No abstract

A procedure has been developed for the exact calculation of the efficiency of thermoelectric generators and cooling devices in which the parameters of the materials have arbitrary temperature dependence. High speed computer techniques are found necessary. Approximate methods are reviewed and their discussion extended. A number of examples are worked out by both the exact and approximate methods. Comparison of these results show that the approximate methods agree with the exact method to about 5% in the case of power generation and to about 15% in the case of the refrigeration coefficient of performance. However, in the case of the maximum heat pumping rate deviations as large as a factor of 2 are found.

A simple scheme is presented that allows the generation and detection of nonclassical states of the electromagnetic (em) field with controllable (predetermined) photon-number and phase distributions. It is based on the two-photon resonant interaction of a single em field mode in a high-Q cavity with initially excited atoms crossing the cavity sequentially (one at a time). The sequence duration should be much shorter than the cavity-mode lifetime. Nonclassical states of the field are generated conditionally, by selecting only those sequences wherein each atom is measured to be in the excited state after the interaction. The field distribution resulting from a sequence of X such measurements is peaked about 2X positions in the phase plane, which evolve sinusoidally as a function of the atomic transit times and are therefore simply controlled. %hen these peaks are chosen not to overlap, the field state constitutes a generalized Schrodinger cat. By choosing them to overlap, we can make parts of the field distribution strongly interfere, giving rise to decimation of the photon-number distribution. In particular, this process can prepare Fock states with controlled photon numbers. The generated phase distribution can be detected by monitoring the pattern of revivals in the excitation of a "probe" atom. PACS number(s): 42.50. -p, 42.52. +x

We put forward a simple, feasible scheme for the preparation and subsequent detection of macroscopic quantum superposition (MQS) states. It is based on the two-photon model which obtains when a cascade of two atomic transitions is resonant with twice the field frequency. The initial conditions amount to a field in a mixed state characteristic of lasers or masers and an excited atom. The MQS is generated by a conditional measurement of the atomic excitation after an interaction time that determines the relative phase of the MQS components. Remarkably, the MQS is subsequently detected and its phase is inferred by measuring the excitation probability of a second, "probe, " atom, as a function of its interaction time. The realization of the scheme in the optical domain, using dielectric microspheres, is discussed. PACS number(s): 42.50.p, 32.80.t, 42.52.+x Superposed macroscopically distinguishable quantum states of the electromagnetic field, hereafter referred to as macroscopic quantum superpositions (MQS), have aroused considerable interest in recent years [1-9]. Their main appeal is that they constitute potentially realizable "Schrodinger cats" that embody the well-known paradoxical aspect of quantum mechanics [4]. Attention has been focused on a single-mode field in a MQS of two coherent quasiclassical states, with identical mean amplitudes and a fixed relative phase [1,3]. Various rather intricate mechanisms have been proposed for the preparation of such MQS, but none of them has been realized thus far. These mechanisms include the following: (a) Nonlinear evolution of the field from an initially coherent state in amplitude-dispersive [5,6], and, particularly, bistably dispersive [7] media. The generation of MQS states in such media is contingent on dissipative losses being negligible. (b) Quantum measurements (photon counting [8] or quadrature detection [9]) of the field, following its preparation in a nonclassical state, e.g. , in a parametric amplifier. Such schemes are hampered not only by dissipation, but also by the performance of photon detectors that falls short of ideal (unit) efficiency.(c) Interactions of a two-level atom with a quantized single-mode field, describable by the Jaynes-Cummings model (JCM). This model is of fundamental importance in quantum optics [10,11] and is realizable to a very good approximation in high-quality resonators [12]. Certain JCM schemes for the generation of MQS rely on the initial preparation of the atom in a polarized state, i.e. , a coherent superposition of the ground and excited states [13,14]. A more recent scheme [15]exploits a remarkable inherent property of the resonant JCM [16],whereby initial preparation of the atom in the ground or excited state and of the field in a quasiclassical coherent state results in the spontaneous disentanglernent of the atomic and field states, with the field forming a MQS of two quasiclassical states. The limitations of this scheme stem from the complexity of the field evolution in the JCM: (i) It is difficult to exactly character...