Re: Quantum computing practically impossible

[Masataka Ohta <[e-mail]>]

Phillip Hallam-Baker wrote:

> It is my understanding of the work on Quantum error correction that it is
> correcting errors in the measurement of quantum states rather than trying
> to compensate for decoherence so the base assumption of the paper seems to
 > be off.

Read the draft and you can find Shor's paper, the first
paper on quantum error correction (reference [1] of the
draft), is titled "Scheme for reducing decoherence in
quantum computer memory".

The idea of Shor was that the measurement will reset randomly
decoherred state to one of a finite set of states. As measurement
result can identify which member of the set the states are
reset by the measurement, the original state can be restored,
which, Shor said, is reduction of decoherence, where reduction
means compensation for single, but not double, qubit errors.

The problem is that if a state is entangled with many unentangled
terms, unless relative coherence between the terms is strictly
retained, which is the impossible assumption of Shor, such
reduction is impossible.

Differently decoherred terms need term specific ways to reduce
decoherence, which is impossible by measuring fixed number of
extra qubits.

> There are good reasons to build quantum computers,

If only it had scaled with quantum error correction.

						Masataka Ohta