A Heisenberg's dog is the central unit in a quantum computer that fact
ors prime numbers. We report the outcome of a proof-of-principle Gedan
kenexperiment in which the number 3 was factored and good agreement wa
s found both with an analytical theoretical prediction and with the re
sults of a quantum Las Vegas calculation. Other future applications of
Heisenberg's dog are hinted at. The anonymous reports of the referees
and our reply to them are reproduced in appendix A and appendix B res
pectively.