The light wasn't actually converted into sodium atoms. Light travels at roughly 300,000 km per second and, by flicking the light on and off quickly you can send pulses - sort of like Morse code. Scientists managed to slow these light pulses right down in a cloud of chilled sodium and then transfer those pulses to the sodium atoms, so they travelled along mimicking the dots and dashed embedded in the light beam. Those sodium atoms moved to the second cloud at a leisurely 200 metres per hours, where they were used to trigger a new light beam containing the same pulses which took off again at 300,000 km an hour.
No sodium atoms were actually created, they were just taken from the cloud, so it doesn't seem right to say scientists converted light to matter. They just transferred information from a light beam to matter and back again.
If you kick a red ball, which strikes a blue ball which in turn pushes another red ball, you can't say you converted a red ball into a blue ball.
All this is obviously an oversimplifaction but if you read the summary in Nature, quoting Harvard physicist Lene Vestergaard Hau, it's clear they didn't actually convert light to matter.
"Hau says that the 'messenger' [sodium] atoms that move between clouds are basically a 'matter copy' of the original light pulse: a piece of light cast in atoms, you could say."
This is still an amazing feature and is another step towards technology such as optical computing, but matter wasn't created out of light and then converted back again. Despite the headlines, no sodium atoms were harmed in the making of this story.