We demonstrate that an evanescent wave spin exists outside a finite quantum well by solving the Dirac equation in a finite cylindrical quantum well. The analytical analysis validates the wavefunction inside an infinite quantum well but recovers a non-zero evanescent wave outside the well. We propose that it is possible to probe or eavesdrop on quantum spin information through the evanescent wave spin without destroying the entire spin state. We argue that a spin-based quantum process or device is deterministic rather than probabilistic.