In Nielsen and Chuang, there's the following paragraph:

The Toffoli gate can be used to simulate NAND gates and can also be used to do FANOUT. With these two operations, it becomes possible to simulate all other elements in a classical circuit, and thus an arbitrary classical circuit can be simulated by an equivalent reversible circuit.

I'm confused about why FANIN isn't required to be able to simulate all other elements in a classical circuit, while FANOUT is required?

  • \$\begingroup\$ If copying text, please provide a reference to the text to avoid plagerisim \$\endgroup\$
    – Voltage Spike
    Commented Aug 19, 2021 at 22:25
  • \$\begingroup\$ Recommend migrating this question over to Quantum Computing SE. \$\endgroup\$
    – Mr. Snrub
    Commented Aug 21, 2021 at 5:45
  • \$\begingroup\$ @Mr.Snrub Thank you for the reply! Just curious, why is this question more suitable for QC SE? I thought what I mentioned in the question also appears in classical information system? \$\endgroup\$
    – Claire
    Commented Aug 23, 2021 at 5:14
  • \$\begingroup\$ My main reason is because you are citing an excerpt from a quantum computing book, so that seems best suited to the quantum computing community. EE.SE does handle some general information theory stuff, but in this case and in your other related question on Toffoli Gates I believe you are asking about "fan-in" and "fan-out" from a EE perspective, and I believe that's completely unrelated to FANOUT and FANIN in the context of Toffoli Gates. (1/2) \$\endgroup\$
    – Mr. Snrub
    Commented Aug 23, 2021 at 5:37
  • \$\begingroup\$ (2/2) (As a side note, is "FANIN" even a valid concept in relation to Toffoli Gates? I did a quick scan of Nielsen and Chuang and couldn't find any mention of it.) As you point out, Toffoli Gates aren't strictly a Quantum Computing thing; but "reversible computation" is still an area of much research & theory, far removed from the education and practice of most EEs. I think your odds of getting a useful answer will be much better over in the quantum computing SE. \$\endgroup\$
    – Mr. Snrub
    Commented Aug 23, 2021 at 5:45


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