Quantum Network Protocols

The physical primitives — Bell pairs, swapping, purification — are the mechanism of a quantum network. Turning them into a usable service requires a protocol stack: layered abstractions that let an application ask for "entanglement between A and B with fidelity $F$ " without knowing anything about lasers, fibres, or timing. This lesson describes how the classical networking idea of layering is adapted to entanglement.

Why borrow the layered model?

The classical internet succeeds because of abstraction: TCP does not care whether the bits travel over copper, fibre, or radio. A quantum network wants the same modularity, but it must abstract a fundamentally different and far more fragile resource — entanglement that decays in time and cannot be copied or stored indefinitely. The proposed quantum stack mirrors the classical layers while replacing "deliver a packet" with "deliver an entangled pair."

| Quantum layer | Responsibility | Classical analogue | |---------------|----------------|--------------------| | Physical | Attempt entanglement generation on a single link | Physical (PHY) | | Link | Deliver robust entanglement across one link, on demand | Link / MAC | | Network | Produce long-distance pairs via swapping across many links | Network (IP) | | Transport | End-to-end qubit transmission (e.g. teleportation) | Transport (TCP) |

The link layer in detail

The link layer is where most of the quantum-specific subtlety lives, because entanglement generation on the physical layer is non-deterministic and heralded: each attempt either fails (most of the time) or succeeds and announces itself. The link layer's job is to hide that randomness behind a clean request/response service. A request typically specifies:

the remote node to entangle with;
the number of pairs wanted;
a minimum fidelity $F_{\min}$ ;
a time window or maximum latency the application can tolerate;
a purpose flag (e.g. measure-immediately vs. store-in-memory).

The link layer repeatedly drives the physical layer, applies purification when needed to meet $F_{\min}$ , and returns each delivered pair with a unique identifier plus the Bell-state label so both nodes agree on which pair, in which state, they now share.

Service types

The Dahlberg et al. link-layer design distinguishes how aggressively the layer schedules requests:

Create-and-Keep (CK). Generate a pair and store both halves in quantum memory for later use, returning when at least one pair is ready. Needed for any protocol that must hold entanglement while other links catch up.
Measure-Directly (MD). Generate and immediately measure each pair, returning correlated classical outcomes. This suits QKD, where the qubits are consumed the instant they arrive and no memory is required.

Exposing both lets applications trade memory requirements against throughput.

Scheduling and fairness

Because entanglement is generated probabilistically and memories decohere, the link layer must also schedule: decide which competing request to serve next, when to purify, and when to give up on a pair whose fidelity has decayed below threshold. This is the quantum counterpart of a MAC layer arbitrating a shared medium — except the "packets" expire on their own if not used in time.

The bigger picture

Above the link layer, the network layer chains links with entanglement swapping to deliver long-distance pairs (the repeater logic), and a transport layer can offer end-to-end qubit delivery via teleportation. The key architectural insight is that each layer asks the one below it only for entanglement of a stated quality, never for the physical details — exactly the separation of concerns that made the classical internet scalable. Designing these protocols so they remain robust as hardware improves is an active research frontier, and a place where classical networking expertise transfers directly into the quantum domain.