Short answer: Superconducting circuits work at millikelvin temperatures, so linking separate refrigerators needs a cold channel for microwave signals. A 2020 paper reported a cryogenic waveguide linking transmon qubits in two dilution refrigerators five metres apart, with state transfer and entanglement fidelities of 85.8 and 79.5 percent. A 2025 paper reported continuous-variable entanglement across a 6.6 metre cryogenic link.
Why does a cryogenic link matter?
The 2020 Physical Review Letters abstract says superconducting circuits are a strong contender for quantum computing, but their cryogenic operating temperatures and the current lack of coherence-preserving microwave-to-optical conversion have hindered quantum networks spanning different cryogenic systems or larger distances. The 2025 npj Quantum Information abstract adds that scaling superconducting information processing is expected to move from single-refrigerator processors toward distributed systems, and that hardware for quantum local area networks compatible with superconducting technology matters for reaching a practical quantum advantage.
What did the 2020 experiment do?
The authors report operating a cryogenic waveguide that coherently links transmon qubits located in two dilution refrigerators separated by five metres. They transfer qubit states and generate entanglement on demand, with average transfer fidelity of 85.8 percent and target state fidelity of 79.5 percent between the two nodes of this elementary network. They say cryogenic microwave links offer a way to scale up quantum computing systems and to create local area superconducting quantum communication networks over length scales of at least tens of meters, which is their outlook and not a demonstrated range.
What did the 2025 experiment do?
The authors present a prototype platform for a microwave quantum local area network, using a cryogenic link between two separate dilution cryostats over 6.6 metres, with a base temperature of 52 mK at the centre. Superconducting microwave coaxial cables form the quantum channel between the nodes. They demonstrate continuous-variable entanglement distribution between the remote refrigerators in the form of two-mode squeezed microwave states, reaching squeezing of 2.10 plus or minus 0.02 dB and negativity of 0.501 plus or minus 0.011. They also show that entanglement is preserved at channel centre temperatures up to 1 K, and describe this as paving the way towards microwave quantum communication at elevated temperatures. They say such a network could form a backbone for future distributed quantum computing, which is a prospect.
What to keep in mind
The two experiments use different encodings. The 2020 work transfers qubit states between transmons, while the 2025 work distributes continuous-variable entanglement as squeezed states, so their figures are not comparable. Neither abstract reports the heralded entanglement scheme the draft topic named, so this post does not claim it. This post is based on abstracts and visible summary text, and full texts were not checked. It makes no claim about products or deployed systems.
FAQ
Why not use optical links?
The 2020 abstract says there is a current lack of coherence-preserving microwave-to-optical conversion, which is why cold microwave links are explored.
How far apart were the refrigerators?
Five metres in the 2020 experiment and 6.6 metres in the 2025 prototype.
What fidelity was reported for state transfer?
An average transfer fidelity of 85.8 percent, with 79.5 percent for target states, in 2020.
Can the link work above millikelvin temperatures?
The 2025 authors show entanglement preserved at channel centre temperatures up to 1 K.
All field notes