AMO In the literature: open

Crystallization of photons in Rydberg media

In plain words

Theory predicts that photons slowed in a Rydberg atom cloud can repel so strongly that they line up like a crystal. This has not been seen for more than a few photons.

Precise statement

Rydberg electromagnetically induced transparency medium with blockade radius $R_{b}$ and optical depth per blockade radius $\mathrm{OD}_{b}$. Predicted crystalline order of polaritons (Otterbach et al., PRL 2013). Measure n-photon correlations g^(n)(tau_1,...) showing periodic order across at least 3 photons. Answer: observed or ruled out at accessible $\mathrm{OD}_{b}$.

What would settle it

Multi-photon correlation measurements showing periodic density order for three or more photons.