Weak measurements and quantum-to-classical transitions in free electron-photon interactions

How does the quantum-to-classical transition of measurement occur?This question is vital for both foundations and applications of quantum mechanics.Here,we develop a new measurement-based framework for characterizing the classical and quantum free electron-photon interactions and then experimentally test it.We first analyze the transition from projective to weak measurement in generic light-matter interactions and show that any classical electron-laserbeam interaction can be represented as an outcome of weak measurement.In particular,the appearance of classical point-particle acceleration is an example of an amplified weak value resulting from weak measurement.A universal factor,exp(-Γ^(2)=2),quantifies the measurement regimes and their transition from quantum to classical,whereΓcorresponds to the ratio between the electron wavepacket size and the optical wavelength.This measurement-based formulation is experimentally verified in both limits of photon-induced near-field electron microscopy and the classical acceleration regime using a DLA.Our results shed new light on the transition from quantum to classical electrodynamics,enabling us to employ the essence of the wave-particle duality of both light and electrons in quantum measurement for exploring and applying many quantum and classical light-matter interactions.

Generating optical orbital angular momentum at visible wavelengths using a plasmonic metasurface

Light beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation.Until recently,it was thought that these two‘rotational’motions of light were largely independent and could not be coupled during light–matter interactions.However,it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling,where a change of the spin angular momentum will modify the orbital angular momentum and vice versa.In this work,we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum.A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas.Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of 62"(with conversion efficiency of at least 1%).This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy,nanoscale sensing and classical or quantum communications using integrated photonic devices.