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All items in the universe can be said to have a De Broglie wavelength which is significant if that wavelength is comparable to the object's size (I don't have to worry about my wavelength diffracting me when I walk through a doorway). What is the general upper limit of size for appreciable diffraction/interference in physics today, and why is it the upper limit (is it technological or physical)?

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As far as I know the largest object that has been successfully diffracted is a oligoporphyrin molecule with a molecular weight of about $25000$. This was done on 2019 and is reported in Quantum superposition of molecules beyond 25 kDa by Fein at al in Nature Physics volume 15, pages 1242–1245.

Quantum behaviour has been demonstrated in larger objects but the behaviour being observed was superposition not diffraction.

To observe diffraction we need the system to remain coherent, but the decoherence time decreases rapidly with increasing size. In principle there is no limit to how how large an object can be and still show quantum behaviour, but in practice the coherence lasts for too short a time to be observed for macroscopic objects.

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Electrons are a relatively light particle, and therefore have a relatively long De Broglie wavelength. Electrons have been diffracted through the spaces between atoms in a crystal.

A proton is about $1800$ times heavier, and has a correspondingly smaller wavelength. It would be hard to find a similar diffraction grating.

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