Alicia Sit

Realization of electron vortices with large orbital angular momentum using miniature holograms fabricated by electron beam lithography

Free electron beams that carry high values of orbital angular momentum (OAM) possess large magnetic moments along the propagation direction. This makes them an ideal probe for measuring the electronic and magnetic properties of materials, and for fundamental experiments in magnetism. However, their generation requires the use of complex diffractive elements, which usually take the form of nano-fabricated holograms. Here, we show how the limitations of focused ion beam milling in the fabrication of such holograms can be overcome by using electron beam lithography. We demonstrate experimentally the realization of an electron vortex beam with the largest OAM value that has yet been reported (L = 1000h\bar), paving the way for even more demanding demonstrations and applications of electron beam shaping.

Experimental quantum cryptography in laboratory, long-distance and underwater conditions using structured light (Conference Presentation)

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 [1]. 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 [2]. 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 [5]. We further test the effect of water turbulences on an underwater quantum channel using twisted photons in an outdoor pool of 3 m length [6]. 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. [1] H. Rubinsztein-Dunlop et al. Roadmap on structured light, Journal of Optics 19, 013001 (2017) [2] M. Erhard, R. Fickler, M. Krenn, A. Zeilinger, Twisted Photons: New Quantum Perspectives in High Dimensions, Nature Light: Science & Applications, 7 17146 (2018) [3] F. Bouchard et al. Experimental investigation of quantum key distribution protocols with twisted photons, arXiv:1802.05773 [4] F. Bouchard, A. Sit, K. Heshami, R. Fickler, E. Karimi, Round-Robin Differential Phase-Shift Quantum Key Distribution with Twisted Photons, arXiv:1803.00166 [5] A. Sit et al. High-Dimensional Intra-City Quantum Cryptography with Structured Photons, Optica 4, 1006 (2017) [6] F. Bouchard et al. Underwater Quantum Key Distribution in Outdoor Conditions with Twisted Photons, arXiv:1801.10299

Quantum cryptography with twisted photons through an outdoor underwater channel

Quantum communication has been successfully implemented in optical fibres and through free-space. Fibre systems, though capable of fast key and low error rates, are impractical in communicating with destinations without an established fibre link. Free-space quantum channels can overcome such limitations and reach long distances with the advent of satellite-to-ground links. However, turbulence, resulting from local fluctuations in refractive index, becomes a major challenge by adding errors and losses. Recently, an interest in investigating the possibility of underwater quantum channels has arisen. Here, we investigate the effect of turbulence on an underwater quantum channel using twisted photons in outdoor conditions. We study the effect of turbulence on transmitted error rates, and compare different quantum cryptographic protocols in an underwater quantum channel, showing the feasibility of high-dimensional encoding schemes. Our work may open the way for secure high-dimensional quantum communication between submersibles, and provides important input for potential submersibles-to-satellite quantum communication.

General lossless spatial polarization transformations

Liquid crystals allow for the real-time control of the polarization of light. We describe and provide some experimental examples of the types of general polarization transformations, including universal polarization transformations, that can be accomplished with liquid crystals in tandem with fixed waveplates. Implementing these transformations with an array of liquid crystals, e.g., a spatial light modulator, allows for the manipulation of the polarization across a beams transverse plane. We outline applications of such general spatial polarization transformations in the generation of exotic types of vector polarized beams, a polarization magnifier, and the correction of polarization aberrations in light fields.