A quantum internet is a network that distributes entanglement and transfers quantum states (qubits) between distant nodes. It does not send ordinary data faster. As of 2026 it exists as research links spanning a few tens of kilometres of deployed fiber, plus lab experiments over longer distances, and it does not replace post-quantum cryptography. This is the quantum internet explained from the physics up: why qubits can’t be amplified, how entanglement distribution and teleportation work on real fiber, where quantum repeaters stand, and what the technology is and isn’t for.<\/p>\n
The classical internet moves bits: copyable, amplifiable values that are either 0 or 1. A quantum internet moves something different. It shares entangled pairs of qubits between nodes, or transfers an unknown quantum state from one place to another.<\/p>\n
None of this makes web pages load faster. Quantum links are slow, lossy and fragile compared with classical links, and every useful quantum protocol still depends on an ordinary classical channel running alongside it. The point is to give distant devices access to shared quantum correlations, which classical networks cannot provide.<\/p>\n
If you want the conceptual background on entanglement itself, our piece on the EPR experiment and what entanglement really implies<\/a> covers it.<\/p>\n Two facts make quantum networking hard. The first is the no-cloning theorem: an unknown quantum state cannot be copied. A classical optical amplifier works by producing more copies of the signal, so the standard trick for long-haul fiber is ruled out for qubits.<\/p>\n The second is fiber loss. Photon loss in optical fiber grows exponentially with distance, so the probability that a single photon survives a long link falls very quickly. Without amplification, the rate of successfully delivered qubits shrinks with every kilometre.<\/p>\n This sets a hard ceiling. Pirandola, Laurenza, Ottaviani and Banchi derived the repeaterless capacity limit (often called the PLOB bound): the best rate any point-to-point quantum link can achieve without intermediate quantum devices. Beating that bound is the benchmark for a working quantum repeater.<\/p>\n The most direct approach is to generate pairs of entangled photons at a source and send one photon each way down the fiber. If both arrive, the two endpoints share an entangled pair they can use, for example, to derive a secret key.<\/p>\n The longest well-documented example of direct distribution over deployed fiber is from IQOQI Vienna. In work published in Nature Communications in October 2022<\/a>, a team led by S. Neumann sent entangled photons from Vienna across 248 km of installed telecom fiber between Austria and Slovakia, with no trusted nodes in between.<\/p>\n That result, now background rather than news, illustrates both the promise and the problem: entanglement can cross a country’s real fiber, but at rates that direct transmission cannot scale much further.<\/p>\n Quantum teleportation moves a quantum state, not matter, from a sender to a receiver who already share an entangled pair. The sender performs a joint Bell-state measurement on the qubit to be sent and their half of the pair. That measurement destroys the original state and yields two classical bits.<\/p>\n The sender transmits those bits over an ordinary channel. Only after the receiver applies a correction based on them does their half of the pair hold the original state. Because the classical message is required, teleportation cannot transmit information faster than light, and the original is never copied, consistent with no-cloning.<\/p>\n A Northwestern University team led by Prem Kumar showed that this can share a cable with normal data. In a paper in Optica (December 2024)<\/a>, they transferred a quantum state over a 30.2 km fiber that simultaneously carried 400 Gbps of classical traffic in the C-band, with the Bell-state measurement performed at the fiber’s midpoint.<\/p>\n The main obstacle was Raman scattering from the bright classical light, which creates noise photons. The team handled it by:<\/p>\n Some coverage described this as teleportation “through internet traffic.” More precisely, the quantum light travelled in a separate wavelength band on the same fiber, as The Quantum Insider’s report<\/a> describes. That is still the practically important finding: dedicated dark fiber is not strictly required.<\/p>\n A quantum repeater does not amplify anything. It splits a long link into shorter segments, creates entanglement within each segment, stores it in quantum memories, and then joins segments by entanglement swapping.<\/p>\n Swapping works like this: if node A shares a pair with B, and C shares a pair with D, a Bell-state measurement on B and C leaves A and D entangled, even though they never interacted. Memories matter because each segment succeeds at random times; the memory holds one segment’s entanglement until its neighbour is ready. The classic blueprint is the DLCZ scheme of Duan, Lukin, Cirac and Zoller (Nature 414, 413, 2001), which uses atomic ensembles as memories.<\/p>\n Three 2024 experiments connected memory-based nodes across real metropolitan fiber:<\/p>\n In August 2026, a USTC team including Bao and Pan published in Physical Review Letters<\/a> the entanglement of two atomic-ensemble quantum memories through 420 km of fiber. They used the DLCZ scheme and converted the memories’ photons to the telecom S-band. The authors state the result beats the repeaterless limit, a key milestone for repeater physics. The preprint has been public since April 2025.<\/p>\nQuantum internet explained: why you can’t copy or amplify a qubit<\/h2>\n
How entanglement distribution over fiber works<\/h2>\n
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How quantum teleportation works over fiber optic cables<\/h2>\n
Teleportation alongside live internet traffic<\/h3>\n
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Quantum repeaters and quantum memories: where things stand<\/h2>\n
Memory nodes on deployed fiber (2024)<\/h3>\n
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The 420 km memory result (August 2026), with an important caveat<\/h3>\n