Quantum Cloning with an Optical Fiber Amplifier

It has been shown theoretically that a light amplifier working on the physical principle of stimulated emission should achieve optimal quantum cloning of the polarization state of light. We demonstrate close-to-optimal universal quantum cloning of polarization in a standard fiber amplifier for telecom wavelengths. For cloning $1\to 2$ we find a fidelity of 0.82, the optimal value being $\frac{5}{6}=0.83$.

Figure 1

A cloning experiment: a state of $N=3$ photons is amplified to a state of $M=6$ photons. Spontaneous emission may create photons in the wrong polarization mode. PBS: polarizing beam splitter.

Figure 2

Scheme of the setup. See text for details.

Figure 3

Inset: ${\mu }_{\mathrm{o}\mathrm{u}\mathrm{t}}$ as a function of ${\mu }_{\mathrm{i}\mathrm{n}}$; the linear fit allows one to extract $G$ and $Q$ defined in (6). Main figure: fidelity (5) as a function of ${\mu }_{\mathrm{i}\mathrm{n}}$. Solid line: $Q=0.8$, best fit with Eq. (7). Dotted lines: upper: $Q=1$ (optimal cloning); lower: $Q=0$ (no cloning).