XPM nonlinearities:Are all involved photons real?

TL;DR

This paper uncovers a symmetry in XPM systems linked to multiphoton processes, predicts cross field noise correlations, and explores high-order nonlinearities and their potential for generating entangled photons and squeezing.

Contribution

It introduces a novel symmetry condition in XPM systems, links it to observable multiphoton processes, and proposes new high-order nonlinear effects for quantum light sources.

Findings

Identified a symmetry condition necessary for multiphoton processes to be visible in multiple fields.

Predicted cross field noise correlations in symmetric XPM systems.

Discovered high-order nonlinearities, such as $$ and $$ resonances, with potential for quantum applications.

Abstract

We have found a novel symmetry for XPM(Cross Phase Modulation) systems .We prove that this symmetry is a necessary condition for a single multiphoton process to be visible in more than one field.We have found, two EIT(Electromagnetically Induced Transparency) systems that show cases of this symmetry.The above process has the possibility of leading to strong sources of temporally entangled photons of different frequency. For cases that do not show this symmetry,more than one "species" of multiphoton processes can take place and each one is visible in one field only. When there is a symmetry, we predict cross field noise correlations for the scattered fields.Cross field noise correlations have recently been seen in a different situation that can provide interesting variation to our study,namely the generated fields in FWM(Four Wave Mixing). We postulate or conjecture the existence of two types of asymmetric multiphoton processes.We show only one of these is possible. One of systems we use for this purpose generates a $\chi^{(9)}$ absorption resonance that is observable on the same trace that shows linear features, in an experiment described in a previous work but the order of nonlinearity newly interpreted using this work.This high order nonlinearity should be very sensitive to cross field correlations.We show how it can be used to test our conjecture of asymmetric processes. We also come across an important $\chi^{(5)}$ absorption,comparable to linear absorption, that is anomalous.The anomaly is that the Imaginary part of this XPM nonlinearity can cause squeezing.We give a qualitative argument which shows that we don't expect the Imaginary part of the XPM nonlinearities we usually come across, to cause squeezing.This promising source of squeezing, is qualitatively different from the other sources hitherto known.