Quantum Entanglement is at the heart of Quantum Information Science, a rapidly developing new field in modern physics which amazes us with fundamentally new ways of communication and computation. This book describes the development and performance of a polarization independent ultrafast interferometric switchable beam splitter, which can be used for several quantum mechanical experiments with photons. It consists of a Mach-Zehnder interferometer with one Pockels cell or Electro Optical Modulator in each arm, which varies the phase of a transmitted beam by an electric field induced birefringence in a crystal. The splitting ratio of the effective beam splitter can be tuned by changing the applied voltage. Such an ultrafast switchable beam splitter can be very useful for the experimental testing of some basic principles of quantum mechanics (e.g. entanglement swapping) and also for practical applications (e.g. quantum cryptography). Furthermore this book describes the realization of two experiments which deal with two fundamental principles of quantum mechanics, complementarity and entanglement.