A moving detector can sort photons by direction without disturbing them

Moving a photodetector at a controlled velocity Doppler-shifts the frequencies of counterpropagating photons differently, letting the detector's finite bandwidth do the sorting: one direction registers, the other doesn't. The photon itself isn't decohered—the directionality comes purely from the measurement basis the motion selects. This offers a velocity-tunable way to read out photon propagation direction, with sharpness set by the detector's quality factor rather than any optical element.