Chinese physicists entangle quantum memories across a record 420 kilometers

[Carol84]

A team led by Xi-Yu Luo at the University of Science and Technology of China in Hefei has entangled two quantum memories across 420 kilometers of optical fiber, more than four times the distance of the previous record and a significant step toward the kind of long distance, matter to matter entanglement that any future quantum internet would ultimately need to rely on. The result, published in Physical Review Letters as an Editors' Suggestion, pushes the same research group's own earlier 2020 record of roughly 50 kilometers out by a factor of roughly eight.

The experiment relies on what is known as the Duan-Lukin-Cirac-Zoller scheme, a well established approach for generating entanglement remotely between two separate atomic ensemble quantum memories, essentially collections of atoms that can each store a shared piece of quantum information. The key trick that let the team stretch the distance this far involves converting photons emitted from the memories into the telecom S band specifically, a wavelength range that experiences unusually low transmission loss when traveling through standard optical fiber, letting the signal survive a journey that would otherwise attenuate far too quickly to remain useful at this range.

Maintaining that fragile entanglement over hundreds of kilometers required extremely careful engineering on the timing and phase side of the experiment as well. The researchers had to stabilize the relative phase of the photons traveling between the two memories using what they describe as full time far off resonant locking to suppress high frequency noise, combined with a separate intermittent dual band locking technique to compensate for slower, low frequency drift over the course of the experiment. Getting both of those stabilization mechanisms to work together reliably across such a long stretch of real world fiber is itself a considerable technical achievement completely separate from the underlying quantum physics.

Perhaps the most consequential detail in the result is that the team's memory to memory entangling probability actually beat the fundamental capacity limit that applies to any repeaterless channel attempting direct entanglement distribution over the same distance. That specific limit represents a hard theoretical ceiling on how efficiently entanglement can be distributed using direct transmission alone, without any intermediate relay stations, which means this demonstration is not just a longer distance record on its own, it is direct experimental evidence that quantum memory based approaches can genuinely outperform the theoretical alternative of simply sending entangled photons directly point to point.

As with any entanglement based communication system, none of this allows information to travel faster than light or bypass the normal rules of physics, since actually using the entanglement to transmit a meaningful message still requires a classical communication channel running alongside it. What this result does meaningfully demonstrate is a genuine, practical building block toward a future quantum network capable of operating well beyond a single metropolitan area, connecting distant nodes with the kind of long haul reach that any real quantum internet would eventually require to actually be useful at continental or global scale.

Physicists Entangle Quantum Memories Kmphys.org