A theater spotlight keeps the actor in its beam no matter where they step. A similar trick now works with terahertz radar: the short light pulse no longer gets lost when the object moves.

Terahertz waves pass through clothing and plastic but do not harm tissues. That is why they are being tested for security screening and medical diagnostics. The usual method measures the shape of a short pulse reflected from the sample. But if the sample shifts even a little, the pulse jumps out of the narrow observation window. Previously, the target had to freeze.

Japanese physicists built a setup with two lasers. One laser rapidly scans a narrow window, and the other, using a feedback signal, shifts that window to keep the pulse peak centered. This allowed them to track a mirror moving at 4 centimeters per second over a distance of 12 centimeters. That is 26 times the window width. The precision in locating the peak reached 225 nanometers, which is less than the wavelength of visible light.

In a model experiment, a gold mirror on a vibration motor mimicked breathing and heartbeat. The system recovered these weak oscillations against a large displacement of up to 10 centimeters. Previously, terahertz spectroscopy required a stationary sample, otherwise the pulse was lost. Now the window itself follows a target moving tens of centimeters, with precision down to fractions of a micron. This paves the way for contactless monitoring of breathing and heartbeat in moving people and for inspecting products on a conveyor belt.