Quantum technologies

Invited speaker: Prof. Han Woerdman
Affiliation: Universität Leiden
Title: Quantum Entanglement of Orbital Angular Momentum
Time and room: 17 h c.t., lecture hall IAP
Abstract: The phase of a light beam can be twisted as a corkscrew around its axis of propagation. Such fields have drawn much attention, in particular since it was realized in 1992 that a twisted light beam carries Orbital Angular Momentum (OAM), namely lħ per photon; here the integer l is the azimuthal mode index of the beam. The concept of OAM has spread into many branches of optics, ranging from optical trapping to quantum information. In optical trapping OAM enables mechanical rotation due to transfer of OAM from light to matter, where the material object can vary from a Bose-Einstein condensate to a living cell. In quantum information OAM allows a high-dimensional alphabet associated with a single photon instead of the usual two-dimensional polarization qubit. Classically speaking, this OAM multiplexing may increase the capacity of a communication channel.

In the talk I will introduce and compare some of the many methods to produce OAM light beams. This serves as a starting point for the main part of the talk which deals with high-dimensional OAM entanglement of two photons. Particularly interesting is the case that the azimuthal index l of the photon state is non-integer; here the half-integer state plays a special role. Such a state corresponds to a high-dimensional superposition of integer OAM states; it can be easily produced in the lab. Quantifying the dimensionality of bipartite entanglement of these non-integer OAM states requires considerable care; for this purpose we introduce the Shannon dimension, in addition to the better known Schmidt dimension. So far we have demonstrated a Shannon dimension up to 6, and a further increase by an order of magnitude seems possible.