Investigation of complex spatial mode structures of Photons

Abstract

Photons are excellent carrier of information in quantum communication or quantum computation. They interact with the environment very weakly, can be controlled very precisely and information can be encoded in several different degrees of freedom such as polarisation or frequency. Furthermore, photons can also have complex spatial structures. This additional degree of freedom leads to a number of fascinating properties such as phase vortices and singularities, orbital angular momentum and the possibility to access higher-dimensional Hilbert spaces. Those complex structures and their properties are the scope of this thesis.<br />After classifying modes with higher-order spatial structure in chapter 2, we analyse one specific family - namely the Ince-Gauss modes - in chapter 3. They possess a complex vortex- and singularity structure and we believe that they could open up exciting possibilities for new fundamental quantum experiments as well as for novel methods in quantum cryptography and quantum information science. We analyse their phase-vortex behaviour and show for the first time the entanglement of Ince-Gauss modes. We also measure an Ince-Gauss specific quantum correlation function, which could be used in enhanced quantum communication protocols. An idea for a very simple protocol that takes advantage of this correlation function can be found in the appendix. In the 4th chapter, we consider a special type of artificial beams, which we call the Maverick beams. We construct and analyse them for the first time, and observe very fascinating intensity distribution of the generated beams, such as spiral patterns or smooth generation of radial rings. With a special method of creating an artificial Bloch sphere, we are able to perform the first quantum experiments with Maverick beams.<br />We show the entanglement of several members of these beams, and analyse the quantum correlation of many other cases. It turns out that the quantum correlation depends on the symmetry of the phase structure of the mode.<br />