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Long-distance quantum chips will need more than a way to send photons between processors. They must generate high-quality entanglement reliably across lossy links, preserve it in quantum memories, and use it for remote operations while controlling errors and adapting to real fiber conditions. Experiments have demonstrated important pieces—including a remote gate between two modules about two metres apart and entanglement over a deployed urban fiber loop—but not a large, fault-tolerant quantum-computing network.
What does it mean to connect quantum chips over long distances?
Here, “quantum chips” means separate quantum-processing modules linked by photons. Each module holds and manipulates quantum information locally; a photonic connection lets distant modules share entanglement and perform distributed operations. The aim is not simply to transmit a qubit intact from one city to another. It is to make remote parts of a quantum computer work together as one computational system.
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A quantum communication demonstration and a scalable distributed quantum computer are different milestones. Entanglement over a long fiber shows that quantum correlations can survive a particular link. A distributed computer must also create those links repeatedly, use them to carry out accurate operations, detect failures, and coordinate many nodes. Neither a long fiber result nor a short-distance computing demonstration alone establishes that complete capability.
Why not just send a quantum state directly?
Optical loss is unavoidable over distance: some photons are absorbed or scattered. If an unknown quantum state is sent directly and its carrier photon is lost, the information cannot simply be copied from a backup; unknown quantum states cannot be perfectly cloned. This makes ordinary “send it and amplify it” networking unsuitable for preserving quantum information.
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A modular architecture instead tries to establish shared entanglement between nodes. Once that link is available, teleportation or quantum gate teleportation can transfer a state or enact a remote operation using local operations and classical communication. The protocol does not remove the effects of loss: a failed photon transmission still means a failed attempt. It changes the strategy by allowing nodes to herald success, store a successful link, and use it when the rest of the operation is ready.
What have experiments demonstrated so far?
The results below demonstrate complementary building blocks, not interchangeable measures of a finished network. Gate fidelity and algorithm success measure computing performance in one experiment; fiber distance and memory storage characterize a different experiment’s link and memory.
| Experiment | What was demonstrated | Reported result | What it does not establish |
|---|---|---|---|
| Main et al., Nature (2025) | Distributed computation using two photonically interconnected trapped-ion modules, separated by about two metres, with heralded remote entanglement. | 86% fidelity for the teleported controlled-Z gate; 71% success rate for the distributed Grover search. | A long-distance deployed network, a multi-hop repeater chain, or fault-tolerant operation. Read the paper in Nature. |
| Knaut et al., arXiv preprint (2024) | Entanglement of nuclear-spin memory nodes over a low-loss telecom fiber spool and a deployed Boston-area urban fiber loop. | 40 km spool; 35 km deployed loop, with reported nuclear-spin entanglement fidelity of 0.69(7) on the deployed link; one-second entanglement storage time in the setup. | A complete repeater chain or distributed quantum-computing operation over those links. This is a preprint result. Read the preprint. |
The two-metre result is notable because it used a link for a computational operation, not only for generating entanglement. Its figures also show why more work is needed: a remote gate with 86% fidelity and an algorithm with a 71% reported success rate are not evidence of the high, repeatable performance a larger error-corrected system would require. The sources do not establish a single universal threshold for that performance.
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Remote links must work accurately and often enough
A useful network link has to do more than produce entanglement occasionally. Its entanglement must be accurate enough for remote gates, and it must be generated at a rate that keeps computation moving. Loss makes attempts fail; imperfect photon sources, interfaces, detectors, and operations can degrade successful links. In a multi-node computation, slow or unreliable links can leave processors waiting and make errors harder to manage.
That is why gate fidelity and repeatability matter alongside distance. Main and colleagues note that scalable distributed quantum computing requires quantum gate teleportation to be deterministic and repeatable; their two-module experiment is a step toward that goal, not evidence that it has already been met.
Photons must match low-loss telecom networks
Long-haul fiber communication is most practical in telecom wavelength bands. A quantum processor may emit photons at a different, platform-specific wavelength, so a system may need either a native telecom-band emitter or a converter that shifts photons into the telecom band. Conversion must preserve the quantum information while avoiding excessive loss and added noise. If too few usable photons reach the other node, a nominally fiber-compatible interface may still yield a poor entanglement rate.
A review by Ying Yu and colleagues explains why telecom compatibility matters for quantum repeaters: it enables use of low-loss fiber and established optical-communications technology. The review focuses on telecom-band quantum-dot technologies, but the broader engineering challenge applies across platforms. See the NIST publication record for the review.
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For a long route, a network cannot rely only on sending a photon directly from end to end. A repeater architecture divides the route into shorter links. Nodes must identify when a link has succeeded (heralding), retain its entanglement while neighboring links are attempted, and coordinate the operations that connect those links. They also need ways to detect and manage errors rather than treating every heralded event as equally useful.
Quantum memories are central because neighboring segments will not necessarily succeed at the same time. The memory must hold a useful state long enough for other segments to come online, with sufficient capacity and low error. The one-second nuclear-spin storage reported by Knaut and colleagues is a promising component result; by itself, it does not demonstrate a multi-hop repeater chain with the required coordination and performance.
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Deployed fiber adds changing conditions
Operational fiber is not a static laboratory connection. Loss, environmental noise, and changes in phase and polarization can affect a photonic link. A system intended for real routes must preserve link quality despite those variations and continuously coordinate quantum operations with classical signals that report success, failure, and timing.
The 35 km Boston-area loop result is useful evidence because it involved deployed fiber rather than only a controlled spool. Its reported fidelity of 0.69(7) belongs to that specific preprint experiment; it should not be read as a general performance figure for quantum networks or as proof that deployed links are already reliable enough for computing.
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Many nodes require an integrated system, not just more processors
Adding nodes increases the number of links, operations, and failure cases that must be managed. A scalable network needs compatible interfaces across modules, photonic routing or switching, stable control and calibration, and protocols that schedule entanglement generation, handle unsuccessful attempts, and coordinate classical feed-forward. Different qubit platforms may each be effective locally but still require difficult interfaces to exchange quantum information.
There is no single agreed performance roadmap established by these experiments. Proposed approaches are best judged across several dimensions rather than by distance alone:
- Channel loss and the rate of usable entanglement generation.
- Remote-gate fidelity and repeatability.
- Memory lifetime, storage capacity, and error detection.
- Telecom wavelength compatibility, conversion efficiency, and conversion noise.
- Resilience to phase, polarization, and environmental drift on deployed fiber.
- Integration complexity and the ability to connect heterogeneous nodes.
What would count as scaling?
Scaling would mean that networked modules can repeatedly establish useful links, execute remote operations with controlled errors, and continue doing so as the number of nodes and route length grow. It would also require the system to cope with loss and link failures without undermining the computation. Current work establishes important ingredients—remote matter-qubit entanglement, heralded links, gate teleportation, long-lived memory in one setup, wavelength conversion, and long-fiber entanglement—but not their reliable integration into a large, fault-tolerant distributed computer.
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