[Cryptography] Quantum computers are thirty years away and will remain thirty years away forever

[jrzx]

All existing quantum computers have not been quantum computers.  They have been demonstrations of the quantum effects that are the physical basis for an actual quantum computer.

A quantum computer is composed of Tofoli gates.  A useful quantum computer will need of millions of Tofoli gates.

For classical logic, nand is the universe gate, from which every logical relationship can be constructed.  For quantum, you need unitary gates, the simplest of which is the Tofoli gate, from which every logical relationship can be constructed.   No Tofoli gates, no quantum computer.

A  Tofoli gate iperforms a unitary transformation on three qbits.  No unitary transformation, no quantum computer.

So far no one has built a quantum Tofoli gate.  They have built quantum CNot gates, but if no unitary transformation of qbits, no quantum computer.

It is claimed that you can build a Tofoli gate out of CNOT gates.  This not true, though it has an element of truth..  Not one Tofoli gate has actually been realized.  You cannot  build a unitary gate out of non unitary gates -- what a quantum CNOT gate does do is demonstrate in principle the quantum physics that you are going to need to build an actual Tofoli gate, which is not in quite the same thing as building it out of CNOT gates.

A Tofoli can be thought of as a collection of CNOT gates, but a collection of separate CNOT gates cannot be a Tofoli gate.

It is not just a matter of feeding the outputs of some CNOT gates into the inputs of others.  You have to physically merge them into one gate.

And so far, not one actual Tofoli gate.
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