On Thu, Nov 28, 2019, 10:37 PM John Rose <[email protected]> wrote:
> It works: > https://science.sciencemag.org/content/356/6343/1140 > Quantum encryption is based on the principle that you can't observe something undetected. You can't eavesdrop without me knowing it. If I want to send messages using single photons, then I can generate entangled pairs, which just means they have to be generated in a way that I know conserves energy, momentum, and angular momentum (spin). If I measure these on one of the photons, I know what you have to measure on the other, regardless of what quantum mechanics says you should measure without that knowledge. A photon has a spin of 1 (in units of reduced Planck's constant) along any axis I choose to measure. Classically this is impossible. But that's because you are thinking of a spinning ball. What really happens is that particles don't exist, only fields. When you solve Schrodinger's equation for a system that includes a propagating electromagnetic field and an observer (a memory device), the solution is the observer observing a photon. The solution is exact (like Einstein said, God does not play dice), but random to the observer because it has limited knowledge. The only problem is the computation is intractable, so we have to use approximations like the Copenhagen interpretation that pretend particles sometimes exist. This leads to all sorts of non intuitive conclusions, like Schrodinger's cat and Bell's inequality, where the approximations don't apply. Anyway, let's say I generate two photons by letting an atom drop to a lower energy state with the same spin. The pair must have opposite spins to conserve angular momentum. If I measure both spins along the same axis, then they are always opposite. But if I measure along different axes, then they can't add to 0 except on average over lots of measurements. I will get random readings as if my device was introducing errors. But that's not the right interpretation, or why would I get no errors when a distant detector was parallel? This was the effect seen over 1200 km away. But it only works with a direct line of sight to the satellite. You can't bounce the photons off mirrors or amplify them because it changes their spin. But you can use this effect to send bits as long as the eavesdropper doesn't know which way to orient the detector. ------------------------------------------ Artificial General Intelligence List: AGI Permalink: https://agi.topicbox.com/groups/agi/T3d8826b1a4fec947-M8da9ba58e6b96c76d8c88a7d Delivery options: https://agi.topicbox.com/groups/agi/subscription
