  In former work, we showed that a quantum algorithm requires the number of
operations (oracle's queries) of a classical algorithm that knows in advance
50% of the information that specifies the solution of the problem. We gave a
preliminary theoretical justification of this "50% rule" and checked that the
rule holds for a variety of quantum algorithms. Now, we make explicit the
information about the solution available to the algorithm throughout the
computation. The final projection on the solution becomes acquisition of the
knowledge of the solution on the part of the algorithm. Backdating to before
running the algorithm a time-symmetric part of this projection, feeds back to
the input of the computation 50% of the information acquired by reading the
solution.
