A Bose-Einstein condensate can be well described by an order parameter called condensate wavefunction. If Bose-Einstein condensation occurs in a superposed single particle state, the condensate wavefunction is correspondingly superposed. Likewise, if two coupled distinguishable or non-overlapping Bose systems condense in an entangled single particle pair state, the condensate wavefunction is co...

We describe the quantum state of a Bose-Einstein condensate at zero temperature. By evaluating the Q-function we show that the ground state of Bose-Einstein condensate under the Hartree approximation is squeezed. We find that multimode Schrödinger cat states are generated as the condensate evolves in a ballistic expansion.

We report on measurements of the excitation spectrum of a strongly interacting Bose-Einstein condensate. A magnetic-field Feshbach resonance is used to tune atom-atom interactions in the condensate and to reach a regime where quantum depletion and beyond mean-field corrections to the condensate chemical potential are significant. We use two-photon Bragg spectroscopy to probe the condensate exci...

Bose-Einstein condensates of quasiparticles such as exciton-polaritons, magnons and massive photons allow for new experimental possibilities as compared to the atomic Bose-Einstein condensates. In this Thesis we focus on a condensate of photons, first created at Bonn University in 2010. One of the recent achievements is the measurement of number fluctuations in such a condensate of photons. We ...

We theoretically study macroscopic entanglement between a magnetically trapped Bose-Einstein condensate and a superconducting loop. We treat the superconducting loop in a quantum superposition of two different flux states coupling with the magnetic trap to generate macroscopic entanglement. The scheme also provides a platform to investigate interferometry with an entangled Bose Einstein condens...

We show that in an atomic Bose gas near a Feshbach resonance a quantum phase transition occurs between a phase with only a molecular Bose-Einstein condensate and a phase with both an atomic and a molecular Bose-Einstein condensate. We show that the transition is characterized by an Ising order parameter. We also determine the phase diagram of the gas as a function of magnetic field and temperat...

State-selected rubidium-87 molecules were created at rest in a dilute Bose-Einstein condensate of rubidium-87 atoms with coherent free-bound stimulated Raman transitions. The transition rate exhibited a resonance line shape with an extremely narrow width as small as 1.5 kilohertz. The precise shape and position of the resonance are sensitive to the mean-field interactions between the molecules ...

In this work the validity of an analytic approximation expression [1] for the critical number of atoms that a Bose-Einstein Condensate can hold before collapsing when the interaction between them is attractive is checked. In order to do this the system is treated in a more precise way by numerically solving the Gross-Pitaevskii equation. One finds that the variational approach underestimates th...

The theory of linear wave structures generated in Bose-Einstein condensate flow past an obstacle is developed. The shape of wave crests and dependence of amplitude on coordinates far enough from the obstacle are calculated. The results are in good agreement with the results of numerical simulations obtained earlier. The theory gives a qualitative description of experiments with Bose-Einstein co...

A New Objective for High Resolution Imaging of Bose-Einstein Condensates In the framework of this thesis a new imaging system for the measurement of the atomic density distribution in Bose-Einstein condensates has been built and characterized. The achieved optical resolution of 1.1 µm according to the Rayleigh criterion makes it now possible to observe structures on the scale of the so-called '...