The Quantum State of a Propagating Laser Field

TL;DR

This paper provides a quantum information-theoretic description of propagating laser fields, clarifying their role in quantum communication and resolving debates about their suitability for quantum teleportation and entanglement generation.

Contribution

It introduces a new framework for describing laser fields that explains their quantum properties and demonstrates their applicability in quantum teleportation and entanglement creation.

Findings

Conventional lasers can be used for continuous-variable quantum teleportation.

Optical coherent states are fundamental in describing propagating laser fields.

The new description resolves previous controversies about laser fields in quantum information.

Abstract

Optical implementations of quantum communication protocols typically involve laser fields. However, the standard description of the quantum state of a laser field is surprisingly insufficient to understand the quantum nature of such implementations. In this paper, we give a quantum information-theoretic description of a propagating continuous-wave laser field and reinterpret various quantum-optical experiments in light of this. A timely example is found in a recent controversy about the quantum teleportation of continuous variables. We show that contrary to the claims of T. Rudolph and B. C. Sanders [Phys. Rev. Lett. 87, 077903 (2001)], a conventional laser can be used for quantum teleportation with continuous variables and for generating continuous-variable quantum entanglement. Furthermore, we show that optical coherent states do play a privileged role in the description of propagating laser fields even though they cannot be ascribed such a role for the intracavity field.