Quantum theory notoriously sets limits on how precisely we can make measurements. But the quirks of the quantum realm can also be turned to advantage. Physicists have now demonstrated a way to almost double measurement precision when using photons to gauge distances.
Like markings on a ruler, the orderly waves of laser light can be used to measure lengths. In an interferometer, a laser beam is split into two beams that take two different paths. The beams bounce off mirrors and converge at the other end of the instrument, where their crests and troughs add together or cancel, depending on how these features line up. The resulting interference pattern reveals tiny differences in how far the two beams traveled. For example, a small displacement of one mirror will cause the interference pattern to shift.