
Dancing with Qubits
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In section 9.8.2, we saw an example of phase kickback while working with oracles. In this section, we look at the concept again in the context of controlled z-rotation gates. We need this within Shor’s factoring algorithm to estimate eigenvalues. phase$kickback
Consider the circuit in Figure 10.8.
As we saw in section 7.6.6, |1⟩ is an eigenvector of Rφz with eigenvalue e φi:
The initial quantum state in Figure 10.8 is |00⟩ = |0⟩⊗|0⟩. We apply H⊗X, yielding
We apply the CRφz:
We began with q0 as the control qubit. The phase from the Rφz gate conditionally applied to q1 ends up causing the phase e φi to appear in the state of q0. The state of q1 was not changed from |1⟩. This is the phase...