Using an electron interferometric technique researchers report a birth time delay on the order of a few hundred zeptoseconds (247 zeptoseconds) between two electron emissions from the two sides of molecular hydrogen, which is interpreted as the travel time of the photon across the molecule. The proposed technique is generally applicable to more complex systems, and further studies are necessary to support this interpretation.

A zeptosecond is a trillionth of a billionth of a second (10^-21 seconds).

A femtosecond equals 0.000000000000001 seconds, or 10^-15 seconds. Light travels 300 nanometers in a femtosecond.
An attosecond is 10^-18 seconds. Light travels 0.3 nanometers in an attosecond.
Light travels 0.07 nanometers or 70 picometers in 247 zeptoseconds.

This is the shortest timespan that has been successfully measured to date.

The scientists carried out the time measurement on a hydrogen molecule (H2) which they irradiated with X-rays from the synchrotron lightsource PETRA III at the Hamburg accelerator centre DESY. The researchers set the energy of the X-rays so that one photon was sufficient to eject both electrons out of the hydrogen molecule.

Electrons behave like particles and waves simultaneously, and therefore the ejection of the first electron resulted in electron waves launched first in the one, and then in the second hydrogen molecule atom in quick succession, with the waves merging.