A unidirectional quantum-mechanical amplifier has been developed in which the amplifying medium consists of a beam of ammonia molecules passing through, in turn, two separated resonant cavities. The cavities are electrically isolated from one another so that the only coupling is provided by the molecular beam. Unlike the various single-cavity amplifiers constructed in other laboratories, in which a common coupling point for input and output signals necessitates the inclusion of isolators or circulators to separate the two signals, the amplifier described herein affords a natural isolation of input and output with independent control of loadings. After a discussion of the physical principles underlying the mechanism of amplification in a two-cavity molecular beam amplifier, a simplified theory of the operation of the two-cavity amplifier is presented, outlining the parameters which influence power gain and noise Figure. The details of an experimental amplifier are described and the results of gain and noise figure measurements are reported.
A unidirectional quantum-mechanical amplifier has been developed in which the amplifying medium consists of a beam of ammonia molecules passing through, in turn, two separated resonant cavities. The cavities are electrically isolated from one another so that the only coupling is provided by the molecular beam. Unlike the various single-cavity amplifiers constructed in other laboratories, in which a common coupling point for input and output signals necessitates the inclusion of isolators or circulators to separate the two signals, the amplifier described herein affords a natural isolation of input and output with independent control of loadings. After a discussion of the physical principles underlying the mechanism of amplification in a two-cavity molecular beam amplifier, a simplified theory of the operation of the two-cavity amplifier is presented, outlining the parameters which influence power gain and noise Figure. The details of an experimental amplifier are described and the results of gain and noise figure measurements are reported.
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.
The subject of masers is introduced by giving an historical account of events which led to the amplification of microwave radiation by an inverted population in a system of weakly interacting ammonia molecules. The introduction also outlines the development of masers up to the present. Sections 2 and 3 are concerned with maser fundamentals. Methods for obtaining inverted populations are discussed in § 2. Section 3 is devoted to the theory of maser gain and noise. This is followed by a discussion of microwave solid state masers. Microwave maser materials and the development of unidirectional travelling wave maser amplifiers are emphasized. Optical masers are treated in § 5. In this section the theory of optical amplifiers and oscillators is outlined. Also included in the theoretical discussion is a rather lengthy consideration of pump power requirements in optically pumped masers. This is followed by a survey of solid state, semiconductor and gaseous optical masers. The review ends with some comments on the extension of masers to new wavelength regions.
Directional amplifiers are crucial nonreciprocal devices in both classical and quantum information processing. Here we propose a scheme for realizing a directional amplifier between optical and microwave fields based on an optomechanical system with optical gain, where an active optical cavity and two passive microwave cavities are coupled to a common mechanical resonator via radiation pressure. The two passive cavities are coupled via hopping interaction to facilitate the directional amplification between the active and passive cavities. We obtain the condition of achieving optical directional amplification and find that the direction of amplification can be controlled by the phase differences between the effective optomechanical couplings. The effects of the gain rate of the active cavity and the effective coupling strengths on the maximum gain of the amplifier are discussed. We show that the noise added to this amplifier can be greatly suppressed in the large cooperativity limit.
Since the demonstration of the possibility of achieving very low noise amplification of signals in the centimetric wavebands, a great deal of work has been carried out in various laboratories with a view to producing and testing devices. The major part of the present article focuses attention on one particular device, the three-level solid-state maser. The physical principles underlying the process of stimulated emission of radiation are in many cases unfamiliar to electronic engineers, and study of the growing literature on the subject can be confusing without a clear understanding of the physics involved. The present article outlines the general theory of emission and radiation processes, particularly as they apply to the microwave spectrum. A quantum mechanical description is employed throughout, and a brief and not very rigorous account is given of the relevant portions of this theory as it applies to the investigation of transistors between energy levels. Well-known expressions are derived for the operating characteristics of a three-level maser, particular attention being given to the significance of the various factors involved and their relationship to easily measured quantities. Little information is included concerning practical maser amplifiers; at the time of writing, few practical devices had been constructed. A survey of recent literature indicates the rapid progress which has since been made, and shows the interest in the development of masers.
The original maser of Gordon, Zeiger and Townes, driven by a focussed stream of ammonia molecules in their antisymmetrical state, provides a conveniently explicit example on which to base a discussion of the principles underlying coherent excitation of a vibrator by stimulated emission. It was shown in chapter 18 how a quadrupole electrostatic lens served to separate symmetric from antisymmetric states, and we shall assume that separation is perfect; it is easy to extend the argument to include a proportion of molecules in the symmetric state. In addition we shall ignore any complications arising from the multitude of rotational states leading to the fine structure shown in fig. 18.8, and shall assume that only one line contributes, for example the strong 3,3 line at 23.9 GHz. Since the microwave cavity resonator, if it is to be excited by the molecules, must normally be very closely tuned to their natural frequency this assumption is realistic. The simplest intuitive approach to the maser is by way of Einstein's treatment of radiation in terms of stimulated and spontaneous processes.† Excited molecules passing through the resonator, when it is already in an oscillatory condition, are stimulated by the field; if the resonator frequency is well matched to the molecular levels they may make a transition down to the ground state and on leaving the resonator have 2Δ 0 less energy than when they entered.
We calculate the input-output characteristics of an optical amplifier that consists of an inverted-population medium placed inside a high-Q Fabry-P\'erot cavity. Expressions are derived for the output second- and fourth-order spectral and temporal correlation functions, and more generally for the output characteristic functional, in terms of the corresponding input quantities. The photocount first and second factorial moments are obtained for both homodyne and direct detection of the amplifier input and output. The general results are applied to several cases of practical interest, including inputs that have coherent or chaotic coherence properties. Particular attention is paid to the effects of amplification on specific nonclassical varieties of input light. It is shown that a maximum of only twofold amplification is permitted if any squeezing present in the input is to survive as squeezing in the output light. Similarly, for the preservation of photon antibunching in amplification, we show, by consideration of a kind of free-space number-state input, that only small gains are allowed. The amplifier model treated here provides a detailed example of the limitations imposed by quantum mechanics on the minimum noise generated by a multimode linear amplifier. In particular, we show that minimum noise occurs in a cavity that is asymmetric with respect to mirror reflectivities, for which we derive the corresponding conditions. In addition, we demonstrate the ability of the amplifier to improve upon signal-to-noise ratio, otherwise limited by low detector quantum efficiency.