  The threading of a polymer chain through a small pore is a classic problem in
polymer dynamics and underlies nanopore sensing technology. However important
experimental aspects of the polymer motion in a solid-state nanopore, such as
an accurate measurement of the velocity variation during translocation, have
remained elusive. In this work we analysed the translocation through conical
quartz nanopores of a 7 kbp DNA double-strand labelled with six markers equally
spaced along its contour. These markers, constructed from DNA hairpins, give
direct experimental access to the translocation dynamics. On average we measure
a 5% reduction in velocity during the translocation. We also find a striking
correlation in velocity fluctuations with a decay constant of 100s of {\mu}s.
These results shed light on hitherto unresolved problems in the dynamics of DNA
translocation and provide guidance for experiments seeking to determine
positional information along a DNA strand.
