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"Spooky" Quantum Data Encryption
Hardy writes "Imagine an encrypted communications channel that immediately notifies the parties if they are being bugged. The American Institute of Physics site is running an article about exploiting what Einstein described as the "spooky" action at a distance properties of quantum entangled particles. The entanglement process can generate a completely random sequence of 0s and 1s distributed exclusively to two users at remote locations. Any eavesdropper's attempt to intercept this sequence will alter the message in a detectable way and enabling the users to discard the appropriate parts of the data. This random sequence of digits is then used to scramble the message. This approach solves the problem of distributing a shared key to both parties without it falling into the wrong hands. This diagram might help.
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This is all very interesting, but it's practical uses are limited by a few factors.
First, the quantum key must be physically transmitted to the receiver. This means that the medium for transmission (in most demonstrations, fiber optics) must be in place between the communicating parties and both parties must have the equipment to detect the value of the key. This equipment must be capable of detecting the polarization of single photons. Not exactly the type of stuff people have just lying around.
Second, there can be no amplification of the signal transmitting the key. Amplification of the signal is equivalent to someone eavesdropping on the key. The usefulness of the key would be destroyed. So forget about using this over normal phone lines or the Internet.
Third, the longer the transmission distance the greater the likeliness of errors in the key. Quantum encryption keys have been successfully transmitted approximately 20 kilometer through fiber optics and 500 meters through the atmosphere, but with about a 2% to 3% error rate. This will probably be acceptable for text messages, but may not be for data streams unless multiple redundent copies of the data or sent, or other error correction techniques are used (adding length to the data transmission). This will work well going from say the White House to the Pentagon, but unless all your secret friends live within 20 kilometers...
Fourth, if transmission speed is a factor for you, quantum encryption poses several problems. Only about 25% of the transmitted quantum key bits will be successfully detected (due to the 4 possible quantum states the photons can be in). This means to have a successful one-time-pad you must generate a key 4 times longer than the message you want to encrypt. Then the receiver has to confirm a sample of the key with the sender to ensure that the key has not been intercepted. Then you can transmit your message with about a 2% error rate.
So this is cool technology, but will really only be useful for military purposes or extremely sensitive corporate secrets.
This is part of the premise. I haven't read the article yet, but I did a term paper on the spooky action (aka EPR paradox) last term. That's half of the picture.
The other half is that the two "entangled" particles have a state which allows someone who reads one particle's state to know EXACTLY what state the other particle is in.
You're familiar with the pop contests that put pictures under the cap right? Imagine a contest with two pictures that form a winning pair. Now, assume the you have a large collection of these pairs. You can split the pairs in order, sending one picture to "Alice", and the second to "Bob". When Alice looks under her liner, she knows exactly which picture Bob has.
This system allows you to generate your encrpytion and decryption keys on-the-fly, while entanglement itself includes conclusive proof that someone "checked under the cap" while the picture was en route.
This seems like a far more likely (and practical) use of the entanglement property then IBM's teleportation from a few years ago. (That was for only a small number of particles at a given time. It was really more of a replication that destroyed the original.)
- W. Blaine Dowler
http://www.bureau42.com
I was at a presentation about this kind of technology several years ago (someone from British Telecom came to give a talk to a bunch of us from University). The basic idea was that you could emit light a photon at a time, and pick this up later on.
If you had a snoop (Eve), the data would be corrupted due to the fact that only one photon existed per data element - later, you could check this and discard any bad data.
You still have to do the actual communication using your favourite strong encryption system. However, this system gets around the problems associated with key distribution over a distance.
So you might say: "well, the laws of physics are changing so rapidly these days that this will soon be a possibility." But revolutions in physics are rarely, if ever, of the sort where all of the old theory is thrown out and a completely new theory is developed. Instead, discrepencies are discovered in some corner of a theory and new a theory is discovered which is a superset of both the old theory and the new data.
Also, "spooky action at a distance" in the form of quantum entanglement was never "impossible," it was just philosophically objectionable to some people, including Einstein. If you mean that "information can never travel faster than the speed of light in vacuum" when you say "faster than light (FTL)" travel, then you are incorrect if Maxwell's equations are to hold. All know examples of FTL (which are trivial and miss the point) violate some aspect of my previous statement in quotations. As for heavier-than-air flight, no rational scientist in any age who has observed a bird would tell you that it's impossible.
It has probably been said a lot before on /. but this is how (I understand that) Quantum Encryption works:
First of all it doesn't send encrypted data. It's just used to send random bits from Alice to Bob. Alice sends for every bit that's 1 a vertical polorised foton and a foton that's turned clockwise 45 for every bit that's 0.
Bob chooses one of two filters for every bit he receives. At random he uses a filter that can either receive a 1 (a filter that's turned counter-clockwise 45) or a filter that can receive a 0 (a filter that's horizontally polorised).
Bob will not receive a foton if he uses the wrong filter, which he will do aproximately half the time. This is because the polarisation direction of the bit and the filter would differ 90.
The interesting thing is that if Bob uses the correct filter, he has only 50 chance that he'll see the foton (can you say 'Quantum effects').
So far Bob knows that:
- he did not receive the bit (because he used the wrong filter or because he had 'bad luck')
- the bit is 1 (by using the correct filter)
- the bit is 0 (by using the correct filter)
Bob should, if knows the value of enough bits (which should be the length of the file to be transimitted), send back the numbers of the bits he received over an unsecure channel.
Alice will then know what Bob is using as a key and she can encrypt the file using XOR. Alice then sends the file over an unsecure channel and Bob can decrypt it.
But what if someone is listening? Let's say that Claude is receiving the bits that Alice send. But Bob will know that Claude is listening because he doesn't receive any bits. The solution would seem that Claude resends the bits to Bob. But there is a problem for Claude here, (s)he did only receive 1/4 of the bits correctly. 37.5% (approximately) will thus be incorrect. In stead of receiving 1/4 of the bits correctly, Bob will only receive 36.5% of 1/4 = 16% of the bits correctly.
But how could Bob and Alice know that not all the bits were received correctly? This is currently solved by sending part of the bits over a quality line (on which Claude could be listening though).
Another problem, letting Bob know that a polorized foton has been send could be solved by sending a pulse of non-polarized light an instance before the polorized foton.
Current results are 48km through optic fiber and 50 meter through the air (3km would do for satelites).
Monkey sense