The Physics of Correlated Quantum Systems

START project Y 48 The Physics of Correlanted Quantum Systems Harald WEINFURTER 28.06.1996 Overview Quantum correlations and entanglement lie at the heart of the basic concepts of quantum mechanics as well as of the new field of quantum information. The goal of this project was to experimentally investigate the fundamental properties and features of entangled photons and their potential applications in quantum communication schemes. Work concentrated in developing the experimental tools to make quantum communication applications feasible and to enable new tests of the foundations of quantum mechanics. As part of the project a quantum cryptography set-up was built which allowed a careful test of various components and synchronization schemes. The experience gained will be used, in the next step, to devise a compact, user-friendly "modem" for a realistic application of quantum key distribution. An important goal was to improve existing down conversion sources of correlated photon pairs. By using a buildup cavity for the UV-pump beam an increase of the output by a factor of seven could be achieved. Concerning the experimental realization of new quantum communication schemes, a detailed study of "remote state preparation" was performed. As could be demonstrated, the state of a photon can be prepared remotely by manipulating its initially entangled twin in an additional degree of freedom. The experience gained here with experimenting in a higher dimensional Hilbert-space could be used for an experimental prove of the contextuality of observations of quantum mechanical systems.