  Magnetic monopoles are hypothesised elementary particles connected by Dirac
strings that behave like infinitely thin solenoids. Despite decades of
searches, free magnetic monopoles and their Dirac strings have eluded
experimental detection, although there is substantial evidence for deconfined
magnetic monopole quasiparticles in spin ice materials. Here we report the
detection of a hierarchy of unequally-spaced magnetic excitations \emph{via}
high resolution inelastic neutron spectroscopic measurements on the quantum
spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$. These excitations are
well-described by a simple model of monopole pairs bound by a linear potential
with an effective tension of 0.642(8) K~$\cdot$\AA$^{-1}$ at 1.65~K. The
success of the linear potential model suggests that these low energy magnetic
excitations are direct spectroscopic evidence for the confinement of magnetic
monopole quasiparticles in the quantum spin ice candidate
Pr$_{2}$Sn$_{2}$O$_{7}$.
