Realization of a Minimal Disturbance Quantum Measurement

We report the first experimental realization of an “optimal” quantum device able to perform a minimal disturbance measurement on polarization encoded qubits saturating the theoretical boundary established between the classical knowledge acquired of any input state, i.e., a “classical guess,” and the fidelity of the same state after disturbance due to measurement. The device has been physically realized by means of a linear optical qubit manipulation, postselection measurement, and a classical feed-forward process.

Figure 1

(a) Diagram of a minimal disturbance measurement (MDM) performed on a single qubit in the state $|\varphi ⟩$. The device provides an output state ${\rho }_F$ with fidelity $F$ and a “classical guess” $G$. (b) Realization of a MDM by the projector $\{E_i\}$, the measurement of the probe qubit, and the classical feed-forward ${\sigma }_Z$.

Figure 2

Optical setup implementing the MDM. The output is characterized adopting the analysis setup illustrated in the dashed box.

Figure 3

(a) Experimental data of the quantum fidelity $F$ versus the classical guess $G$ for an arbitrary input qubit. The fidelities have been averaged over the six states $\{|H⟩,|V⟩,|L_{\pm }⟩,|C_{\pm }⟩\}$; solid line: optimal trade-off between $F_{\mathrm{univ}}$ and $G_{\mathrm{univ}}$ [Eq. (1)]. (b) Experimental data of the quantum fidelity $F$ versus the classical guess $G$ for an equatorial input qubit. The fidelities have been averaged over the four states $\{|H⟩,|V⟩,|L_{\pm }⟩\}$; solid line: optimal trade-off between $F_{\mathrm{cov}}$ and $G_{\mathrm{cov}}$ [Eq. (2)].