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The Toffoli gate takes in three inputs and gives out three outputs, and is often referred to as the quantum AND gate.

It takes in a,b,c and gives out a, b, c XOR (a AND b).

Why does it do that, instead of just giving out a,b,b AND c? or a,b,(a AND b)?

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    $\begingroup$ in a word, reversibility! $\endgroup$
    – vzn
    Commented Oct 21, 2015 at 15:08
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    $\begingroup$ That's approximately the 7th question you ask about quantum computation in the span of the last two week. there's no magic to understanding it but spending a lot of time, reading about it (maybe using a textbook) until you get the basics. Try to use this site for asking questions after you exhausted all your other immediate resources. $\endgroup$
    – Ran G.
    Commented Oct 21, 2015 at 20:34
  • $\begingroup$ Are you sure that's the intent of the site? Isn't it to have good answers to questions that people are likely to ask about? If everyone exhausted all resources before coming here, there would be no site. (Perhaps a better meta post, if this hasn't already been chatted about before) $\endgroup$
    – Alan Wolfe
    Commented Oct 21, 2015 at 21:02
  • $\begingroup$ @AlanWolfe, feel free to ask that question on meta -- I suspect you'll get some informative answers. You might like to take a look at cs.stackexchange.com/help/how-to-ask and meta.stackoverflow.com/q/261592/781723, to get some background. $\endgroup$
    – D.W. ♦
    Commented Oct 22, 2015 at 23:19

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As the comments said, if it worked like i asked, it would neither be reversible, nor a unitary matrix.

Both things are required for quantum computing!

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