Scientists have developed a novel interferometric technique capable of measuring time delays with zeptosecond (a trillionth of a billionth of a second) resolution. They have used this technique to measure the time delay between extreme ultraviolet light pulses emitted by two different isotopes of hydrogen molecules — H2 and D2 — interacting with intense infrared laser pulses. This delay was found to be less than three attoseconds (one quintillionth of a second long) and is caused by slightly different motions of the lighter and heavier nuclei.