Effects of entanglement in an ideal optical amplifier

TL;DR

This paper demonstrates that entanglement can cause significant decoherence in ideal optical amplifiers, even with negligible added noise, especially affecting macroscopic superposition states.

Contribution

It reveals that entanglement between the input and medium can induce large decoherence in ideal amplifiers, challenging the traditional noise-based explanation.

Findings

Entanglement leads to exponential decoherence in ideal amplifiers.

Small gain can cause significant which-path information leakage.

Input/output relations are incomplete when post-selection is considered.

Abstract

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.