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Journal of Modern Optics

Original studies on quantum coherence

Kōichi Shimoda

Published Aug 15, 2005• 0 citations
Publisher

Abstract

The earliest works that demonstrated quantum coherence of masers and lasers are described. Phase coherence of the maser was studied by heterodyning outputs of two independent masers in 1955, and that of lasers in 1963. We also studied coherent superposition of two eigenstates by observing the induced dipole moment of ammonia in the downstream of the maser, oscillating at the frequency of the maser.

DOI
10.1080/09500340500165150

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In 1954 the invention of the ammonia molecular beam maser oscillator, through Charles Hard TOWNES and co-workers and Nikolai Gennadievich BASOV and Alexander Mikhailovich PROKHOROV, opened a new area of applied physics, the field of quantum electronics. Ten years later the importance of the invention of the new device which consists essentially of a microwave resonator and excited ammonia molecules which pass through the resonator was acknowledged by the award of the 1964 NOBEL Prize in physics to the above workers. \nSoon after the invention of the maser principle, an ammonia beam maser utilizing separated emission fields was brought into operation, a device which consists of two separated microwave resonators through which the ammonia beam maser passes in succession. 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It is shown that the double-hump detuning phenomenon is produced by a fundamental radiation process occurring in the first emission coil and 1 therefore, that it is not a second-order effect such as could occur because of the non-uniform velocity distribution of the excited particles. \nIt is also shown that for the two-resonator ammonia beam maser the detuning phenomenon can be explained in terms of quantum mechanical probabilities, when a uniform velocity distribution of the ammonia molecules is assumed. Those results agree qualitatively with the results which have been obtained using BLOCH's equations and which have been verified experimentally by means of the two-coil nuclear maser. \nA second distinctive feature of the conventional two-cavity ammonia beam maser is that if the first resonator is sufficiently detuned, generation in the second resonator can take place simultaneously at two different frequencies. 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Chapter II contains a review of previous work on two-cavity ammonia beam masers. \nChapter III reviews the conventional theory of nuclear masers. In Chapters IV to IX the two-coil nuclear maser is investigated. Chapter X contains a general discussion of the two-resonator maser problem in terms of quantum-mechanical probabilities. Some general remarks concerning practical applications of two-resonator masers \nare made. The conclusions are summarized in a final section.

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Quantum Electronics and Laser Science Conference•May 12, 2000•0 citations
The earliest works that demonstrated quantum coherence of masers and lasers are described. Phase coherence of the maser was studied by heterodyning outputs of two independent masers in 1955, and that of lasers in 1963. We also studied coherent superposition of two eigenstates by observing the induced dipole moment of ammonia in the downstream of the maser, oscillating at the frequency of the maser.
Some very interesting pure non-Gaussian states in quantum optics have already been produced experimentally (with one pulsed laser): the single-photon-added coherent states, among others. Important interference phenomena by superposing beams from two independent masers or lasers have been investigated earlier experimentally and theoretically. By pursuing on both subjects altogether, we propose a possible new experiment to generate single-photon-added coherent states, by employing two independent laser sources, both in continuous regime and having approximately equal frequencies and coherence times tc: we expect that such generations could occur during times of the order of tc/3 (possibly, a bit shorter). This expectation follows from a fully quantized multimode analysis of the temporal mode structure, which extends previous studies of parametric down conversion and balanced homodyne detection
The theory of stimulated emission was put forwarded by A. Einstein (1917), which predicted that as light passed through a substance it could stimulate the emission of more light. In 1954, this phenomenonwas used by Charles Townes andArthur Schawlow in the construction of a microwave amplification by stimulated emission of radiation (maser) using ammonia gas and microwave radiation. Later, this operating principle of maser was put forth to the optical frequencies by them from the Bell Telephone laboratories in 1958, which led to the realization of the device, known as light amplification by stimulated emission of radiation (laser; Townes and Schawlow, 1958). A laser is a quantum optical device that produces a nearly monochromatic (single wavelength), and coherent beam of light by exciting atoms to a higher energy level and causing them to radiate their energy in phase.
The equations are generalized governing coherent emission from quantum mechanical amplifiers (microwave, infrared, and optical masers) using either electric or magnetic dipole transitions. Earlier analyses by Gordon, Zeiger and Townes, Feynman, Vernon and Hellworth, and Wang and Singer, are incorporated in this generalization. The principal new result of this analysis is that an amplitude modulation of the output is to be expected from all maser oscillators excepting those in which excited atoms are supplied at a notably greater rate than the depopulation rate due to coherent induced emission. This analysis also suggests that an energy population inversion of electric dipole moments can be performed using coherent light sources in the same manner as with magnetic dipole moments. It appears therefore that modulation of an optical or infrared maser (e.g., for communication purposes) may be easily accomplished by varying the population of excited states through control of the pumping energy. This modulation may be both by absorption and amplification. (Author)
In 1954 the invention of the ammonia molecular beam maser oscillator, through Charles Hard TOWNES and co-workers and Nikolai Gennadievich BASOV and Alexander Mikhailovich PROKHOROV, opened a new area of applied physics, the field of quantum electronics. Ten years later the importance of the invention of the new device which consists essentially of a microwave resonator and excited ammonia molecules which pass through the resonator was acknowledged by the award of the 1964 NOBEL Prize in physics to the above workers. \nSoon after the invention of the maser principle, an ammonia beam maser utilizing separated emission fields was brought into operation, a device which consists of two separated microwave resonators through which the ammonia beam maser passes in succession. It is now well known that this two-cavity variant has great practical advantages over the original one-cavity ammonia beam maser. \nHowever, not all of the macroscopic effects observed using the two-cavity ammonia beam maser could be explained in a straight­forward manner. One of the more obscure effects presents itself as follows: If the frequency of the auto-oscillation in the first resonator is detuned from one side of the possible frequency range in which generation occurs to the other then the intensity of the electromagnetic radiation in the second cavity can go through two maxima, with a gap in the middle. \nThis effect, the "double-hump detuning phenomenon", which was first observed by the Russian research group of BASOV in 1962, has since then been a challenge to several theoretical and experimental workers. BASOV and co-workers themselves made two different attempts in order to explain this phenomenon. At the Third International Conference on Quantum Electronics, held in Paris in 1963, this team reported that the effect might occur because the transition probability of the molecules passing through the second resonator depends on the degree of excitation of the first cavity. However, in 1967 BASOV and co-workers published a paper where it is suggested that the double-hump detuning phenomenon might well arise because of the interference which occurs between the emissions from molecules which move with different velocities and radiate fields with different phases. \nOn the other hand, research activities in fields other than microwave spectroscopy have made it possible to devise masers which are similar to the ammonia beam maser in that excited particles pass through a tuned resonant component, but which emit at quite different frequencies, e.g. in the audio-frequency ranee or in the mm wave range. However, up to now none of these different masers has been furnished with a successive resonator. \nIt is the purpose of this thesis to report for the first time the successful operation of a tandem maser which is not a two-cavity molecular beam maser. The author devised and investigated a maser where excited protons in water pass successively through two tuned solenoids. \nThe system is based on a conventional proton magnetometer maser oscillating in the magnetic field of the earth at a frequency near to 2 kHz. \nThe general result of the investigation is that despite \nthe enormous frequency difference (audio-frequency compared with a microwave frequency) the two coil nuclear maser behaves in much the same way as a two cavity ammonia beam maser. For the two-coil nuclear maser the double-hump detuning phenomenon can be explained \nin terms of the motion of the macroscopic nuclear magnetization, and an analysis by means of BLOCH'S equations is possible. It is shown that the double-hump detuning phenomenon is produced by a fundamental radiation process occurring in the first emission coil and 1 therefore, that it is not a second-order effect such as could occur because of the non-uniform velocity distribution of the excited particles. \nIt is also shown that for the two-resonator ammonia beam maser the detuning phenomenon can be explained in terms of quantum mechanical probabilities, when a uniform velocity distribution of the ammonia molecules is assumed. Those results agree qualitatively with the results which have been obtained using BLOCH's equations and which have been verified experimentally by means of the two-coil nuclear maser. \nA second distinctive feature of the conventional two-cavity ammonia beam maser is that if the first resonator is sufficiently detuned, generation in the second resonator can take place simultaneously at two different frequencies. The analogous effect has been observed using the two-coil nuclear maser. \nThe conventional one-coil nuclear maser which is currently used as a magnetometer has the disadvantage that it does not offer a suitable criterion for tuning the oscillation frequency accurately to the LARMOR frequency. For the two-cavity ammonia beam maser it has previously been proposed to modulate mechanically the inter-cavity distance and to use zero phase modulation as a means of tuning the maser frequency accurately to the centre frequency of the molecular transition. It is shown that by modulating mechanically the inter­coil distance of the two-coil nuclear maser. a suitable tuning criterion can be obtained in an analogous manner. A second tuning criterion is offered by the double-hump detuning phenomenon. \nIn Chapter I the maser principle is discussed using a classical model, and the different maser types which are of interest in the context of this thesis are enumerated. Chapter II contains a review of previous work on two-cavity ammonia beam masers. \nChapter III reviews the conventional theory of nuclear masers. In Chapters IV to IX the two-coil nuclear maser is investigated. Chapter X contains a general discussion of the two-resonator maser problem in terms of quantum-mechanical probabilities. Some general remarks concerning practical applications of two-resonator masers \nare made. The conclusions are summarized in a final section.
Summary form only given, as follows. Much (indeed most!) of quantum mechanics is concerned with determining the eigenvalues and eigenstates of the system of interest and scattering rates between these states. But in many cases (e.g. in superconductors and lasers) coherent superpositions of states are required to understand the physics. Furthermore some of the deepest issues of physics arise from correlations and coherences between subsystems (e.g. the EPR paradox and the Bell inequalities). Quantum optics is at the heart of such such considerations. On the one hand, we find that the application of atomic coherence concepts yields lasers that operate without inversion, light pulses that propagate at a few meters/sec, and new tools for the detection of Bell states applied to quantum dense coding and teleportation. On the other hand, quantum optics yields experimental and conceptual insights into many of the counterintuitive and puzzling aspects of quantum mechanics.