  Fu and Kane have taught us that a Majorana zero-mode appears on the quantum
spin Hall edge at the interface with a superconductor. If a magnetic scatterer
is placed on the edge, the zero-point energy of massless edge excitations
exerts a force on the scatterer. This is the fermionic analogue of the
electromagnetic Casimir effect. We show that the Majorana zero-mode produces a
repulsive Casimir force, pushing the scatterer away from the superconductor.
Unlike some other signatures of Majorana zero-modes, the repulsive Casimir
force is directly tied to the topological invariant of the system (the sign of
the determinant of the reflection matrix from the superconductor).
