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Entanglement of optical photons using non-linear crystals has been around for a long time. Macroscopic entanglement using diamonds recently reported in the literature and receiving considerable attention. Quantum mechanics in biology has been the subject of fascinating research. Has anyone demonstrated entanglement of low frequency (i.e., microwave) photons? I see no theoretical objection, has it ever been demonstrated? Ditto entanglement at xray?

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The first report on entangled microwaves seems to be A clear proof of spatially separated entanglement with degenerate frequencies is reported in

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Hi Bob. If you want to merge your 2 accounts go here. – Qmechanic Feb 5 at 12:14

You can certainly entangle microwave photons with other quantum objects:

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Well but my question is can you entangle two photons at microwave frequencies? – carl Oct 6 '12 at 2:47
It is very difficult to measure single microwave photons. Compare the per photon energy to that of a visible photon, and to the room temperature energy scale, $kT$. – user2963 Oct 6 '12 at 3:23

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