When I put each single qubit within a quantum register using Hadamard gates in superposition, how does it work that the whole register quantum-state is in superposition?

On the math-level the register-state is the tensor-product of the single qubit-states; but how is this realized on the physical level? How do the qubits interact? Looking on entanglement I take a CNOT and I know this gate is realized on the physical level. But what about the tensor-product on the physical level?

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    $\begingroup$ Welcome to QCSE!, could you be more specific on what you mean by "how does it work"? $\endgroup$
    – R.W
    Commented Apr 19, 2022 at 12:43

1 Answer 1


According to the Solovay-Kitaev theorm, the Clifford gates (which are generated by $H$, $S$ and $CNOT$), along with a non-Clifford gate - say the $T$ gate, can be used to approximate any unitary down to arbitrary precision. The state of the register is a pure-state so it can be “reached” using only these gates. Notice that the only 2 qubit gate here is the $CNOT$.

What this implies is that it is enough to physically implement the $CNOT$ gate as the only 2 qubit gate, (as well as the mentioned single qubit gates) to reach the entangled state of both registers.

I hope this andswers your question.


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