Apply CNOT
Quantum enhanced phase measurements
All qubits are initially prepared in the state
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The CNOT gates create an N-qubit entangled state that is super sensitive to rotations in the xy-plane.
Apply phase
The phase shift produces the same tiny rotation as before on each qubit in the xy-plane.
A second round of CNOT gates disentangles the N qubits, and adds all the phase shifts to the central qubit.
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Measuring the central qubit (“+” or “–") reveals the presence of a tiny phase shift with a much greater sensitivity than can be achieved without entanglement.