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In the Qiskit textbook, at step 4, Bob who has received the classical bits from Alice then needs to apply a X and/or Z gate depending on what the classical bits received are, why is that?

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Without that fixup, the state of Bob's qubit can be in any one of the states $\alpha |0\rangle + \beta |1\rangle$ (the state we actually wanted to teleport) or $\alpha |0\rangle - \beta |1\rangle$, $\beta |0\rangle + \alpha |1\rangle$ or $\beta |0\rangle - \alpha |1\rangle$ (the states we don't want to end up with). X and Z gates are applied to fix the resulting state so that it ends up in $\alpha |0\rangle + \beta |1\rangle$ - otherwise we can't count our teleportation a a success.

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