Schematic figure of several single-molecule experiments. (a) Förster resonance energy transfer can occur between the donor and the receptor within a certain range. (b) Optical tweezers. Double DNA is attached to an optically trapped bead, and the free end is attached to the surface of the trapping chamber. (c) Magnetic tweezers base on permanent magnets. A pair of small permanent magnets produces a magnetic field gradient along the axial direction, and the rotation of magnets produces rotation of the magnetic bead. (d) Atomic force microscopy. A cantilever with a sharp tip holds above a piezoelectric scanning stage.