  Feynman's model of a quantum computer provides an example of a
continuous-time quantum walk. Its clocking mechanism is an excitation of a
basically linear chain of spins with occasional controlled jumps which allow
for motion on a planar graph. The spreading of the wave packet poses
limitations on the probability of ever completing the $s$ elementary steps of a
computation: an additional amount of storage space $\delta$ is needed in order
to achieve an assigned completion probability. In this note we study the END
instruction, viewed as a measurement of the position of the clocking
excitation: a $\pi$-pulse indefinitely freezes the contents of the input/output
register, with a probability depending only on the ratio $\delta/s$.
