Bose-Einstein condensate

Bose-Einstein condensate (BEC) is a state of matter that forms with certain elements near absolute zero with identical enough atoms. Temperatures usually have to be so low that movements of elements slow below 1 mm per second compared to usual 0,5 km/s at room temperature. BEC requires thorough vacuum as BEC's produced usually have around 1000 atoms and if they move so slowly then 1 room temperature air molecule can increase their average speed a lot.  Special thing about BEC is that it can slow light down to speeds people can run like 25 km/h and it seems to have 0 viscosity. Bose sent this idea about existence of BEC to Einstein around 1924 and temperature needed to create it was reached in 1995 with few thousand rubidium atoms cooled to 170 nanokelvins. Using this calculator it looks like rubidium would have to move move 0,57 mm/s (20 meters per hour). In 1938 helium-4 that got cooled to 2,17 K (about 100 m/s) showed also partial BEC attributes like lack of viscosity and slowed light but less than 10% of gas was considered BEC.
During cooling atoms go to minimal energy they are known to have so far and this allows them to go together more densely in one place. Experiments with lithium seemed to show that they also form dense BEC due to their attractive forces (unlike rubidium atoms that repel each other) but after some maximum number of lithium atoms is reached their attraction can cause sudden collapse somewhat similar to star imploding shortly before supernova explosion. In case of rubidium strong magnetic fields can cause sudden attractive forces that imploded atoms enough to hide them temporarily and then they got energy to fly away from each other.
One way to slow light is to make material transparent to very narrow wavelength while it blocks other wavelengths.  

In 2009 a 0,1 mm cloud of sodium in form of BEC seemed to store light for 1,5 seconds.

From 1999 article. BEC state seems to turn entire gas cloud into single atom or somewhat liquid "laser" with waves similar to radio waves shared by atoms spreading through it. Vacuum has to be at least hundreds of trillions times thinner than atmospheric pressure.

2001 article. Sodium BEC seems mostly nontransparent but lasers can affect transparency. Photons are fast due to their zero or near-zero mass but BEC and atoms have mass. If that electromagnetic fluctuation common to light photon gets absorbed with atoms it forms "polariton" that carries electromagnetic activity but is much slower due to extra mass and in case of BEC entire gas cloud can turn into polariton with further increased mass and slowed light speed (additional BEC mass seems to help in this slowing until cloud is too large to stay BEC). Lasers had to be adjusted (weakened) to increase transparency of BEC and proportion of polaritons that were atoms while reducing proportion of photon type polaritons. After atomic polaritons formed they released their light after turning up the laser. Kinda like low light charges them with light and extra energy makes it release again but such light storage happens at low temperatures.

Possible simplified explanation

BEC could work because while atomic nuclei are moving at ~1 mm/s their electrons will have plenty time to respond to each others aligning electrons so voltage differences between atoms would minimize. As they cool and get closer they also get their electrons closer to each other which could make them react to each other faster increasing conductivity with that. If photons reach conductive enough material they give their electromagnetic fluctuations over to that material. In metal this absorbed photon energy could move fast but in very cold gas with electrons bit more further from each other they could respond by slowly giving their electric field to neighboring atoms.

Consider that almost all elements (except helium) used for making BEC had 1 outer electron to manipulate with laser as they were from 1st periodic table group like lithium, sodium, rubidium and cesium. They get minimal energy they would align their only electron in ways that there would be least pushing between them. If laser forces electrons to one side they could all do it at almost same time like single atom trying to align its electron with outer electric field and they lose this energy as electromagnetic radiation when they fall down to their minimal energy state. 

Size of atoms was probably major reason why lithium and rubidium behaved differently. Lithium is relatively smaller and bit more electronegative making it attract electron more due to smaller distance between positive charge of atomic nucleus and atom surface while in rubidium there are many more negative electrons between rubidiums nucleus and surface making it likelier that atoms repel.