Millions of atoms entangled in record-breaking quantum tests

In a feat of quantum one-upmanship, two teams of scientists have staked new claims of linking whopping numbers of atoms at the quantum level.

Researchers from Geneva demonstrated quantum entanglement of 16 million atoms, smashing the previous record of about 3,000 entangled atoms (SN Online: 3/25/2015). Meanwhile, scientists from Canada and the United States used a similar technique to entangle over 200 groups of a billion atoms each. The teams published their results online March 14 in a pair of papers posted at arXiv.org.

Through quantum entanglement, seemingly independent particles become intertwined. Entangled atoms can no longer be considered separate entities, but make sense only as part of a whole — even though the atoms may be far apart. The process typically operates on small scales, hooking up tiny numbers of particles, but the researchers convinced atoms to defy that tendency.

“It’s a beautiful result,” says atomic physicist Vladan Vuletić of MIT, who was part of the team that previously demonstrated the 3,000-atom entanglement. Quantum effects typically don’t appear at the large scales that humans deal with every day. Instead, particles’ delicate quantum properties are smeared out through interactions with the messy world. But under the right conditions, quantum effects like entanglement can proliferate. “What this work shows us is that there are certain types of quantum mechanical states that are actually quite robust,” Vuletić says.

Both teams demonstrated entanglement using devices known as “quantum memories.” Consisting of a crystal interspersed with rare-earth ions — exotic elements like neodymium and thulium — the researchers’ quantum memories are designed to absorb a single photon and reemit it after a short delay. The single photon is collectively absorbed by many rare-earth ions at once, entangling them. After tens of nanoseconds, the quantum memory emits an echo of the original photon: another photon continuing in the same direction as the photon that entered the crystal.

By studying the echoes from single photons, the scientists quantified how much entanglement occurred in the crystals. The more reliable the timing and direction of the echo, the more extensive the entanglement was. While the U.S.-Canadian team based its measurement on the timing of the emitted photon, the Swiss team focused on the direction of the photon.

The quantum memories used to entangle the atoms aren’t new technologies. “The experiments are not complicated,” says physicist Erhan Saglamyurek of the University of Alberta in Canada, who was not involved with the research. Instead, the advance is mainly in the theoretical physics the researchers established to quantify the entanglement that was expected to arise inside such quantum memories. This allowed them to actually prove that such large numbers of particles were entangled, Saglamyurek says.

Scientists from the two research teams declined to comment, as the papers reporting the work are still undergoing peer review by a journal.

The results don’t have any obvious practical use. Instead, the work grows out of technology that is being developed for its potential applications: Quantum memories could be used in quantum communication networks to allow for storage of quantum information.

Eventually, physicists hope to push weird quantum effects to larger and larger scales. For quantum entanglement, “it would be a dream if you could make that visible to the naked eye,” says quantum physicist Jakob Reichel of École Normale Supérieure in Paris. The latest results don’t go that far.

“It’s not a revolution,” Reichel says. But, “I think it helps us [get] a better feeling for entangled states.”