Entanglement between the signal and the amplifying medium can produce a large amount of decoherence in an ideal optical amplifier even when the gain is arbitrarily close to unity and the added noise is negligible.
In an ideal linear amplifier, the output signal is linearly related to the input signal with an additive noise that is independent of the input. The decoherence of a quantum-mechanical state as a result of optical amplification is usually assumed to be due to the addition of quantum noise. Here we show that entanglement between the input signal and the amplifying medium can produce an exponentially-large amount of decoherence in an ideal optical amplifier even when the gain is arbitrarily close to unity and the added noise is negligible. These effects occur for macroscopic superposition states, where even a small amount of gain can leave a significant amount of which-path information in the environment. Our results show that the usual input/output relation of a linear amplifier does not provide a complete description of the output state when post-selection is used.
In a linear amplifier, the output signal is linearly related to the input signal with an additive noise that is independent of the input. Here we show that entanglement between the input signal and the amplifying medium can produce exponentially-large decoherence in an ideal optical amplifier even when the gain is arbitrarily close to unity and the added noise is arbitrarily small. These nonlinear effects show that the usual input/output relationship of a 'linear' amplifier does not provide a complete description of its performance.
In this paper we evaluate experimentally measured signal-to-noise ratio performance of different types of optical amplifiers [V. Josse, M. Sabuncu, N. Cerf, et al., Phys. Rev. Lett. 96 163602 (2006); Z.Y. Ou, S.F. Pereira and H.J. Kimble, Phys. Rev. Lett. 70 3239 (1993)]. We pay particular attention to the low gain regime where the quantum limit of the amplifier varies with gain. Independent of the amplifier type the noise performance is remarkably close to the quantum limit. Low gain optical amplifiers are of interest for quantum communication.
Quantum mechanical arguments show that any optical amplifier must add noise to the amplified beam of light.For the case of a high gain amplifier with a coherent (Poissonian) input, the signal-to-noise ratio of the amplified beam must be at least two times smaller than that of the incident beam.We review the theoretical basis of this prediction.We also describe the results of our experimental investigations of the noise properties of optical amplifiers that utilize the nonlinear optical response of strongly driven atomic transitions.