Light with a complex amplitude structure invokes interesting fundamental properties such as phase and polarization singularities, which also enables novel applications in classical and quantum optical experiments . One feature, namely a twisted phase front and its orbital angular momentum, attracted a lot of attention due its broad range of applications. In the quantum domain, structured photons are highly beneficial since they serve as a physical realizations of high-dimensional states, which allow for example an enlarged information content per single carrier and are known to have a better noise resistance in quantum cryptography applications .
At first, I will present a set of laboratory experiments, in which we investigate different quantum cryptographic protocols. Our versatile approach relies on a heralded single photon source, a preparation stage at Alice’s sender, a 1 m-long quantum channel, and a detection stage at Bob’s receiver unit. Because the generation and detection is performed using computer generated, re-programmable holograms displayed on spatial light modulators, the same setup can be used to experimentally survey different quantum key distribution techniques and compare their benefits and deficiencies. The investigated protocols are all based on high-dimensional quantum states and include the seminal protocol of Bennett & Brassard, tomographic protocols, and recently introduced differential phase shift protocols [3,4]. We compare the performance of the different approaches in terms of noise resistance and secret key rates. Our study highlights the benefits of using structured photons and high-dimensional quantum states for different implementations and channel conditions.
In a second series of experiments, we get a step closer to real world implementations and investigate long distance and underwater quantum cryptography using high-dimensional quantum information encoded on structured light. We establish an approx. 280m long intra-city quantum link and study the influence of turbulence on achievable key rates . We further test the effect of water turbulences on an underwater quantum channel using twisted photons in an outdoor pool of 3 m length . Although we are able establish a secure channel with three dimensional quantum states, we find mode deformations and vortex splitting due to strong turbulent conditions most probably caused by local variations in temperature. We perform a detailed analysis of the observed turbulence and find that underwater channels may give rise to turbulent conditions that are fundamentally different in terms of temporal and spatial disturbance from those present in a free-space channel.
 H. Rubinsztein-Dunlop et al. Roadmap on structured light, Journal of Optics 19, 013001 (2017)
 M. Erhard, R. Fickler, M. Krenn, A. Zeilinger, Twisted Photons: New Quantum Perspectives in High Dimensions, Nature Light: Science & Applications, 7 17146 (2018)
 F. Bouchard et al. Experimental investigation of quantum key distribution protocols with twisted photons, arXiv:1802.05773
 F. Bouchard, A. Sit, K. Heshami, R. Fickler, E. Karimi, Round-Robin Differential Phase-Shift Quantum Key Distribution with Twisted Photons, arXiv:1803.00166
 A. Sit et al. High-Dimensional Intra-City Quantum Cryptography with Structured Photons, Optica 4, 1006 (2017)
 F. Bouchard et al. Underwater Quantum Key Distribution in Outdoor Conditions with Twisted Photons, arXiv:1801.10299
Tuning vector vortex in spatially coherent multicolored beam is studied. A wavelength filter based on the tunable modal properties of light is experimentally demonstrated, and the polarization topology of optical beam profile is mapped. A hybrid mode-wavelength division multiplexing (HMWDM) scheme is proposed. In this scheme information is encoded in the wavelength of light, and the spatial mode and polarization modulation about the optical mode is used to turn on and off frequency channels. This scheme is applicable to increase information capacity of light and in high resolution microscopy.
A spatially incoherent white light optical vortex is generated using a tunable liquid crystal q-plate and white lamp source. This work investigates the propagation of incoherent vector vortex to the far field, and makes comparisons with a coherent optical vortex at a particular wavelength. The contrast ratio between the vortex’s ring and core darkness is determined, and the polarization of the vortex s mapped. For the keywords, select up to 8 key terms for a search on your manuscript's subject.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information
field. In this paper we report the characterization in the quantum regime of a recently introduced
optical device, known as q-plate. Exploiting the spin-orbit coupling that takes place in the q-plate, it is possible
to transfer coherently the information from the polarization to the orbital angular momentum degree of freedom,
and viceversa. Hence the q-plate provides a reliable bi-directional interface between polarization and orbital
angular momentum. As a first paradigmatic demonstration of the q-plate properties, we have carried out the
first experimental Hong-Ou-mandel effect purely observed in the orbital angular momentum degree of freedom.