Quantum enhanced phase measurements
All qubits are initially prepared in the state
The CNOT gates create an N-qubit entangled state that is super sensitive to rotations in the xy-plane.
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.
Measuring the central qubit (“+” or “–") reveals the presence of a tiny phase shift with a much greater sensitivity than can be achieved without entanglement.