  The ability to coherently transport electron-spin states between different
sites of gate-defined semiconductor quantum dots is an essential ingredient for
a quantum-dot-based quantum computer. Previous shuttles using electrostatic
gating were too slow to move an electron within the spin dephasing time across
an array. Here we report a nanosecond-timescale spin transfer of individual
electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation
rates at a so-called `hot spot', we can upper bound the shuttle time to at most
150 ns. While actual shuttle times are likely shorter, 150 ns is already fast
enough to preserve spin coherence in e.g. silicon based quantum dots. This work
therefore realizes an important prerequisite for coherent spin transfer in
quantum dot arrays.
