  Improving the secret key rate is one of the vital issues in practical
applications of quantum key distribution (QKD). In this paper, we propose an
experimental scheme of high-dimensional measurement-device-independent quantum
key distribution (MDI-QKD) based on spatial basis aiming to increase the key
rate. Two groups of discrete position basis and momentum basis are applied as
conjugate spaces to generate quantum secret key. The position states are
transmitted by single-mode fibers and represented by the index of the fiber,
while the momentum state is a coherent superposition of the position states
characterized by the phase gradient. The measurement of each momentum basis is
realized by multi-slit diffraction. This experimental proposal can be
implemented with standard optical elements. In addition to an enhanced key rate
in a higher dimension, this high-dimensional MDI-QKD exhibits a comparable
security level/performance compared to conventional polarization-based MDI-QKD.
