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I agree that this is the basic principle. Since we can now be in a superposition of a |O> and a |1> state (no matter how we have defined them), we are able to explore several paths in a binary tree at once, and thus to reach extreme degrees of parallelism.

The problem is when I try to go beyond that basic principle...

Apparently, the simple fact that most classical operations like NAND, OR, and XOR are irreversible makes them unsuitable for quantum computing. Thus, we need to re-think computing in terms of fully reversible operations like the ones at http://en.wikipedia.org/wiki/Quantum_gate . This is where things start to feel strange.

Moreover, extreme parallelism is more the realm of computers than it is the one of human minds. Trying to picture oneself what "You get a quantum machine to generate every possible alternate password and have them used AND changed all at the same time, at once, in a single instance." means in practice is difficult. How exactly could I be using several passwords at once ? I mean, as soon as an intruder measures my stored data, I have only used one single password on it, right ?

The problem is when I try to go beyond that basic principle...

Moreover, extreme parallelism is more the realm of computers than it is the one of human minds. Trying to picture oneself what "You get a quantum machine to generate every possible alternate password and have them used AND changed all at the same time, at once, in a single instance." means in practice is difficult. How exactly could I be using several passwords at once ? I mean, as soon as an intruder measures my stored data, I have only used one single password on it, right ?

Guessing a password is not a computational problem, but decryption is. Suppose that your data is encrypted with a really really really hard to determine password. Also, suppose that there is a reasonably reliable test to determine when data is decrypted successfully.

With conventional computing devices, an attacker must try to guess your password. For every

**guess**, he/she must attempt to decrypt your data and determine if the decryption is successful. This takes a lot of time and computing resources (this is essentially the justification for password encryption - decryption without knowing the password is too much trouble to be worth the time of an attacker).

A possible fear (and a valid one, IMHO) is that an attacker with a quantum computing device can skip the step of guessing your password. He/she can attempt decryption by

**all**password possibilities simultaneously.

In this way, a whole level of complexity (and a whole order of magnitude) would be removed.

Make more sense?

Member since:

2005-07-07

'Quantum computing' really just means following all possible paths in a branch simultaneously. So for example:

We're writing a program to search a binary tree for a value:

- In a 'standard' program, each step involves deciding whether to look at the right or left half of the current tree.

- In a 'quantum' program, we always look at both halves simultaneously.