We perform an experiment in which a quantum heat engine works under two reservoirs, one at a positive spin temperature and the other at an effective negative spin temperature i.e., when the spin system presents population inversion. We show that the efficiency of this engine can be greater than that when both reservoirs are at positive temperatures. We also demonstrate the counter-intuitive result that the Otto efficiency can be beaten only when the quantum engine is operating in the finite-time mode.

We propose an implementation of the parametric amplification of an arbitrary radiation-field state previously prepared in a high-Q cavity. This nonlinear process is accomplished through the dispersive interactions of a single three-level atom (fundamental |g , intermediate |i , and excited |e levels) simultaneously with i) a classical driving field and ii) a previously prepared cavity mode whose state we wish to squeeze. We show that, in the adiabatic approximantion, the preparation of the initial atomic state in the intermediate level |i becomes crucial for obtaing the degenerated parametric amplification process.

In this letter we present a scheme for the implementation of frequency up-and down-conversion operations in two-mode cavity quantum electrodynamics (QED). This protocol for engineering bilinear two-mode interactions could enlarge perspectives for quantum information manipulation and also be employed for fundamental tests of quantum theory in cavity QED. As an application we show how to generate a two-mode squeezed state in cavity QED (the original entangled state of Einstein-Podolsky-Rosen).PACS numbers: 32.80. 42.50.Ct, 42.50.Dv Parametric frequency conversion has been a major ingredient in quantum optics. Employed in the generation of squeezed and two-photon states of light to test sub-poissonian statistics [1] and Bell's inequalities [2], parametric down-conversion (PDC) has been constantly revisited through the work by Louisell et al. [3]. Sub-poissonian statistics, one of the characteristics of squeezed light, has deepened our understanding of the properties of radiation [1] and its interaction with matter [4]. It has provided an unequivocal signature of the quantum nature of light, disputed since the discovery of the photoelectric effect, and has continued to motivate fundamental works up to the present [5]. Apart from fundamental phenomena, the potential application of PDC in technology is also striking, ranging from improvements in the signal to noise ratio in optical communication [6] to the possibility of measuring gravitational waves through squeezed fields [7].The combination of simplicity and comprehensiveness exhibited by the frequency-conversion mechanisms applied in some of the recent proposals of quantum information theory [8] has motivated the goal of the present letter: the implementation of the frequency up-and downconversion operations in two-mode cavity quantum electrodynamics (QED). With this protocol to engineer twomode interactions, it would be possible to map into cavity QED some of the proposals for quantum logical processing originally designed for travelling fields. This protocol may be useful for scalable quantum computation and communication proposals [9], besides enlarging such perspectives, it may also be employed for fundamental tests of quantum theory [10].The parametric frequency conversion operations are accomplished through the dispersive interactions of the cavity modes with a single three-level-driven atom injected into the cavity, which works as a nonlinear medium. Although considerable space has been devoted in the literature to the interaction between a three-level atom and two cavity modes [12], the issue of tailoring the bilinear Hamiltonians of frequency conversion processes in cavity QED has not been addressed.Parametric up conversion (PUC). We envisage working with Rydberg atoms in the microwave regime. Starting with the PUC, the energy diagram of the Rydberg three-level atom is in the Λ configuration as sketched in Fig. 1a. The ground (|g ) and excited (|e ) states are coupled through an auxiliary more-excited level (|i ). The cavity microwave modes of frequenc...

In this work we show how to engineer bilinear and quadratic Hamiltonians in cavity quantum electrodynamics (QED) through the interaction of a single driven two-level atom with cavity modes.The validity of the engineered Hamiltonians is numerically analyzed even considering the effects of both dissipative mechanisms, the cavity field and the atom. The present scheme can be used, in both optical and microwave regimes, for quantum state preparation, the implementation of quantum logical operations, and fundamental tests of quantum theory.