Single Photon Reveals Quantum Entanglement of 16 Million Atoms

Scientists have demonstrated entanglement between 16 million atoms in a crystal crossed by a single photon, reinforcing the quantum theory that entanglement can persist in macroscopic physical systems.

The theoretical study of large-scale entanglement has to be followed by an experimental demonstration, consisting of two basic steps: the preparation of an entangled system and a subsequent appropriate measurement verifying the presence of entanglement. In the context of entanglement in large systems, the preparation of entanglement is generally simpler than its verification.

This is a partial view of the source producing the single photons that were stored in the quantum memory to produce entanglement between many atoms inside the memory. Courtesy of UNIGE.

“It’s impossible to directly observe the process of entanglement between several million atoms since the mass of data you need to collect and analyze is so huge,” said researcher Florian Fröwis.

Scientists at the University of Geneva (UNIGE) developed an entanglement witness for quantifying a large number of genuinely entangled particles based on the collective effect of directed emission, combined with the nonclassical nature of the emitted light. They examined the characteristics of light re-emitted by the crystal and analyzed its statistical properties and probabilities, following two principles: that light is re-emitted in a single direction rather than radiating uniformly from the crystal and that it is made up of a single photon.

A rare-earth-ion-doped crystal was used to absorb and re-emit light at the single-photon level, where at least
40 billion atoms were collectively interacting with the optical field. Using the measured photon number statistics of the re-emitted light, scientists collected partial information about the quantum state of the atomic ensemble before emission. Then, they showed that certain combinations of re-emission probabilities for one and two photons implied entanglement between a large number of atoms. With the measured data from solid-state quantum memory, the scientists demonstrated inseparable groups of entangled particles containing at least
16 million atoms.

“We haven’t altered the laws of physics. What has changed is how we handle the flow of data,” said researcher Mikael Afzelius.

The team believes that its work demonstrates that large entanglement depth is experimentally certifiable even with large atomic ensembles and low detection and re-emission efficiencies. It has shown that entanglement between many atoms is necessary for the functioning of quantum memories that are based on collective emission, because the combination of directed emission (i.e., high-memory efficiency) and preservation of the single-photon character imply large entanglement depth.

The work also demonstrated the fundamental difference between various manifestations of large entanglement. The scales at which the UNIGE researchers observed entanglement depth appeared to be unattainable for other types of large entanglement, such as Schrödinger-cat states.

Particle entanglement is a prerequisite for the approaching quantum revolution. The relationship between two particles at the quantum level is much stronger than the correlations proposed by the laws of traditional physics.

In a parallel research effort, scientists at the University of Calgary in Canada demonstrated entanglement between many large groups of atoms.

The UNIGE research was published in Nature Communications (doi:10.1038/s41467-017-00898-6).