Abstract
Matter–wave interferometry has been used extensively over the last few years to demonstrate the quantum-mechanical wave nature of increasingly larger and more massive particles. We have recently suggested the use of the historical Poisson spot setup to test the diffraction properties of larger objects. In this paper, we present the results of a classical particle van der Waals (vdW) force model for a Poisson spot experimental setup and compare these to Fresnel diffraction calculations with a vdW phase term. We include the effect of disc-edge roughness in both models. Calculations are performed with D2 and with C70 using realistic parameters. We find that the sensitivity of the on-axis interference/focus spot to disc-edge roughness is very different in the two cases. We conclude that by measuring the intensity on the optical axis as a function of disc-edge roughness, it can be determined whether the objects behave as de Broglie waves or classical particles. The scaling of the Poisson spot experiment to larger molecular masses is, however, not as favorable as in the case of near-field light-grating-based interferometers. Instead, we discuss the possibility of studying the Casimir–Polder potential using the Poisson spot setup.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. From the experiments by Davisson and Germer in 1927, we know that material particles such as electrons behave like waves. However, the question remains: how large, massive or complex can a particle be before it stops exhibiting wave properties? At the beginning of the 19th century the wave nature of light was demonstrated in the famous Poisson spot experiment, where light diffracts into a bright spot at the center of the shadow of a circular object. We have previously used this experimental configuration to demonstrate the wave nature of deuterium molecules. In this article, we explore the feasibility of scaling the experiment to large molecules such as the fullerene C70, and thus use it to explore the wave properties of larger molecules.
Main results. The classical van der Waals force, which attracts neutral particles towards the shadow-casting object, can also result in a bright spot at the center of the shadow. We show that a clear distinction between the two models (particle or wave) can be drawn if one considers diffraction from circular discs with varying amounts of edge roughness. The effect from the close-range van der Waals force is more significantly affected by these imperfections in the circular object.
Wider implications. In future Poisson spot experiments with large molecules, we hope to see contributions from the longer range Casimir–Polder force, which also attracts molecules towards the circular object.
Figure. The Poisson spot experiment: a light point source illuminates a circular object, casting a shadow onto a screen. At the center of the shadow a bright spot can be observed—this is the Poisson spot.