, is mainly because of the unfortunate sequence of events. Feynman and Gell-Mann published their work in January 1958, which actually took the idea from a presentation at a conference in Padua by Sudarshan and Marshak in July 1957, as a paper in a prominent journal. A short paper by Sudarshan and Marshak appeared only March 1958 and the conference proceedings itself appeared only in May 1958. Of course, Feynman has graciously acknowledged this in his famous remark in 1974: "We have a conventional theory of weak interactions invented by Marshak and Sudarshan, published by Feynman and Gell-Mann and completed by Cabibbo. I call it the conventional theory of weak interaction, the one that is described by the V - A Theory."

However, V - A Theory never made it to the Nobel Prize, even though it is a landmark development in the path towards unification of forces. The best time was the early 1980s when Sudarshan's name was, indeed, nominated to the Nobel Committee by well-known Indian physicists. But the possibility was overtaken by events with the latter development of Weinbarg-Salam-Glashow unification of electromagnetism and weak interactions making it to the Nobel grade.

But in the case of quantum optics, there appears to be a genuine case of denial and omission by the Nobel Committee. Even the gracious gesture of Feynman would not seem to have been emulated by Glauber, let alone the Nobel Committee.

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