The idea of a global quantum internet moved another step toward reality after scientists successfully entangled two quantum memories through 420 kilometers of optical fiber.
- Scientists at USTC achieve quantum entanglement between two physical quantum memories separated by standard optical fiber.
- The physical transmission span reaches 420 kilometers, setting a record-breaking distance for physical quantum memory networks.
- Telecom operators face infrastructure overhauls as quantum key distribution approaches integration with standard fiber-optic lines.
Researchers led by Jian-Wei Pan at the University of Science and Technology of China demonstrated entanglement between two atomic-ensemble quantum memories over the distance, according to a study published in Physical Review Letters on Aug. 11. The experiment was designed to address one of the central problems facing long-distance quantum networks: preserving quantum information while sending it through lossy optical fiber.
The result is significant because the researchers were not simply distributing entangled photons. They linked quantum memories, which can store a quantum state and later retrieve it. That storage capability is important for future quantum communication systems that will need to connect multiple network segments.
How The 420 Km Link Worked
The experiment used two memory nodes, known as Alice and Bob, with a middle station called Charlie.
The memories used laser-cooled atomic ensembles. When a memory generated a photon, the researchers converted the photon’s wavelength from 780 nanometers to about 1,522 nanometers, placing it in the telecom S-band. That allowed the photons to travel through optical fiber with substantially lower attenuation. The researchers reported fiber losses of about 0.17 decibels per kilometer for the ultra-low-loss fiber used to extend the test.
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→ Submit a Press ReleaseThe team also had to stabilize the relative phase of the two memory systems. Small fluctuations in temperature and other conditions can disrupt the interference needed to establish and verify entanglement over long fiber paths.
At the full 420 km distance, the experiment recorded a memory-memory entangling probability of about 1.09 millionth per trial. The researchers nevertheless verified genuine entanglement at that distance.
More importantly, the measured entangling probability exceeded the PLOB bound, the theoretical limit governing direct transmission of quantum information through a lossy channel without a repeater. The advantage appeared once the fiber distance passed roughly 230 km in the experiment.
The 420 Km Figure Needs Context
The headline distance does not mean two independent quantum computers were positioned 420 km apart and connected through existing commercial infrastructure.
Both memory nodes were located in a laboratory at the University of Science and Technology of China. The setup used two 10.1 km deployed fiber links between the laboratory and the central Charlie station, while coiled ultra-low-loss fiber was used to extend the experimental path to 420 km.
That distinction matters when considering what the experiment says about a future global network.
The researchers themselves described the system as a test bed for studying quantum-network applications beyond metropolitan scale. Their work does not demonstrate an operational intercity quantum network, much less a finished global internet.
Why Quantum Memories Matter
Conventional optical-fiber transmission becomes increasingly difficult as distance increases because photons are lost along the way. Direct transmission therefore suffers an exponential decline in efficiency.
Quantum repeaters are intended to address that problem by dividing a long connection into shorter segments, establishing entanglement across those segments and then extending it through entanglement swapping.
Quantum memories are a key component of that architecture because they can store the quantum state while neighboring links are established. The researchers said their system could therefore serve as a platform for applications ranging from metropolitan networks to longer-distance connections.
The experiment also points to a possible path toward more advanced applications, including quantum key distribution and distributed quantum computing. But those systems require additional advances in reliability, storage, network architecture and entanglement generation.
The 420 km demonstration represents a narrower achievement: researchers showed that matter-based quantum memories can remain entangled across a fiber path long enough to move beyond the metropolitan scale.
The global quantum internet is still a long way from deployment. But the experiment removes one more obstacle from the road.
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