Quantum Memory Node

Sync wait — right node delay 180 ms

Coherence time T2 240 ms

Write efficiency 85%

Read efficiency 90%

Live state

Left fidelity

–

Right fidelity

–

Swap attempts

Swap success rate

–

Fire a photon into each memory node, then watch the fidelity decay while the far node's photon is still in flight — a real repeater swap only succeeds if both memories survive the wait.

How it works

Each node is an atomic-ensemble quantum memory. A photon carrying a qubit arrives and is written into the ensemble with probability equal to the write efficiency — real absorptive memories lose some photons to scattering and off-resonant coupling. Once written, the stored state's fidelity decays toward the maximally-mixed value as environmental noise dephases it:

F(t) = 0.5 + 0.5 · e^(−t / T2)

A quantum repeater needs both memories to still hold a usable state when the slower photon (the "sync wait") finally arrives, so it can attempt an entanglement swap. The chance the swap actually succeeds is the product of both write efficiencies, the read-out efficiency, and both memories' fidelity at the moment of the swap:

P(swap) = η_write² · η_read · F_L(t) · F_R(t)

Sync wait — how long the right-hand photon takes to arrive after the left one; the longer this is, the more the left memory decoheres before it can be used.
Coherence time T2 — how forgiving the memory is; longer T2 means fidelity survives longer waits (this is the number labs are racing to push into seconds).
Write / read efficiency — the round-trip photon-to-atom-to-photon conversion efficiency; today's real devices sit well under 100%, so it directly caps the usable swap rate even with perfect coherence.

Real-world relevance: this write → store-while-you-wait → read cycle is exactly the bottleneck in building long-distance quantum networks and repeaters — storage time and fidelity trade off against each other, and both are capped by imperfect write/read efficiency.

Photon in flight Stored qubit (Bloch arrow) Swap success Swap failure

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