# quantum entanglement at microwave frequencies

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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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