Observation of micro-macro entanglement of light

TL;DR

This paper reports the first experimental demonstration of micro-macro entanglement of light, involving over a hundred million photons on the macro side and single photons on the micro side, confirming quantum correlations across vastly different scales.

Contribution

It provides the first experimental evidence of micro-macro entanglement with large photon numbers, bridging microscopic quantum states and macroscopic light fields.

Findings

Micro-macro entanglement demonstrated with over 100 million photons.

Correlations between microscopic quadrature differences and macroscopic photon number variance.

Entanglement verified through quantum state tomography after bringing the macro state back to the single-photon level.

Abstract

Schroedinger's famous thought experiment involves a (macroscopic) cat whose quantum state becomes entangled with that of a (microscopic) decaying nucleus. The creation of such micro-macro entanglement is currently being pursued in several fields, including atomic ensembles, superconducting circuits, electro-mechanical and opto-mechanical systems. For purely optical systems, there have been several proposals to create micro-macro entanglement by greatly amplifying one half of an initial microscopic entangled state of light, but experimental attempts have so far been inconclusive. Here we experimentally demonstrate micro-macro entanglement of light. The macro system involves over a hundred million photons, while the micro system is at the single-photon level. We show that microscopic differences (in field quadrature measurements) on one side are correlated with macroscopic differences (in the photon number variance) on the other side. On the other hand, we demonstrate entanglement by bringing the macroscopic state back to the single-photon level and performing full quantum state tomography of the final state. Our results show that it is possible to create and demonstrate micro-macro entanglement for unexpectedly large photon numbers. Schroedinger's thought experiment was originally intended to convey the absurdity of applying quantum mechanics to macroscopic objects. Today many quantum physicists believe that quantum principles in fact apply on all scales. By combining the present approach with other (e.g. mechanical) systems, or by applying its basic ideas in different contexts, it may be possible to bring quantum effects ever closer to our everyday experience.