Without feedback/loop how can we build a sequential circuit? The basic feature of sequential circuit is that is depends not only on the current inputs but also on the previous inputs/outputs. I've gone through few papers on reversible gates and I'm so confused. Is it possible to create a quantum circuit (which is obviously reversible) which is a sequential circuit?

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    $\begingroup$ Would it be possible for you to give an example of something you'd like to accomplish using a sequential circuit? Or an example of a circuit you have in mind? $\endgroup$ Jun 12, 2019 at 10:56
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    $\begingroup$ hmmm... a simple flipflop? I'm actually a beginner so I just got this doubt while reading book. $\endgroup$ Jun 12, 2019 at 21:26
  • $\begingroup$ This may be implementation dependent, but it is possible that it is trivial in qubits. Because the bit '1' is implemented as current flowing, and '0' as current not flowing, to hold that state when there is no input needs some extra mechanism, like the feedback mechanism in latches, which is the basis of sequential circuits. Here, I believe the qubits are held in their state in most implementations. $\endgroup$ Jun 14, 2019 at 9:15
  • $\begingroup$ So you want your circuit to change based on a previous result? $\endgroup$
    – draks ...
    Apr 27, 2020 at 10:28
  • $\begingroup$ What do you mean by previous outputs? If there are measurements taking place midway through the circuit, the circuit will not be reversible; then, classical control can always be used to change the circuit based on measurement results. If there are operations that change the state of some ancillary qubits, one can control subsequent quantum gates using those ancillae. If you do allow for feedback then you can actually undo arbitrary unitary operations, per doi.org/10.1103/PhysRevLett.123.210502 and doi.org/10.1103/PhysRevA.102.012602 $\endgroup$ May 10, 2021 at 21:04


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