Curriculum / Entanglement & Protocols / Distributed Entanglement
Distributed Entanglement
Understand how entanglement is distributed across nodes in a quantum network.
Distributed Entanglement
Creating a Bell pair in a lab is straightforward. Distributing entanglement between geographically separated parties, Alice in London and Bob in Tokyo, requires entirely new infrastructure.
Why is shared entanglement worth this much engineering? Because once a high-quality Bell pair connects two sites, everything else in this track follows: teleport arbitrary qubits, run device-independent QKD, synchronize distributed sensors. A Bell pair is the universal fuel of quantum networking, so the entire field reduces to one problem: deliver entanglement over distance, and keep it alive.
The Core Challenge
Photons are the natural carriers of entangled quantum states over long distances. But:
- •Photon loss in fiber optic cable is exponential with distance (~0.2 dB/km at telecom wavelengths)
- •At 100 km (20 dB of loss) only 1% of photons survive. At 200 km it is 0.01%. At 1,000 km the transmission factor is 10⁻²⁰: even firing ten billion photons per second, you would wait roughly three centuries for a single arrival
- •Unlike classical signals, you cannot amplify quantum states (no-cloning)
Because arrival is so rare, real links are heralded: a successful detection event announces "a pair now exists" to both endpoints, and everything downstream (swapping, purification, teleportation) waits for that herald. Loss then costs rate, not correctness, the pairs you do catch are still good.
Entanglement Distribution Methods
Direct Transmission Generate an entangled photon pair at one node, send one photon to the remote node. Practical for distances under ~100 km.
Entanglement Swapping
[animation: quantum-network] Use intermediate nodes (quantum repeaters) to stitch together short-range Bell pairs into a long-range entangled pair. Alice-Relay + Relay-Bob → Alice-Bob entanglement.
Satellite-Based Distribution China's Micius satellite demonstrated 1,200 km entanglement distribution in 2017. Satellites avoid atmospheric loss in the "free-space" regime.
Most of a satellite link travels through vacuum, where photons do not attenuate at all; only the last ~10 km of atmosphere absorbs and scatters. The Micius experiment distributed pairs to ground stations 1,200 km apart at roughly one detected pair per second. Tiny by classical standards, yet through fiber of the same length the rate would be smaller by many orders of magnitude. Beam spreading, weather, and daylight background remain the practical limits.
This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.
How this lesson works
A guided reading lesson with interactive knowledge checks. Concepts are explained step by step with circuit diagrams and runnable examples, and you confirm understanding before moving on.
Part of: Entanglement & Protocols
Master multi-qubit systems, quantum teleportation, and cryptographic protocols.
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