Enhancing quantum entanglement by photon addition and subtraction

TL;DR

This paper systematically investigates how local photon addition and subtraction affect entanglement in two-mode squeezed vacuum states, revealing that such non-Gaussian operations generally enhance entanglement, with balanced operations yielding the highest improvements.

Contribution

The study provides an analytical and numerical analysis of photon addition and subtraction effects on entanglement, highlighting optimal strategies and the nuanced relationship between entanglement and non-Gaussianity.

Findings

Entanglement increases with the number of photon operations.

Equal sharing of operations between modes maximizes entanglement.

Photon addition often yields stronger entanglement enhancement than subtraction.

Abstract

The non-Gaussian operations effected by adding or subtracting a photon on the entangled optical beams emerging from a parametric down-conversion process have been suggested to enhance entanglement. Heralded photon addition or subtraction is, as a matter of fact, at the heart of continuous-variable entanglement distillation. The use of such processes has recently been experimentally demonstrated in the context of the generation of optical coherent-state superpositions or the verification of the canonical commutation relations. Here, we carry out a systematic study of the effect of local photon additions or subtractions on a two-mode squeezed vacuum state, showing that the entanglement generally increases with the number of such operations. This is analytically proven when additions or subtractions are restricted to one mode only, while we observe that the highest entanglement is achieved when these operations are equally shared between the two modes. We also note that adding photons typically provides a stronger entanglement enhancement than subtracting photons, while photon subtraction performs better in terms of energy efficiency. Furthermore, we analyze the interplay between entanglement and non-Gaussianity, showing that it is more subtle than previously expected.