Quantum entanglement distance
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Quantum Entanglement Distance: Experimental Achievements
Long-Distance Quantum Entanglement via Satellite and Free-Space Links
Quantum entanglement has been successfully distributed over very large distances using both ground-based and satellite-based methods. The most notable achievement is the distribution of entangled photons between two ground stations separated by more than 1,200 kilometers using the Micius satellite, demonstrating that quantum entanglement can survive and be verified over global distances. This experiment showed a clear violation of Bell's inequality, confirming the presence of entanglement even after transmission through the atmosphere and space, and highlighted the potential for a global quantum communication network . Earlier, free-space experiments had managed to distribute entanglement over 600 meters between independent receivers with no line of sight, marking an important step toward satellite-based quantum networks .
Fiber-Based Quantum Entanglement Distribution
On the ground, optical fibers have been used to distribute entanglement over significant distances. Entanglement between matter (such as trapped ions or atomic ensembles) and light has been observed over 50 kilometers of optical fiber, which is two orders of magnitude further than previous achievements and is a practical distance for building large-scale quantum networks . Additionally, entanglement between two quantum memories has been demonstrated over 22 kilometers of field-deployed fibers and up to 50 kilometers of coiled fibers, showing that such distances are feasible for future quantum networks connecting remote quantum processors . In metropolitan settings, entanglement between quantum memories has been achieved over 12.5 kilometers, supporting the development of multi-node quantum networks .
Overcoming Distance Limitations: Quantum Repeaters and Purification
The main challenge in distributing entanglement over long distances is loss and noise in the communication channels, which typically limits ground-based entanglement to about 100 kilometers 29. To overcome this, quantum repeaters and entanglement purification techniques are being developed. Quantum repeaters use quantum memories and entanglement swapping to extend the range, while purification protocols can distill high-quality entanglement from noisy channels, as demonstrated over 11 kilometers of multicore fiber . Recent advances in repeater protocols suggest that, with certain network designs, it may be possible to generate entanglement at a rate that does not decrease with distance, which is a significant step toward scalable quantum networks .
Fundamental Nature of Entanglement and "Spooky Action at a Distance"
Quantum entanglement is often described as "spooky action at a distance" because measurements on one particle instantly affect the state of another, no matter how far apart they are. This nonlocal property has been confirmed in all the above experiments, with violations of Bell's inequality serving as strong evidence that entanglement persists over large distances 28.
Conclusion
Quantum entanglement has been experimentally distributed over distances ranging from a few meters to more than 1,200 kilometers, using both free-space and fiber-optic links. Satellite-based experiments have pushed the boundaries to global scales, while advances in quantum repeaters and purification are addressing the challenges of loss and noise in long-distance transmission. These achievements are paving the way for practical, large-scale quantum networks and a future quantum internet 1234+4 MORE.
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