Quantum enhancement of signal-to-noise ratio with a heralded linear amplifier

Amplification of signals is an elemental function for many information processing systems and communication networks. However, optical quantum amplification has always been a technical challenge in both free space and fiber optics communication. Any phase-insensitive amplification of quantum light states would experience a degradation of signal-to-noise ratio as large as 3 dB for large gains. Fortunately, this degradation can be surmounted by probabilistic amplification processes. Here we experimentally demonstrate a linear amplification scheme for coherent input states that combines a heralded measurement-based noiseless linear amplifier and a deterministic linear amplifier. The amplifier is phase-insensitive and can enhance the signal-to-noise ratio of the incoming optical signal. By concatenating the two amplifiers, it introduces flexibility that allows one to tune between the regimes of high gain or high noise reduction and control the trade-off between these performances and a finite heralding probability. We demonstrate amplification with a signal transfer coefficient of T _s > 1 with no statistical distortion of the output state. By partially relaxing the demand of output Gaussianity, we can obtain further improvement to achieve a T _s = 2.55 ± 0.08 with an amplification gain of 10.54. Since our amplification scheme only relies on linear optics and a post-selection algorithm, it has the potential of being used as a building block in extending the distance of quantum communication.