@phdthesis{Pagel2015,
  author    = {Daniel Pagel},
  title     = {Quantum dissipation and entanglement generation in photonic systems},
  journal    = {Quantendissipation und Erzeugung von Verschr{\"a}nkung in photonischen Systemen},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-002333-2},
  year      = {2015},
  abstract  = {The realistic description of the physical processes in quantum optical systems requires careful investigation of the interplay between quantum dissipation and entanglement generation. In this thesis, we have considered from a microscopical perspective the entanglement generation in semiconductor microcavities at short times, the dissipative evolution of the quantum harmonic oscillator towards a stationary state, and the nonclassical properties of the asymptotic states of different photonic systems. In our description of two-dimensional semiconductor microcavities we showed that two different pump configurations can be used to stimulate parametric scattering processes between polaritons that lead to the generation of internal polariton entanglement. A moving polariton induces an ultrafast electric polarisation as a source of light that serves as a probe of the internal entanglement properties. The identification of the nonclassical correlations of the emitted photons is based on entanglement witnesses that can also be used for the quantification of entanglement, e.g., in terms of the Schmidt number. The simultaneous creation of multiple branch entangled photon pairs renders it possible to generate an arbitrary number of entangled qubit states. By adjusting the number of pump beams and their spectral properties, one can optimize the Bell-type correlations within one ore more of those entangled qubits. Quantum dissipation can be studied in a microscopic setting with the well known model of a central oscillator coupled linearly to a bath of harmonic oscillators. We showed that equilibration of the central oscillator is the generic behaviour, which is prevented only in situations in which the classical oscillator equation of motion possesses undamped oscillatory solutions. Because of its localised spectral function, the infinite linear harmonic chain is an example for this behaviour. Thermalisation of the central oscillator depends on additional conditions. Equipartition of kinetic and potential energies requires the weak damping limit but is independent on the initial condition. The initial bath preparation enters the asymptotic temperature. Essential for the thermalisation of several oscillators is, that the asymptotic temperature is independent of the central oscillator frequency, which is fulfilled if the initial bath energy distribution matches that of a thermal state. Nevertheless, because this condition involves the sum of kinetic and potential energy, full thermalisation is possible in environments with nonthermal individual energy distributions, even in those far from thermal equilibrium. We showed, that even in the absence of full thermalisation the fluctuations of the central oscillator follow a generalised fluctuation dissipation theorem that reduces to the well known thermal result whenever the central oscillator thermalises in the strict sense. Photonic systems such as two-level emitters in a cavity or semiconductor microcavities are employed in quantum optics applications. The realistic theoretical description of the physical processes requires the use of methods from quantum optics as well as fromthe field of quantum dissipation. Our focus was on the correct theoretical description of the emission from systems with strong coupling. The analysis of the light generated by emitters in a cavity reveals a non-trivial dependence of the photon statistics on the light-matter coupling and temperature. Clearly identifiable parameters regimes with sub- and super-Poissonian photon statistics appear at strong and ultrastrong coupling, and lie immediately next to each other. We provided an approximate rule to relate the emission characteristics for a single emitter to those obtained for few emitters under an appropriate scaling of the emitter-cavity coupling. In accordance with this rule, the generation of noncassical light is easier with more emitters. The outright failure of the quantum optical master equation at predicting any of the features observed in the emission statistics shows that using the correct master equation is essential in all situations. Including internal dissipation channels we showed that a continuously driven semiconductor microcavity generates entangled light even at infinitely large times. The entanglement generation is thus robust against decoherence under realistic experimental conditions. Because the pair correlations between polaritons can sustain over long times and distances in these solid-state devices, a microcavity is a highly efficient source of entangled light and therefore well suited for quantum optics applications.},
  language  = {en}
}