Quantum Spring
  In this paper, we will give a short review on \textit{quantum spring}, which is a Casimir effect from the helix boundary condition that proposed in our earlier works. The Casimir force parallel to the axis of the helix behaves very much like the force on a spring that obeys the Hooke's law when the ratio $r$ of the pitch to the circumference of the helix is small, but in this case, the force comes from a quantum effect, so we would like to call it \textit{quantum spring}. On the other hand, the force perpendicular to the axis decreases monotonically with the increasing of the ratio $r$. Both forces are attractive and their behaviors are the same in two and three dimensions. 
Chao-Jun Feng, Xin-Zhou Li
hep-th quant-ph