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I have seen a question about the other way (CSWAP from CCNOT). Both are 3 qubit gates.

Is there a way to simulate CCnot from Fredkin gates?

user206904
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  • @CraigGidney is that answer even correct ? I remember reading somewhere that it IS possible. Also your answer does not point out what is wrong with the circuit given by OP... Thanks – user206904 Nov 25 '21 at 06:48
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    Their circuit is wrong because it doesn't restore the ancilla qubits. It leaks information, which will prevent interference from working correctly later. – Craig Gidney Nov 25 '21 at 06:52
  • @CraigGidney, ok, I see why their circuit is wrong. Thanks. But can you develop a bit on the part where you mentioned might work logically but not physically? – user206904 Nov 25 '21 at 19:48
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    If you use a dual rail encoding where logical 0 is physical 01 and logical 1 is physical 10 then a physical cswap can implement the logical toffoli, because there's no need to change the number of physical 1s. – Craig Gidney Nov 25 '21 at 20:27

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