Quantum watch timestamps time through helium interference

A team at Uppsala University demonstrates a “quantum watch” that reads elapsed time from evolving interference patterns among many excited helium states (Rydberg states) instead of counting clock ticks. By matching short slices of the changing quantum pattern to theory, they achieve femtosecond precision and uncover small drifts in conventional lab timing, offering a new tool for ultrafast experiments where the exact start time is hard to define.