Really short version: day-um.
And what would you do with pulses of light that short? Jonathan Marangos at Imperial College London, UK, says the super-short flashes could let researchers image the movement of electrons around large atoms. Can you dig the kind of impact that could have, to better understand the basic building blocks of matter? They've already used it to take take a pic of a couple oscillations of the original laser pulse.
Right now, the "atomic unit of time" is 24 attoseconds, which is the time it takes an electron to travel from one side of a hydrogen atom to the other. Marangos thinks even shorter pulses are possible, saying zeptosecond pulses (trillionths of a billionth of a second!) might be possible. These would be capable of imaging the movement of nuclear particles like protons.
Science marches on.