Spatial entanglement and optical singularities (Wolfgang Löffler)

oam wavefront A standard Gaussian light beam has plane wave fronts. But we can also have light fields where the wavefront is twisted; in this case a spiraling Poynting vector can lead to the appearance of orbital angular momentum (OAM). In 1992, it was found in Leiden that a single photon has an OAM of oam ellhbar where oam ell is usually an integer number. This enables control over the huge Hilbert space of the spatial degrees of freedom, and is experimentally accessible with computer-controlled spatial light modulators.


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Fiber transport of spatially entangled photons

oamfiber How can such spatially OAM-entangled photons be transported practically? Clearly, fiber-based distribution would be very important for application in quantum communication. However, by definition, one needs a multi-mode fiber. It is well known that in conventional multi-mode fibers, uncontrolled coupling of the modes quickly destroys coherent mode superpositions. As a consequence, the quantum information, which is encoded in the spatial degrees of freedom, would suffer from quick decoherence. In this project, we investigate how this can be overcome by using hollow-core photonic crystal fibers in collaboration with P. Russell's group at the MPI in Erlangen. The figure shows an electron microcraph of the cross-section of a hollow-core photonic crystal fiber. The sketchy wave fronts indicate a superposition of photons with different orbital angular momentum


Scattering of Twisted Light by Chiral Particles

twisted How is twisted light scattered by helical particles? The interesting case is that of very small particles, let's say, of the order of a few optical wavelengths. Clearly, the twisted light, which we scatter from such particles, has to be focussed strongly, and this regime is not well known. Helical particles (left) appear often in nature: Many building blocks, i.e., molecules, are chiral, and self-assembly frequently leads to a helical super-structure. Twisted light (right) is light with helical wavefronts.


clc As a first step (and to contribute to an ongoing discussion), we asked ourselves if the circular dichroism of a cholesteric polymer (a solid form of a cholesteric liquid crystal) is sensitive to the light's twist under strong-focussing condition. The latter point makes this in particular interesting, because in focussed light beams, the spin (polarization) couples to the orbital (phase) angular momentum.