Short answer: Entanglement purification, also called entanglement distillation, turns several low-quality entangled pairs into fewer, higher-quality ones using local operations and classical communication. It matters because noise degrades entanglement as it is shared between distant nodes in a quantum network. A 2017 Science experiment demonstrated distillation on a small two-node network primitive, and 2023 papers review the protocols and study how noisy gates and memory limits affect purification in network designs.
Why entanglement needs purifying
A 2023 review on arXiv (2304.12679) says entanglement is a central resource in quantum key distribution, quantum secret sharing, teleportation, quantum computation and quantum networks. It is also fragile: noise in quantum channels and imperfect operations degrade maximally entangled states into low-quality mixed states, which limits applications. The review calls entanglement purification a powerful tool to distil high-quality entangled states from low-quality ones.
The review covers the basic theory and protocols using linear optics, cross-Kerr nonlinearities, hyperentanglement, deterministic schemes and measurement-based schemes. It also surveys experimental progress with linear optics and discusses the outlook. It says the review may pave the way for practical implementations in long-distance quantum communication and networks.
Purification inside a network
A 2023 Physical Review Research paper describes purification in a network whose repeaters have limited processing fidelity. It explains that large-scale quantum networks use quantum repeaters because photons are inevitably lost in transmission. Repeaters help with entanglement routing through entanglement swapping, improve quality through entanglement purification, and store entanglement in quantum memories for synchronization. The authors explore how noisy gates, depolarizing channels and finite memory storage time affect the performance of purification, which in turn affects entanglement routing. They apply custom purification protocols to routing across different network topologies, aiming to inform future design choices. This is a study of network design, not of a deployed network.
A laboratory demonstration
A 2017 Science paper by Kalb and colleagues is summarized by the journal as describing an entanglement distillation protocol that could enhance the purity and robustness of entanglement between nodes of a primitive quantum network. The authors realized distillation on a network primitive of two distant nodes, each containing an electron and a nuclear spin as a two-qubit node. They generated two copies of a remote entangled state, mediated by single photons, with robust storage in the nuclear spins. After local two-qubit gates, single-shot measurements herald the distillation of an entangled state with increased fidelity that is available for further use. They say combining generating, storing and processing entangled states should enable exploring multiparticle entanglement on an extended quantum network.
What this does not tell you
These sources do not report a large working quantum network. The experiment involved a two-node primitive, and the network study is a design analysis under assumed noise models. Purification consumes entangled pairs, so the gain in quality comes at a cost in resources, which the network study treats as part of the design question.
FAQ
What is entanglement purification?
A process that produces fewer, higher-quality entangled pairs from several noisy ones.
Is it the same as entanglement distillation?
Yes. The terms are used interchangeably in the 2023 network paper.
Why do quantum networks need it?
Noise and loss degrade entanglement shared between distant nodes.
Has it been demonstrated?
A 2017 Science paper demonstrated it on a small two-node network primitive.
What limits purification in a network?
The 2023 network paper studies noisy gates, depolarizing channels and finite memory storage time.
All field notes