A negative time delay may seem paradoxical, but what it means is that if you built a 'quantum' clock to measure how much time atoms are spending in the excited state, the clock hand would, under certain circumstances, move backward rather than forward.
Photons can get absorbed by the atoms they travel through. When this happens, the energy they carry causes the atoms' electrons to jump to a higher energy state.
One of the ways the atoms return to a ground state is that the energy gets re-emitted as photons. To an observer, this looks like the light that traveled through the medium was delayed.
According to the researchers, they were stunned that there was no 'expert consensus' on what actually happened to an individual photon during this delay.
Collection
[
|
...
]