«Imagine, if you will, a collection of many photons. Now imagine that they have mass, repulsive interactions, and number conservation.»

The new era of polariton condensates. D. W. Snoke and J. Keeling in Physics Today 70:54 (2017).  What the paper says!?

This is a nice, brief overview of polariton condensates for a broad audience, still with some details on, e.g., the material aspects.

Polariton BECs are said to be useful for

optical devices, in cluding low-threshold lasers, superfluid photonic circuits, and all-optical transistors.

This claim:

full thermal equilibration is necessary for true superfluidity.

A list of important results:

The latest experiments have also produced a wide range of condensation-related phenomena beyond those seen in early demonstrations, including Josephson oscillations of two coupled condensates, phase locking of two or more condensates, quantized vortices, and the superfluid-like suppression of scat tering from defects.

What seems most "transformative" to the authors:

Perhaps no development would be more transformative than fabrication of a polariton-based transistor.