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Single Photons Bounced Off Orbiting Satellite
KentuckyFC writes "If we're ever going to benefit from the perfect security of quantum communication, we're going to need ways of transmitting entangled photons around the globe and certainly further than the current record of 144km through the atmosphere. Anton Zeilinger at the University of Vienna and colleagues have taken an important step towards this by bouncing individual photons off the Ajisai geodetic satellite (essentially a space-based disco ball) which is orbiting at 1400km. The group says the experiment is an important proof of principle for satellite-based quantum communications."
From my understanding it does serve a practical purpose in that intercepting the message changes it. Thus while you can't stop people from tapping into your message, you do have instant feedback about when that happens.
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Real security improvements. There is no proof that there is not a trivial way to factorise multiples of large prime numbers, which is the basis of most current encryption standards. There are alternatives, but again there is no proof that these cannot be cracked quickly.
;-)
Even though it is unlikely that someone will have a mathematical breakthrough that would allow your PDA to break 2kb keys, we know that a lot (maybe all) of these algorithms could be cracked with a quantum computer. It is possible that the US NSA already has such a computer, maybe together with Russia, China and Bill Gates
Quantum encryption is proven to be uncrackable without showing that someone is listening. With a preamble of two-way communications you can have a connection that is proven to be absolutely secure, and no breakthrough in mathematics or technology will break it.
Breaking quantum cryptography is not hard, it is impossible. The security is guaranteed by the laws of physics. Unless quantum mechanics is flat out wrong, it can never be broken, period. And saying quantum mechanics could be wrong is like saying gravity could be wrong.
About quantum computing, it's actually closer to providing new computational powers than you might think. In terms of a powerful, programmable computer that can factor large numbers, we are a long way off. But in terms of being able to simulate certain quantum systems that current supercomputers cannot, we are fairly close.
"You need the photons to be "entangled". That means effectively in their own little world, not intereacting with the universe in any way."
What are you smoking? Where did you get that definition of entanglement?
Read up on the topic.
Pay special attention to the "faster than light discussion" parts to see why they need to send the photon.
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Hi,
I am a theoretical particle physicist, and I understand what you are trying to say, and I understand what the replies are saying as well.
You are correct that the 'basic' quantum cryptography that is taught can be hacked. This is just because a simplified version is used in books, because it's confusing enough.
Others who point out the no cloning principle are exactly right. You cannot read and then reemit a photon with the same polarization (for example). Basically the way it work is like this:
Alice picks one of four polarizations at random, out of vertically polarised (0 degrees, say), horizontally polarised 90 degrees), or polarised at 45 degrees, or at -45 degrees.
Bob then picks, at random, whether to measure the polarization horizontally/vertically or to rotate his polariser 45 degrees and measure at 45 degrees or at -45 degrees.
If Bob chooses the 'wrong' direction, he'll get a random result. If he chooses the 'right' direction, then he'll get the correct result. After this is done, Alice tells Bob how she sent it, and Bob can discard all the measurements in which he chose the wrong direction to measure in. If Bob tells Alice the same results that Alice sent, then they know there was no interference.
There is no way to measure the 'true' polarization of the light that Alice sent, because you have to have your polariser either measuring horizontal/vertical, or measuring at 45/-45. You can't have it do both. So noone can monitor and then reemit a photon with the same polarization.
The satellite is covered with retroreflectors, aka corner cubes. It is a property of three perpendicular planes that any light is returned along its incident path. Reflectors are used on bikes and cars, and highway signs (and high-gain front projection screens).
http://www.af.ca/halifax/sciences/gim/LAGEOS-NASA.jpg
The sphere, LAGEOS, is covered with corner cubes. For scale, I think it's about 60cm in diameter. To send a single photon up and receive it is amazingly accurate, and lucky. Divergence of a laser results in many orders of magnitude loss of signal over those distances. I believe we know the orbits of these inert satellites to better than 5cm, at least we did two decades ago when I worked on TLRS and MOBLAS station stuff at Goddard. We would send 200 ps pulses (Nd:YAG laser) and then gather the returns in a telescope - sometimes of only several photons out of the entire pulse - which were then used to calculate time-if-flight and determine the exact location on earth. This was done to track motion of the tectonic plates.
Is it just my observation, or are there way too many stupid people in the world?