Bad Qubits — Learn Quantum by Doing

|q⟩ Bad Qubits

Multiple choice

Per this lesson, applying the controlled-S gate (cp(pi/2, 0, 1)) twice in a row implements which controlled operation?

Controlled-S, because the second application cancels the first
Controlled-T, because squaring halves the phase angle
Controlled-S^2 = CZ, because the phase angles add to P(pi) = Z
The identity, because S is its own inverse