A new “quantum watch” uses changing interference patterns inside excited helium atoms to determine how much time has passed without counting from a defined time zero. Uppsala University researchers found that short slices of these quantum patterns can serve as distinctive timestamps, with estimated timing accuracy of about 8 femtoseconds. The method exposed a small drift in conventional laboratory timing equipment and could help ultrafast experiments where establishing an exact starting time is difficult. A clock normally needs a beginning. Set the starting point, count the ticks and calculate how much time has passed. A quantum system built from excited helium atoms could offer a different approach. Researchers at Uppsala University have shown that complex interference patterns inside highly excited atoms can act as timestamps. Instead of counting regular oscillations from a known zero point, the method reads a distinctive quantum pattern and matches it to theory to determine how much time has elapsed. The team calls the method a “quantum watch,” rather than a clock. Its key signal comes from quasiunique beat signatures, or …