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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: 2) by wonkey_monkey on Wednesday August 17 2016, @02:36PM
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.
It's not a quantum state that you need to observe, though, is it? - it's a classical one. To do so means collapsing the quantum state which (as I understand it) breaks the entanglement.
Once one measurement has been made on the photon, any subsequent measurements (unless they happen to be exactly the same kind of measurement, of which there is no guarantee) will result in uncorrelated results. The parties concerned will notice this (because they randomly make different kinds of measurement and only use the results where they happened to make the same measurement).
For that matter, can you even make the measurement without causing the destruction (absorbing) of the photon? Never been clear on that.
systemd is Roko's Basilisk
(Score: 0) by Anonymous Coward on Wednesday August 17 2016, @05:20PM
Like this won't make Denial Of Service the choice for horking up these "quantum secure channel" communications systems...
(Score: 2) by bob_super on Wednesday August 17 2016, @05:25PM
How would the satellite know whether the person observing the photon is legit or not?
I'm wondering whether China just launched yet another boring spy satellite, and just decided to distract us by trolling everyone...
(Score: 2) by Capt. Obvious on Wednesday August 17 2016, @06:07PM
There's an unencrypted side-channel. No secret information there, but (a) it is required to read the encrypted data and (b) it includes information from the receiver on whether the data was eavesdropped.
(Score: 3, Informative) by Capt. Obvious on Wednesday August 17 2016, @06:16PM
If you could, then this encryption would be worthless. It's postulated as impossible, but sometimes in physics that just means "not yet". There has been some announcements about the ability to do so, but they have the credibility of cold fusion: those who claim it's there/almost there are huckseters, and those who claim to be working on it but are serious are looking at career defining/decades long work/Nobels.
That said, because that would break this encryption wide open, I doubt the government of whomever discovered this would allow it to be published.
(Score: 2) by Scruffy Beard 2 on Wednesday August 17 2016, @08:07PM
According the the Quantum Eraser Experiment, you are allowed to observe, so long as you throw away the observation.
(Score: 2) by Scruffy Beard 2 on Wednesday August 17 2016, @08:10PM
Should have used pre-view :P
How the Quantum Eraser Rewrites the Past | Space Time | PBS Digital Studios [youtube.com]
(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.