  The nitrogen-vacancy (NV) center in diamond has a wide range of potential
applications in quantum metrology, communications and computation. The key to
its use lies in how large the optical spin contrast is and the associated
fidelity of spin state readout. In this paper we propose a new mechanism for
improving contrast with a spin-to-charge protocol that relies on the use of an
external electrode and cryogenic temperatures to discretize the diamond
conduction band for spin-selective resonant photoionization. We use effective
mass theory to calculate the discrete eigenenergies in this new system and use
them to formulate a new spin-to-charge protocol that involves resonant
photoionization out the NV ground state into the diamond conduction band. The
major sources of broadening are also addressed which guide the design of the
experiment. With this mechanism we theorise an optical spin contrast that and
an associated spin readout fidelity of 85%. This significant improvement can be
applied to a number of cryogenic quantum technologies.
