Figure 2. Sketch of the experimental set-up.

(a) One strand of DNA is stretched to about 90% longitudinal extension between the nanocapillary and the optical trap; tension is fully equilibrated, that is, . (b) After turning off the external potential at time t=0, the polymer retracts and the tension decays, allowing the attached bead to slowly relax towards its equilibrium position under the influence of the linear trap force f_trap(t), the polymer backbone tension f_P(L,t), and the (small) friction generated by the bead itself.