A macroscopic quantum state analysed particle by particle

TL;DR

This paper demonstrates the direct observation of large-scale entanglement in a macroscopic quantum state using photonic technology, revealing how microscopic entanglement underpins macroscopic quantum phenomena.

Contribution

It introduces a novel experimental approach combining photonic technology and quantum state tomography to analyze entanglement in macroscopic quantum states at the particle level.

Findings

All photons within the squeezing coherence time are entangled.

Entanglement monogamy reduces entanglement with higher photon density.

Increased squeezing can decrease bipartite entanglement.

Abstract

Explaining how microscopic entities collectively produce macroscopic phenomena is a fundamental goal of many-body physics. Theory predicts that large-scale entanglement is responsible for exotic macroscopic phenomena, but observation of entangled particles in naturally occurring systems is extremely challenging. Synthetic quantum systems made of atoms in optical lattices have been con- structed with the goal of observing macroscopic quantum phenomena with single-atom resolution. Serious challenges remain in producing and detecting long-range quantum correlations in these systems, however. Here we exploit the strengths of photonic technology, including high coherence and efficient single-particle detection, to study the predicted large-scale entanglement underlying the macroscopic quantum phenomenon of polarization squeezing. We generate a polarization-squeezed beam, extract photon pairs at random, and make a tomographic reconstruction of their joint quantum state. We present experimental evidence showing that all photons arriving within the squeezing coherence time are entangled, that entanglement monogamy dilutes entanglement with increasing photon density and that, counterintuitively, increased squeezing can reduce bipartite entanglement. The results provide direct evidence for entanglement of macroscopic numbers of particles and introduce micro-analysis to the study of macroscopic quantum phenomena.