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The IBM roadmap for scaling their quantum computers refer to a quantum communication link to combine together multiple smaller quantum chips. What is the technology that enables connecting together multiple smaller superconducting quantum systems? Are there any nice papers which describe them?

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There are lots of ways of moving quantum information, so it's not clear exactly what IBM means when they say "quantum communication link". At small scales, like within the same fridge (they almost certainly mean something within one fridge), it could be as "simple" as small wires between the two chips.

Within the same building people have made cryogenic pipes connecting two fridges, with waveguides along the pipe. You apply a pulse at one end to swap the qubit's information into the waveguide, which the information travels down, and then you apply a pulse at the other end at the right time to swap it out.

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Between buildings you would probably want to somehow transduce your qubit into an optical photon, so you could send the photon down a room temperature fiber optic cable, and then transduce the photon back into your type of qubit at the other side. Actually, at this scale you probably only transduce into machines and never out of machines: have a source of entangled photons sending to both sides and use that entanglement to perform teleportation. This has the enormous benefit that you can purify the entanglement before using it, reducing noise. (I don't think anyone has demonstrated the ability to transduce from optical photons into superconducting qubits with anywhere near sufficient fidelity to have purification help instead of hurt.)

Between cities or continents you'd probably still distribute entanglement via photons. At this scale you'd for sure have to be purifying it. But the photons would travel from satellites through air instead of along fiber lines. Alternatively, you might cross big distances using a series of short fiber link hops, with purification at every step to avoid noise accumulating with distance.

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  • $\begingroup$ Thanks! This is exactly what I was looking for. $\endgroup$ Commented Aug 8, 2023 at 4:41
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The blogpost Expanding the IBM Quantum roadmap to anticipate the future of quantum-centric supercomputing adds more context to that roadmap. Since it is planned for 2024, my understanding is that the technology is being developed at the moment.

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  • $\begingroup$ That's right, but I guess there would be some theoretical studies out there describing possible designs for the communication link. $\endgroup$ Commented Aug 8, 2023 at 4:41

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