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We demonstrate the relevance of entanglement, Bell inequalities and decoherence in particle physics. In particular, we study in detail the features of the “strange” system as an example of entangled meson–antimeson systems. The analogies and differences to entangled spin-1 2 or photon systems are worked out, the effects of a unitary time evolution of the meson system is demonstrated explicitly. After an introduction we present several types of Bell inequalities and show a remarkable connection to violation. We investigate the stability of entangled quantum systems pursuing the question of how possible decoherence might arise due to the interaction of the system with its “environment”. The decoherence is strikingly connected to the entanglement loss of common entanglement measures. Finally, some outlook of the field is presented.

In this article we are concerned with some possible physical realizations of quantum gates that are useful for quantum information processing. After a brief introduction into the subject, in Sect. 2 we focus on a particular way of using atoms in one-dimensional optical lattices as carriers of the quantum information. As an alternative, in Sect. 3 the information carriers are photons that interact via effective nonlinearities which arises from mixing at passive linear optical elements and postselection through photodetection. These two seemingly different implementations have in common that their decoherence mechanism is described by a single theory, namely that of quantum electrodynamics in causal media which will be the subject of Sect. 4. Figure 1 should serve as an overview of the subject areas covered.

This chapter is an overview of the research on spin-based quantum dot quantum computation, which covers two most prominent architectures, one based on GaAs quantum dots, the other on phosphorus donors in Si. Particular topics include the electron spin coherence in GaAs and Si, the conditions for the Heisenberg Hamiltonian to be a satisfactory description of two-electron interaction in a GaAs double dot, the possibility of using multi-electron quantum dot for quantum computing, the implication of band structure in case of Si, schemes of spin detection, schemes to measure single electron properties in ensemble-averaged experiments, and other related issues. Current status of research in several experimental fronts that are related to quantum dot quantum computing are also discussed.

The derivation of a microscopic many-body theory for the nonlinear optical response of semiconductors is reviewed. At the Hartree–Fock level, the semiconductor Bloch equations include many-body effects via band gap and field renormalization. These equations are sufficient to describe excitonic resonances as they appear already in the linear absorption spectra. An adequate description of nonlinear optical effects in semiconductors beyond the Hartree–Fock level includes Coulomb interaction induced carrier correlations. Different schemes have been developed to treat such correlation effects. As two examples, the second-order Born approximation and the dynamics-controlled truncation scheme are introduced and analyzed. In addition to the derivation of the equations of motion, a few examples are presented which highlight important signatures of many-body correlations in the optical response of semiconductors.

These are the notes for a course on a theoretical description of condensation of excitons and exciton-polaritons in semiconductors. A special emphasis is made on the case of quantum wells. We start by presenting a standard theory that can be found also in several excellent books and reviews. We concentrate in the question of detecting condensation without paying attention to the open problem of the dynamics of condensation appearance. Special care is devoted to the emission of light. The recently studied case of condensation of magnetoexcitons is briefly discussed. Since excitons have the third component of the total angular momentum as an internal degree of freedom, this opens the extremely interesting possibility of multicomponent condensates which we discuss in some detail. In particular, we show the appearance of an interesting behavior of the polarization of the light emitted by this multicomponent condensate.