C. J. Villas-Boas

We afford an experimentally feasible platform to study Boltzmann negative temperatures. Our proposal takes advantage of well-known techniques of engineering Hamiltonian to achieve steady states with highly controllable population inversion. Our model is completely general and can be applied in a number of contexts, such as trapped ions, cavity-QED, quantum dot coupled to optical cavities, circuit-QED, and so on. To exemplify, we use Hamiltonian models currently used in optical cavities and trapped ion domain, where the level of precision achieved the control of the freedom degrees of a single atom inside a cavity/trapped ion. We show several interesting effects such as absence of thermalization between systems with inverted population and cooling by heating in these unconventional systems.

A class of Hamiltonians that are experimentally feasible in several contexts within quantum optics and lead to so-called cooling by heating for fermionic as well as for bosonic systems has been analyzed numerically. We have found a large range of parameters for which cooling by heating can be observed either for the fermionic system alone or for the combined fermionic and bosonic systems. Analyzing the experimental requirements, we conclude that cooling by heating is achievable with present-day technology, especially in the context of trapped-ion and cavity QED, thus contributing to the understanding of this interesting and counterintuitive effect.

Here we show how to generate a dark two-mode squeezed state of a trapped ion, employing a three-level ion in a V configuration with a strong decay of the excited states. The degree of squeezing can be manipulated by choosing the intensity of the driving fields. Our scheme is robust against the usual dissipation mechanism and could be implemented with present-day technology. The validity of the approximations employed in this work was tested by numerical calculations, which agreed completely with the analytical solutions.

In this paper we extend current perspectives in engineering reservoirs by producing a time-dependent master equation leading to a nonstationary superposition equilibrium state that can be nonadiabatically controlled by the system-reservoir parameters. Working with an ion trapped inside a nonindeal cavity we first engineer effective Hamiltonians that couple the electronic states of the ion with the cavity mode. Subsequently, two classes of decoherence-free evolution of the superposition of the ground and decaying excited levels are achieved: those with time-dependent azimuthal or polar angle. As an application, we generalise the purpose of an earlier study [Phys. Rev. Lett. 96, 150403 (2006)], showing how to observe the geometric phases acquired by the protected nonstationary states even under a nonadiabatic evolution.

We show how to generate quadratic and bi-quadratic phonon-photon interactions through a driven three-level ion inside a cavity. With such a system it is possible to squeeze the cavity-field state, the ion motional state or even the entangled phonon-photon state. We present a detailed analysis of the cavity-field squeezing process, distinguishing three different regimes of this amplification mechanism: the subcritical, critical, and supercritical regimes, which depend, apart from the coupling parameters, on the excitation of the vibrational state. As an application of the engineered Hamiltonians, we show how to implement a Fock-state filter for the vibrational mode. New aspects of the technique of adiabatic elimination emerge in this analysis.

We describe a proposal to probe the quantum tunneling mechanism of an individual ion trapped in a double-well electromagnetic potential. The time-evolution of the probability of fluorescence measurement of the electronic ground state is employed to characterize the single-particle tunneling mechanism. The proposed scheme can be used to implement quantum information devices.