Origin of Quantum Noise and Decoherence in Distributed Amplifiers

TL;DR

This paper investigates how distributed optical amplifiers cause quantum noise and decoherence, showing that entanglement with the environment, rather than vacuum fluctuations, is the main source of noise affecting quantum information transmission.

Contribution

It provides a detailed analysis of decoherence mechanisms in distributed amplifiers using perturbation theory, highlighting the role of entanglement with atomic media.

Findings

Decoherence reduces photon number spread and increases phase uncertainty.

Amplifier noise stems from entanglement with the environment, not just vacuum fluctuations.

The system's state becomes a mixture of squeezed states due to atomic interactions.

Abstract

The use of distributed amplifiers may have some potential advantages for the transmission of quantum information through optical fibers. In addition to the quantum noise introduced by the amplifiers, entanglement between atoms in the amplifying media and the optical field corresponds to which-path information that can further reduce the coherence. Here we analyze the effects of decoherence in a phase-insensitive distributed amplifier by using perturbation theory to calculate the state of the entire system including the atomic media. For an initial coherent state, tracing over the atomic states allows the reduced density matrix of the field to be expressed as a mixture of squeezed states with a reduced spread in photon number and an increased phase uncertainty. The amplifier noise and decoherence can be interpreted as being due to entanglement with the environment rather than the amplification of vacuum fluctuation noise.