Physicists have long known that quantum mechanics allows for a subtle connection between quantum particles called entanglement, in which measuring one particle can instantly set the otherwise uncertain condition, or "state," of another particle — even if it's light years away. Now, experimenters in Israel have shown that they can entangle two photons that don't even exist at the same time. ...I don't doubt this experiment, but the explanation is really misleading. Quantum mechanics teaches that photons never exist as particles. They have some particle properties and some wave properties.
The experiment shows that it's not strictly logical to think of entanglement as a tangible physical property, Eisenberg says. "There is no moment in time in which the two photons coexist," he says, "so you cannot say that the system is entangled at this or that moment." Yet, the phenomenon definitely exists. Anton Zeilinger, a physicist at the University of Vienna, agrees that the experiment demonstrates just how slippery the concepts of quantum mechanics are. "It's really neat because it shows more or less that quantum events are outside our everyday notions of space and time."
So what's the advance good for? Physicists hope to create quantum networks in which protocols like entanglement swapping are used to create quantum links among distant users and transmit uncrackable (but slower than light) secret communications. The new result suggests that when sharing entangled pairs of photons on such a network, a user wouldn't have to wait to see what happens to the photons sent down the line before manipulating the ones kept behind, Eisenberg says. Zeilinger says the result might have other unexpected uses: "This sort of thing opens up people's minds and suddenly somebody has an idea to use it in quantum computing or something."
A lot of these quantum paradoxes depend on you thinking of photons as particles, analogous to macroscopic particles with which we have personal experiment, like marbles or ping pong balls. Think of photons as particles, and almost everything about light is very mysterious. Stop thinking about them as particles, accept quantum mechanics, and light is not so strange.
There are no applications to uncrackable secret communications or to quantum computing. This is just quantum mechanics, not some great new physics.
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