by Benjamin Johnson
Aleksandr M. Prokhorov
Nobel Prize in Physics 1964 together with Charles H. Townes and Nicolay G. Basov
"for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".
Alexander Prokhorov was a Russian physicist born on July 11, 1916, in Atherton, Queens- land, Australia. In 1964 he received the Nobel Prize for physics for contributions to the development of the maser and laser along with Nikolai Basov and Charles Townes. Prokhorov was the son of immigrants who fled the tsarist government in 1911 and returned to Russia in 1923 after the Russian Revolution. He attended Leningrad State University from 1934 until 1939 where he received his Bachelor’s degree with honors in physics. Afterward, he moved on to begin graduate work at the P.N. Lebedev Institute in Moscow in the oscillation laboratory studying the propagation of radio waves until 1941 when he was called into the Red Army. Active in the Second World War until 1944, Prokhorov was discharged after his second injury. Returning to the Lebedev Institute, he began to study frequency stabilization in vacuum-tube oscillators and received his Candidate’s (Master’s) degree in 1946. Furthering his studies in 1947, Prokhorov studied coherent centimeter radiation (microwaves) from electrons in a synchrotron and received his Ph.D for this work in 1951.
Prokhorov continued research on microwave spectroscopy, studying molecular rotational and vibrational spectra as well as absorption. The latter led to his initial collaborations with Nikolai Basov and research on molecular oscillators.
The Soviets began to investigate the details of such oscillators in the years after the Second World War, the first theoretical results coming from Basov and Prokhorov in a presentation in 1952 with a publication of the research following in 1954. The two initially referred to the device as a molecular amplifier and generator (MAG); the name “maser” was coined later in the United States.
While a maser is in operation, the active medium, which contains atoms or molecules in an excited state, is traversed by photons (light). The photons interact with the medium and cause transitions of the electrons from the excited to the lower electronic levels, whereby a photon identical to the initial photon is emitted (stimulated emission). If this phenomenon takes place in a resonant cavity with walls of high reflectivity and the active medium can be continually re-excited, the photons may oscillate through the medium, causing the continued release of radiation. Therefore, one initial photon can lead to the exponential creation of new photons and a beam of intense, coherent electromagnetic radiation.
In the 1950s, it was not lost on those who made early contributions to the development of the maser oscillator that the necessary physics to achieve such a device had existed for about 40 years. Since Einstein formulated the idea of stimulated emission in 1916, there had been several publications on the subject. In 1924, Richard Tolman published a paper at the California Institute of Technology on “negative absorption,” with later contributions to the same subject coming from Willis Lamb and Robert Retherford in 1950. At the University of Maryland, Joe Weber published what is often taken to be the first complete theoretical description of the maser in 1952. But the first unequivocally accepted solution to the problem came in 1954 when James Gordon, Herbert Ziegler and Charles Townes published their description of the working maser based on an active medium of ammonia molecules. A short time later in the same year, Prokhorov and Basov published their first theoretical report, although it is clear the Soviets had been independently advancing their own ideas on negative absorption since at least the late 1930s. Prokhorov and Basov chose to examine an active medium of CsF, not ammonia, which required resonator walls of impractical reflectivity. In spite of also experimenting with the ammonia molecule, the focus on CsF may have delayed the Soviet attempts to build a functioning maser despite being in possession of the necessary basic knowledge.
Reflecting back on the story connecting Einstein to the birth of the maser, their are reasons why its development was delayed. In 1916 and 1917, Einstein published a pair of articles on the quantum theory of radiation (zur Quantumtheorie der Strahlung). Therein, he attempted a derivation of Planck’s black body emission formula based only on the novel quantum description of the interaction of light and matter and Boltzmann’s distribution from statistical mechanics. Describing a molecule in thermodynamic equilibrium in an electromagnetic field, Einstein envisioned three processes at work. Two of these were the well-known absorption and spontaneous emission of radiation. The third was the new concept of spontaneous emission, which described radiation interacting with an atom or molecule in an excited state and stimulating the emission of new radiation, the direction, frequency, phase and polarization of which should be exactly the same as the original radiation. This would lead to increased radiation density and high intensities.
If the system is in thermodynamic equilibrium, the probability of the first two processes outweigh that of stimulated emission to such an extent, that its scarcity caused controversy whether the process even existed. However, Einstein’s correct derivation of Planck’s formula depended on the inclusion of the term describing stimulated emission. This led many physicists to question whether the term had any physical significance at all or whether it was a mathematical trick to reach the desired result. This skeptical attitude toward stimulated emission, a key building block of the maser, can help to explain why progress before the time of Prokhorov was minimal.
The completion of quantum mechanics in the mid 1920s gave rise to still another hurdle to the development of the maser. Specifically, Heisenberg’s uncertainty principle had played a central role in changing physicists’ view of the nature of the universe. Here it was stated that a more exact knowledge about one characteristic of a system meant a loss of knowledge about a related characteristic. An increased knowledge about the energy of a system leads directly to a loss of knowledge about how long the system will remain at that energy. In the first masers based on ammonia, the molecules remained inside the resonant cavity for less than one ten-thousandth of a second with any spontaneous emission having to take place in less time than this. From the uncertainty principle, modern physicists considered the small time available for photon emission would lead directly to a large distribution of emitted energies, or a large resulting beam bandwidth. A monochromatic beam would be an impossibility as would, for that reason, a maser. Prokhorov, Basov and Townes, however, were able to empirically establish the veracity of their assertions about their devices. And still, some scientists, especially those of the theoretical bent, were forever unable to accept the existence of the maser.
Despite these doubts, the real problem turned out to be how one can bring a system into a thermodynamic state known as population inversion. This ensures that a sufficient number of the atoms or molecules composing the active medium are in excited states so that the probability of stimulated emission becomes significant. A term coined to describe this non-equilibrium scenario is “negative temperature,” because mathematically, such a system can be described by an absolute temperature less than zero.
To reach and maintain population inversion, early masers exploited inhomogeneous magnetic fields which directed excited molecules toward the resonant chamber while rejecting molecules in the ground state. The setups used by the Soviets in the early 1950s were less efficient at selecting excited molecules than those of the Americans and led to slower initial development.
But after learning about the working maser from Columbia, the Soviets easily made up for lost time and were able to quickly offer their own contributions. And even prior to this, still in 1954, Prokhorov and Basov published on the subjects of optical pumping to obtain inversion and on an active medium based on an energetic three level system. Attesting to the Soviet’s ability to achieve independent results, these ideas became important during the subsequent development of the solid state laser.
Prokhorov and Basov were initially informed of Townes’ achievement in 1955 during a visit to Cambridge, England, where they presented their own working concept of an ammonia-based laser. Watching from the audience, Townes commented on the importance of the Soviets’ work before stating that he and his group at Columbia had indeed achieved a working device based on these same concepts. Clearly, neither side had had prior knowledge of the others’ work and the meeting led to a long scientific and personal relationship. Each side was eager to hear of the experiences of the other, how problems had been dealt with and how further progress could be made.
At a conference on quantum electronics in the United States in 1959, Prokhorov and Basov had the opportunity to present further results and finally to tour Townes’ laboratory at Columbia and even make a visit to Townes’ home to meet his family. The friendship endured much in the following years, as efforts began to secure recognition for the invention of the maser, and lasted up until the end of Prokhorov’s life.
Prokhorov’s work led to the field of quantum electronics which continues to be an active, modern and important field of study which has garnered continued recognition through further Nobel Prizes. From supermarket cash registers to ultra-precise time keeping, research laboratories and surgery, the spoils of quantum electronics contribute to daily experiences in our lives. Prokhorov continued his work on masers and later also made contributions to the laser, studying different active media and types of resonators, also with application to thermonuclear fusion. One key contribution was the ruby laser which was able to emit in the microwave and visible wavelength regions. In 1972 Prokhorov became deputy director of the Lebedev Institute.
In his career, Prokhorov received the Lenin Prize (together with Basov), became a member of the Russian Academy of Sciences, an honorary member of the American Academy of Arts and Sciences and was editor in chief of the Great Soviet Encyclopedia. Beginning in 1950 he was an active member of the communist party.
Prokhorov married Galina Alekseevna Shelepina in 1941 with whom he had one son. He died on 8 January, 2002 in Moscow at the age of 85.
 Nobel Prize Winners, Tyler Wassan, Ed., The H.W. Wilson Company, New York (1987)
 Making Waves Charles H. Townes, American Institute of Physics Press (1995)
 How the Laser Happened Charles H. Townes, Oxford University Press (2002)
 Albert Einstein, Strahlungs-Emission und Absorption nach der Quantentheorie, Verhandlungen der Deutschen physikalischen Gesellschaft 13/14, 318-323 (1916)
 Albert Einstein, Zur Quantentheorie der Strahlung, Physikalische Zeitschrift 18, 121-128 (1917)
 James P. Gordon, Herbert J. Ziegler, Charles H. Townes, Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3, Physical Review 95, 282-284 (1954)
 Nikolai Basov, Alexander Prokhorov, Journal for Experimental and Theoretical Physics USSR 27, 433- 438 (1954)
 Charles H. Townes, Nobel Prize Acceptance Speech (1964)
 Nikolai Basov, Nobel Prize Acceptance Speech (1964)
Alexander Prokhorov, Nobel Prize Acceptance Speech (1964)
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