Low-Noise Amplification of a Continuous-Variable Quantum State

We present an experimental realization of a low-noise, phase-insensitive optical amplifier using a four-wave mixing interaction in hot Rb vapor. Performance near the quantum limit for a range of amplifier gains, including near unity, can be achieved. Such low-noise amplifiers are essential for so-called quantum cloning machines and are useful in quantum information networks and protocols. We demonstrate that amplification and "cloning of one half of a two-mode squeezed state is possible while preserving entanglement. The inseparability criterion between the two original modes remains satisfied for small to large gains, while the EPR criterion is satisfied for a smaller range. This amplification of quantum correlations paves the way for optimal cloning of a bipartite entangled state.