Most quantum key distribution protocols using a two-dimensional basis, such as HV polarization as first proposed by Bennett and Brassard in 1984, are limited to a key generation density of 1 bit per photon. We increase this key density by encoding information in the transverse spatial displacement of the used photons. Employing this higher-dimensional Hilbert space together with modern singlephoton-detecting cameras, we demonstrate a proof-of-principle large-alphabet quantum key distribution experiment with 1024 symbols and a shared information between sender and receiver of 7bit per photon.

We have studied the dynamic properties of hybrid devices in which the piezoelectric material lead zirconate titanate is integrated with silicon photodiodes on-chip. Such an integrated system enables direct conversion of light energy into mechanical deformation and motion, opening up new pathways for light propulsion in microrobots and nanorobots. By operating our devices under alternating illumination and simultaneously recording the time-dependent deformation and surface potential, we were able to derive frequency and voltage dependent time constants and phase relations between photovoltage and deformation. We observed that the silicon top contact resistance limits the response time to 6 ms in small area devices in which the capacitance is low. Furthermore, we observed a phase transition at low frequency that seems to be consistent with the occurrence of a negative capacitance. Our method of using time-dependent atomic force and Kelvin probe force microscopy proves to be suitable for the investigation of nanoscale, dynamic properties of light-driven piezo systems and can lead the design of next generation devices.

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