Realization of the optimal universal quantum entangler

We present an experimental demonstration of the “optimal” and “universal” quantum entangling process involving qubits encoded in the polarization of single photons. The structure of the “quantum entangling machine” consists of the quantum injected optical parametric amplifier by which the simultaneous realization of the $1\to 2$ universal quantum cloning and of the universal NOT (U-NOT) gate has also been achieved.

Figure 1

Schematic diagram of the self-injected optimal parametric amplifier. The universal optimal quantum entangler is realized on the cloning (C) channel (mode $k_1$). Micrometric adjustments of the coordinate $\mathbf{Z}$ of the UV mirror $M_p$ ensured the time superposition in the active NL crystal of the UV 140 femtosecond pump pulses and of the single-photon pulse injected via back reflection by the fixed mirror $M$.

Figure 2

Probability distribution for the variables $XYZ$ where $X$, $Y$, and $Z$ are the polarization $\vec{\pi }$ state detected, respectively, by the detector pair (a) on the mode $k_1$, the detector pair (b) on the mode $k_1$, and the detector pair (2) on the mode $k_2$. Each correlation data has been measured in a time of $2400\mathrm{s}$.

Figure 3

Coincidence counts $(D_T,D_2,D_a,D_b)$ versus the position $Z$ of the UV mirror $M_p$ The enhancement in the coincidence counts is a signature of the entanglement of the state ${\mid \Phi ⟩}_{\mathit{out}}$.