Albert Einstein called it "spooky action at a distance". He hated the concept of quantum mechanics, saying "God does not play dice with the Universe". Unfortunately, Einstein was quite wrong.

The idea is that pairs of photons can be generated and can go virtually any distance, and then a measurement made on one of these photons essentially instantaneously determines the equivalent value of the other photon. There is apparently no limit to the distance over which this can be done. A few years ago I read that the apparent velocity of this connection is at least 10,000 times c, or the speed of light in a vacuum. https://blogs.scientificamerican.com/news-blog/quantum-weirdnes-wins-again-entangl-2008-08-13/#:~:text=(In%202007%2C%20researchers%20transmitted%20entangled,two%20of%20the%20Canary%20Islands.)&text=But%20in%20reality%2C%20no%20experiment,times%20the%20speed%20of%20light.

"The photons were indeed entangled, the group reports in Nature. But in reality, no experiment is perfect, so what they end up with is a lower limit on how fast the entanglement could be traveling: 10,000 times the speed of light."

I'm not a good person to be explaining this to you: My degree is in chemistry, not physics.

But read up on Bell's inequalities. (different Bell!) https://en.wikipedia.org/wiki/Bell%27s_theorem

"Bell's theorem proves that quantum physics is incompatible with local hidden variable theories. It was introduced by physicist John Stewart Bell in a 1964 paper titled "On the Einstein Podolsky Rosen Paradox", referring to a 1935 thought experiment that Albert Einstein, Boris Podolsky and Nathan Rosen used to argue that quantum physics is an "incomplete" theory.^[1]^[2] By 1935, it was already recognized that the predictions of quantum physics are probabilistic. Einstein, Podolsky and Rosen presented a scenario that, in their view, indicated that quantum particles, like electrons and photons, must carry physical properties or attributes not included in quantum theory, and the uncertainties in quantum theory's predictions are due to ignorance of these properties, later termed "hidden variables". Their scenario involves a pair of widely separated physical objects, prepared in such a way that the quantum state of the pair is entangled. " [partial quote]

Quantum communication over fiber optics has been done to a record distance of 50 kilometers.

https://www.sciencedaily.com/releases/2019/08/190829150732.htm#:~:text=For%20the%20first%20time%2C%20a,for%20a%20future%20quantum%20internet.&text=FULL%20STORY-,For%20the%20first%20time%2C%20a%20team%20has%20sent%20a%20light,50%20km%20of%20optical%20fiber.

Ironically, I have actually made a major contribution to this field, although almost nobody realizes it yet. The limit to the distance of quantum entanglement, 50 kilometers above, is based on the amount of optical loss present in the fiber. Prior to my invention, and currently, the record for a low-loss single-mode optical fiber is by Sumitomo Electric, and is 0.1419 db/kilometer. https://global-sei.com/company/press/2017/03/prs029.html#:~:text=Now%20Sumitomo%20Electric%20has%20advanced,lowest%20loss%20of%20optical%20fiber.

Fiber optics scientists and engineers achieved a fiber loss of about 0.200 db/km in 1978, and about 0.160 by the mid-1980's. They are apparently approaching a limit asymptotically, the limit defined by the presence of natural-quantities of spin-containing isotopes in silica.

In about December 2008, while stuck in a Federal prison cell at USP Tucson, Arizona, I realized that the cause of the residual loss in these fibers is the Si-29 and O-17 isotope atoms, and for Corning-type fibers (containing germania, or GeO2, dopant), the Ge-73 isotope atoms. Remove these spin-containing isotopes from optical fiber silica, and it should be possible to drop the loss by at least a factor of 100, or to about 0.001db/kilometer.

(Corning-type fibers use a core of 8% GeO2 and 92% SiO2, and pure silica in the cladding. Sumitomo-type fibers use(s) a pure-silica core, and a fluorine-doped cladding. Since there are fewer spin-containing isotopes in pure silica than in germania-doped silica, Sumitomo had/has a small loss advantage over Corning.)

If this fiber is used for comunication between New York and Ireland, the hop should be achievable by a single, continuous fiber. It will not need to contain the 40-odd EDFA https://en.wikipedia.org/wiki/Optical_amplifier amplifiers currently employed.

Similarly, if such fiber is used for a quantum link, it should be possible to do the link at least at a distance of 5000 kilometers, 100x better than the recent (2019) record.

The Chinese have been doing entanglement experiments on a satellite named Micius. https://en.wikipedia.org/wiki/Quantum_Experiments_at_Space_Scale

"QUESS is a proof-of-concept mission designed to facilitate quantum optics experiments over long distances to allow the development of quantum encryption and quantum teleportation technology.^[7] Quantum encryption uses the principle of entanglement to facilitate communication that is totally safe against eavesdropping, let alone decryption, by a third party. By producing pairs of entangled photons, QUESS will allow ground stations separated by many thousands of kilometres to establish secure quantum channels.^[3] QUESS itself has limited communication capabilities: it needs line-of-sight, and can only operate when not in sunlight.^[8] "

Jim Bell