He was a physicist from God. The words "outstanding", "talented" are only a pale reflection of the gift that nature has endowed him with. It seemed that generating new ideas and discovering new phenomena was a natural function of his organism. And no vicissitudes of fate could interfere with this process ...

David Nikolayevich was born on May 21, 1929. In 1947 he graduated from the famous Moscow School No. 110 and entered the Faculty of Physics of Moscow State University, from where, however, he was forced to leave: the echo of the repressions of the 30s, in which his parents suffered, reached him. But David Nikolaevich nevertheless became a certified physicist, having graduated from the Physics Department of the University in Gorky, where he moved with the Peshkov family, in which he was brought up after the arrest of his parents. After graduating from the university, David Nikolaevich returned to Moscow and began working at the IRE. From the first steps of his scientific activity until the end of his life, David Nikolayevich linked his fate with quantum radiophysics. First it was masers, then lasers, laser physics and quantum optics.

True, in the years 55-56 he became seriously interested in the ocean, went to work at the Department of Marine Physics of the Faculty of Physics of Moscow State University, tried to get on an expedition to Antarctica. But it was an episode, although the craving for the ocean remained and was nevertheless realized after 20 years. And in 1958, David Nikolayevich returned to quantum radiophysics and headed experimental research in the laboratory of quantum radiophysics, which had just been created by S. D. Gvozdover, at the Department of Microwave Radiophysics. It is worth noting here that D.N. Klyshko was at that time an active high-class experimenter. He continued to consider himself an experimenter when he became a world-class theorist. He completed all his theories by considering specific experiments that were implemented by his students.

In 1964, D. N. Klyshko defended his thesis on "Multiquantum and many particle transitions in radio spectroscopy and quantum radiophysics". Both opponents and the opposing organization rated this work as a doctoral one. However, procedural difficulties did not allow D. N. Klyshko to be awarded the degree of Doctor of Science. He defended his doctoral thesis on "Multiphoton processes in the optical and radio range" in 1972. In this dissertation, D.N. Klyshko not only summed up his research in the field of multiphoton processes, but took the first, decisive step in creating a new direction in the then young science - quantum optics. We are talking about spontaneous parametric down-conversion of light (SPDC), which was predicted by D.N. Klyshko in 1966. It was David Nikolayevich's finest hour, when not only his enormous creative potential was manifested, but also his mighty will and endurance, because then events developed according to a very dramatic scenario. Here is how David Nikolayevich himself sparingly describes it in his book "Photons and Nonlinear Optics", published in 1980 and still unsurpassed in terms of breadth of coverage and depth of analysis of the quantum nature of light: 1967 (in fact, 1966, when D.N. Klyshko reported this result at a seminar at the Institute of Solid State Physics in Chernogolovka and at a seminar by R.V. Khokhlov, and in 1967 an article by D.N. Klyshko was published in Letters to JETP ") gave an unexpectedly large value with an effective temperature of ~ 10³ K even when pumped with a power of 1 W, and it was not clear why the effect had not been noticed earlier in frequency subtraction and parametric amplification experiments. As a result, the experimenters addressed by the author did not believe into the reality of these mysterious quantum noises, and they were discovered at Moscow University by accident during experiments with a parametric light generator." The dramatic nature of the situation was that if it were not for the persistence of D.N. Klyshko, Russian science would lose priority in the discovery of the SPDC phenomenon, since in March 1967 at a symposium on modern optics in the USA, simultaneously with our report (its co-authors were both D. N. Klyshko, and V.V. Fadeev and O.N. Chunaev, who really accidentally discovered the mysterious glow, but not by chance, thanks to D.N. Klyshko, who interpreted it as an SPDC), reports were made by two American groups that independently discovered this phenomenon. In 1974, the prediction and detection of SPDC was recognized as a discovery. In 1984 D.N. Klyshko (together with A.N. Penin and V.V. Fadeev) was awarded the State Prize "for the discovery and study of the phenomenon of spontaneous parametric down-conversion and its application in spectroscopy and metrology." Undoubtedly, the leading role in the discovery of SPDC as a new type of light scattering belongs to D.N. Klyshko, and in the history of science his name should be on a par with the scientists who discovered other types of light scattering: Rayleigh, Mie, Mandelstam, Brillouin, Raman. It would be logical and fair to refer to the SPDC as the "Klyshko scattering".

David Nikolayevich himself rejected this idea. He opposed in every possible way any campaigns aimed at obtaining titles and titles for him. For example, he could automatically (as the author of the discovery) become an academician of the Russian Academy of Natural Sciences at the moment when it was created, but did not want to put forward his candidacy. His nomination as a corresponding member of the Russian Academy of Sciences was undertaken by his colleagues and students, in fact, against his wishes. David Nikolaevich tried to stay away from pseudo-scientific fuss, which could interfere with his work, violate the comfortable state in which he was doing science, or rather, living science.

For more than 30 years - from 1967 to the day of his untimely death - this is a period in the life of David Nikolayevich, when he, in one breath, in a state of continuous creative inspiration, created the foundation of quantum optics and solved its specific problems. There is a whole layer in this science, which was raised by him. Ideas and predictions of new effects poured out of his creative laboratory at such a speed that the experimenters did not keep up with this flow, they still have to master the legacy of David Nikolayevich for a long time to come.

D.N. Klyshko created a school of quantum optics in Russia. From this school came 6 doctors of science and a large number of candidates. His pedagogical style was as idiosyncratic as his scientific style. His students received from him not only knowledge, but also that which is difficult to express in words, which distinguishes a real scientist from an artisan in science. They received this from David Nikolaevich in communication with him - at his lectures, seminars, at "gatherings" over a cup of tea in the laboratory. He was very fond of these "gatherings" and was often their initiator, wherever he was. The participants of the famous expedition to the Pacific Ocean (in 1977) on the research ship "Dmitry Mendeleev" probably remembered D.N. Klyshko not only as a scientist, but also as the "owner of the salon", in which those close to him people. This outwardly very restrained, perhaps somewhat stern person instantly attracted the people around him and became the center of the company, without putting any effort into it.

Not only the department, not only the Faculty of Physics and the University, but also the world community have lost one of the brightest scientists and teachers, one of the "last Mohicans" of the outgoing tribe of highly intelligent Russian intellectuals. They are replaced by their students, also talented, but in many ways different. I would like to believe that they absorbed the valuable things that David Nikolaevich unobtrusively taught them.

Abstract of the main works of D.N. Klyshko in the field of quantum optics.

Speaking about the contribution of D.N. Klyshko to quantum and nonlinear optics, one should first of all mention the theory of spontaneous parametric down-conversion (SPDC) developed by him in 1967. The SPDC effect was predicted by D.N. Klyshko, "Coherent Photon Decay in a Nonlinear Medium", Sov.Phys.JETP Lett., 6, 23 (1967) and observed experimentally in the same year. Spontaneous parametric down-conversion occupies a central place in modern quantum optics, because to date, this is the simplest and most efficient way to generate correlated (entangled) states of photons, or biphotons, as D.N. Klyshko called them. The unique statistics of the fields emitted during SPDC was first discussed in D.N.Klyshko and B.Ya.Zel'dovich, "Statistics of Field in Parametric Luminescence", Sov.Phys.JETP Lett.,9, 40 (1969) . The SPDC effect has replaced the two-photon decay of excited atoms, which was previously used to demonstrate the EPR paradox and experimental verification of Bell's inequalities, and thus gave a powerful impetus to the development of quantum optics. More recently, this rapidly developing field of science has led to such unexpected and beautiful effects as anti-correlation, quantum cryptography, and quantum teleportation, all of which rely heavily on SPDC. Shortly after the theory of SPD, D.N. Klyshko developed the theory of hyper-Raman scattering in D.N. Klyshko and N.I. Nazarova, "Scattering of Light by Light in a Noncentrally Symmetrical Medium", Sov.Phys.JETP, 31, 472 (1970).

D.N. Klyshko was the first to realize that spontaneous parametric down-conversion and Raman scattering by polaritons (an effect known even before the discovery of SPD) have a common nature and are limiting cases of the same effect (DNKlyshko, "Scattering of Light in a Medium With Nonlinear Polarizability", Sov. Phys. JETP, 28, 522 (1968)). He proposed a unified description for them based on the effective cubic susceptibility model. The new theory stimulated work on the spectroscopic application of SPDC. The theory of D.N. Klyshko also included a description of the active spectroscopy of polaritons (D.N. Klyshko, "Active spectroscopy of polaritons", Quantum electronics, vol. 2, No. 2, (1975)).

An important contribution of D.N. Klyshko to modern optics was the methods of quantum metrology developed by him on the basis of SPDC: absolute measurement of the spectral brightness of radiation (DNKlyshko, "Utilization of Vacuum Fluctuations as an Optical Brightness Standard", Sov.J. Quantum Electronics, 7, 591 (1977)) and absolute calibration of photodetectors (DNKlyshko, "On the Use of Two-Photon Light for Absolute Calibration of Photodetectors", Sov. J. Quantum Electronics, 10, 1112 (1980)). The first method is based on the existence of a universal brightness level, which can be called the "vacuum brightness" and which, after multiplication by the parametric conversion coefficient, determines the brightness of the SPDC in the absence of idler radiation at the input to the nonlinear crystal. If idler radiation is added to the input, its brightness can be determined by comparing the output signal of the SPDC in both cases. The second method, the absolute calibration of photodetectors, is based on the correlation between the signal and idler photons emitted during the SPDC process and makes it possible to measure the quantum efficiency of photodetectors without using any reference radiation sources or receivers. Both methods of quantum metrology were experimentally implemented in the early 1980s.

In all the works of D.N. Klyshko, there is a close connection with the experiment. He had a wonderful understanding of the experiment, incredible for a theoretician - that is why his experimental predictions were so accurate, giving an almost one hundred percent guarantee of success to the experimenters. And his works, devoted to the analysis of experiments already carried out, always brought clarity to the most confusing issues. Some beautiful experiments, once proposed by D.N. Klyshko, are still waiting for their implementation, such as, for example, the study of the correlation between the Stokes and anti-Stokes components in Raman scattering (D.N. Klyshko, "Correlation of the Stokes and anti-Stokes components in inelastic scattering Light", Quantum Electronics, 4, 6 (1977)) and measurement of odd field moments in thermal radiation (D.N. Klyshko, "On some features of thermal radiation", Doklady USSR Academy of Sciences, 244, 3 (1979)).

D.N. Klyshko developed several new theoretical approaches in quantum and nonlinear optics: a generalization of the Kirchhoff law to non-geometric and nonlinear optics ("D.N. Klyshko, "Generalized Kirchhoff laws and absolute quantum photometry", Proceedings of the USSR Academy of Sciences, 46, 8 (1982)), theory of nonunitary transformations in optics (DNKlyshko, "Nonunitary Transformations in Quantum Optics", Phys.Lett. A, 137, 334 (1989)), advanced wave model for describing correlation and interference experiments in quantum optics (DN Klyshko, Combined EPR and Two-Slit Experiments: Interference of Advanced Waves", Phys. Lett. A, 132, 299 (1988)).

It is characteristic that the effect of interference of non-classical fields emitted during SPDC in the second order in the field was noted in the early works of D.N. Klyshko (for example, DNKlyshko, "Scattering of Light in a Medium With Nonlinear Polarizability", Sov.Phys.JETP , 28, 522 (1968)) - it is this factor that determines the shape of the SPDC line in a spatially limited crystal. Almost unnoticed at first, this effect is remarkable in that the interference here manifests itself in spontaneous emission, the "seed" of which is vacuum noise. Later, second- and fourth-order interference in SPDC was analyzed in detail by D.N. Klyshko and generalized to the case of scattering in several spatially separated crystals (AVBurlakov et.al., "Interference effects in spontaneous two-photon parametric scattering from two macroscopic regions", Phys. Rev. A, 56, 3214 (1997)).

All areas of modern quantum optics were the subject of constant close attention of D.N. Klyshko. His work has made an invaluable contribution to the consideration of such issues as the Berry phase, compression, quantum cryptography, quantum teleportation. In his recent works, D.N. Klyshko paid much attention to the problems of methodology and interpretation of quantum mechanics and quantum optics, such as the EPR paradox, which he interpreted as a violation of the Kolmogorov axioms in quantum mechanics, the justification of the postulate of the reduction of the wave function, for which he proposed experimental verification, operational criteria for the nonclassical nature of light, and much more. Remarkable are its fundamental parallels and classical analogies, which are involved in the discussion of experiments and have always served the same goal: bringing maximum clarity to the formulation and solution of physical problems.