  The basic operating element of standard quantum computation is the qubit, an
isolated two-level system that can be accurately controlled, initialized and
measured. However, the majority of proposed physical architectures for quantum
computation are built from systems that contain much more complicated Hilbert
space structures. Hence, defining a qubit requires the identification of an
appropriate controllable two-dimensional sub-system. This prompts the obvious
question of how well a qubit, thus defined, is confined to this subspace, and
whether we can experimentally quantify the potential leakage into to states
outside the qubit subspace. In this paper we demonstrate that subspace leakage
can be quantitatively characterized using minimal theoretical assumptions by
examining the Fourier spectrum of the Rabi oscillation experiment.
