Is there a quantum logic gate (or any sequence of QLGs) that takes as input two Qubits and outputs one Qubit?

  • If yes, could you please write it down in the simple example?
  • If no, could you explain why?

PS: Sorry for stupid question


1 Answer 1


Quantum gates are reversible and hence must have a one to one correspondence between the input and output bits. Another way of thinking about reversibility is that there is a way to run the circuit backwards from the set of outputs and get back all possible inputs. This means that you have the same number of input and output bits.

However, do note that the way circuits are implemented, you will not necessarily use all the output bits. Say, you're adding two bits. Your "answer" is a single bit and the remaining output bits are irrelevant.

All of the above holds for classical reversible computation as well!

  • $\begingroup$ Thanks for brief answer. I see there is a link between Quantum gates and Charles Bennetts discovery of reversible computation. I also partially see the I/O correspondence is implied by Unitary operation limitations. But one could also say that the behavior of classical (non-reversible) gates is caused by the fact, that information is lost somewhere (in the computation). $\endgroup$ Feb 26, 2019 at 12:03
  • $\begingroup$ The lost information corresponds to (roughly speaking) the heat generated by irreversible computation. For a more precise statement, see en.wikipedia.org/wiki/Landauer%27s_principle $\endgroup$
    – rnva
    Feb 26, 2019 at 13:35

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