Quantum Nondemolition Circuit for Testing Bipartite Complementarity

We present a quantum circuit that implements a nondemolition measurement of complementary single- and bipartite properties of a two-qubit system: entanglement and single-partite visibility and predictability. The system must be in a pure state with real coefficients in the computational basis, which allows a direct operational interpretation of those properties. The circuit can be realized in many systems of interest to quantum information.

Figure 1

Quantum circuit for QND measurement of concurrence. $|\chi ⟩$ is the two-qubit input state, the ancilla qubit is initially in the state $|0⟩$, and $R_x(\pi /2)=\exp (-i\pi {\sigma }_x/4)$.

Figure 2

Universal quantum circuit for QND measurement of concurrence, visibility and predictability. The dashed box is the ancilla state preparation and $R_{{\overrightarrow{\theta }}_i}=\exp (-i\overrightarrow{\sigma }·{\overrightarrow{\theta }}_i)$.