FQxI physicists Aephraim Steinberg, Howard Wiseman and colleagues have been in the news for the past couple of weeks thanks to their recent preprint, "Experimental evidence that a photon can spend a negative amount of time in an atom cloud." Coverage has appeared in New Scientist, Scientific American and The Independent.
From The Independent:
Photons absorbed by a material experience a time delay – also known as a group delay – before exiting the material due to their interactions with atoms within it. To better understand this process, the researchers came up with an experiment to shoot photons through a cloud of ultracold atoms and observe the atomic excitation. To their surprise, the transit time of some of the photons through the atoms concluded before the atomic excitation had finished, giving it a negative value and making it appear as though the photons exited the material before entering it.
Writing on Twitter/X, Steinberg said, “Woo-hoo! It took a positive amount of time, but our experiment observing that photons can make atoms seem to spend a *negative* amount of time in the excited state is up! It sounds crazy, I know, but check it out!"