Nonclassicality of axion-like dark matter through gravitational self-interactions

Understand

Axion-like particles (ALPs) are promising dark matter candidates.

• They are typically described by a classical field, motivated by large phase space occupation numbers.
• Here we show that such a description is accompanied by a quantum effect: squeezing due to gravitational self-interactions.
• For a typical QCD axion today, the onset of squeezing is reached on $\mathrm{\mu s}$-scales and grows over millennia.