Free-energy recovery in a classical system in a 20 base-pairs-long DNA hairpin mechanically unfolded by optical tweezers . (Upper panel) Experimental setup. The hairpin is inserted between two handles and the trap pulls it at a constant speed. (Lower panel) Work distributions can be measured by repeated stretching (continuous lines)/releasing (dashed lines) cycles at three different speeds: slow (blue), green (medium), and red (fast). According to Eq. (2), the stretching (forward process) and the releasing (reverse process) work distributions cross at a value of the work W, which is independent of the pulling speed and equal to the total free-energy difference ΔG. (The range of crossing is indicated by vertical dashed lines.) This property should be observed in quantum systems as well.