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from the new-toys-are-all-made-in-china dept.
China has launched a satellite that will beam entangled photons to base stations on Earth:
China has successfully launched the world's first quantum-enabled satellite, state media said. It was carried on a rocket which blasted off from the Jiuquan Satellite Launch Centre in China's north west early on Tuesday. The satellite is named after the ancient Chinese scientist and philosopher Micius. The project tests a technology that could one day offer digital communication that is "hack-proof". But even if it succeeds, it is a long way off that goal, and there is some mind-bending physics to get past first.
The satellite will create pairs of so-called entangled photons - tiny sub-atomic particles of light whose properties are dependent on each other - beaming one half of each pair down to base stations in China and Austria. This special kind of laser has several curious properties, one of which is known as "the observer effect" - its quantum state cannot be observed without changing it. So, if the satellite were to encode an encryption key in that quantum state, any interception would be obvious. It would also change the key, making it useless.
(Score: 0) by Anonymous Coward on Wednesday August 17 2016, @10:13PM
The canonical (aka "Copenhagen") explanation is that, before a measurement has been made on the photon, it is in a nebulous superposition of states. For instance, if you're talking about polarization, it is in a linear combination of states where it has a probability of 0.5 of being in either state (just like Schrodinger's Cat). When a polarization measurement is made, the wavefunction "collapses" and the photon has a well-defined polarization. Now, since it was one-half of an entangled pair, that means the other photon's wavefunction colllapses and the other photon instantly gets the other polarization state. Here, when they say that its quantum state cannot be observed without changing it, they are talking about measuring one of its quantized states, such as its polarization or spin, which takes on discrete values (as differentiated from, say, its position). The "spooky action-at-a-distance" comes in when you consider how can the other photon ever know what polarization it is supposed to have because if nothing can travel faster than light, how can the information get from the measured photon to the other one?
You are correct in that once the measurement has been made, the state of the two particles have been fixed, and the fates of their QM states are no longer tied together. You cannot measure its state without affecting its QM state.