Origin of quantum noise and decoherence in distributed amplifiers

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 phaseinsensitive 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.

Effects of entanglement in an ideal optical amplifier

“…For an OPA, 21 in N g q = − − , where ˆi n q is the x quadrature in the idler mode. The same results apply to any ideal linear amplifier, including those based on an inverted atomic medium [9]. We will use an OPA to illustrate the effects of entanglement on a quantum signal.…”