Bad Qubits — Learn Quantum by Doing

Multiple choice

In the one-bit teleportation gadget, an input

|\psi\rangle

on wire 1 is entangled with an ancilla

|+\rangle

on wire 2 via controlled-

Z

, then wire 1 is measured in the

X

basis with outcome

s

. What state does wire 2 hold?

$Z^s |\psi\rangle$ — only a phase byproduct, with no gate applied
$|\psi\rangle$ unchanged, since measurement cannot apply a gate
$H |\psi\rangle$ exactly, with no outcome dependence at all
$X^s H |\psi\rangle$ — the Hadamard is always applied, with only an outcome-dependent Pauli byproduct $X^s$