Theoretical Breakthrough For Quantum Cryptography 116
KentuckyFC writes "Quantum cryptography uses the quantum properties of photons to guarantee perfect secrecy. But one of its lesser known limitations is that it only works if Alice and Bob are perfectly aligned so that they can carry out well-defined polarization measurements on the photons as they arrive. Physicists say that Alice and Bob must share the same reference frame. That's OK if Alice and Bob are in their own ground-based labs, but it's a problem in many other applications, such as ground-to-satellite communications or even in chip-to-chip communications, because it's hard to keep chips still over distances of the order of the wavelength of light. Now a group of UK physicists have developed a way of doing quantum cryptography without sharing a reference frame. The trick is to use entangled triplets of photons, so-called qutrits, rather than entangled pairs. This solves the problem by embedding it in an extra abstract dimension, which is independent of space. So, as long as both Alice and Bob know the way in which all these abstract dimensions are related, the third provides a reference against which measurements of the other two can be made. That allows Alice and Bob to make any measurements they need without having to agree ahead of time on a frame of reference. That could be an important advance enabling the widespread use of quantum cryptography."
Re:You lost me at hello... (Score:5, Informative)
I'll give it a shot.
Alice wants to get out of her car and into Bob's car. In laboratory conditions both cars are perfectly still so it's easy. Out on the freeway travelling at high speeds it's a recipe for disaster.
But these clever engineers have come up with a wonderful design for a semi-trailer that both cars can sit on while being driven down the freeway. Now Alice and get out of her car and into Bob's car for that secret rendezvous. In the middle of the freeway.
Re:You lost me at hello... (Score:3, Informative)
Quantum physics does not have a car analogy. Cars cant be mixed up and then split so each part has a bit of the other, and not just physically. if one car starts, it means its parts in both entangled sets start and the moment you go and look witch of the cars you have it becomes one or another, instantly causing the other entangled car thingy to become the car you didn't get. Also, fu Eve.
Hungarian Physicists and Automotive Engineers are closer to tackling that problem:
http://tech.slashdot.org/story/10/02/24/1614245/Hungarian-Electric-Car-Splits-Into-Two-Smaller-Cars [slashdot.org]
Re:You lost me at hello... (Score:3, Informative)
Here's my own amateur shot at it. If I'm wrong, I'm sure someone who knows better will correct me. Oh, and fuck the car analogy.
The quantum entanglement measurements will only work with two entangled photons when the velocities and accelerations of the two parties involved are the same. But if you're doing it with two objects with different motion, say a person on the ground and a satellite orbiting the earth, it won't. The satellite is in free fall and, according to general relativity, not in an accelerated reference frame. A person on earth, though, is feeling a constant acceleration coming up from the ground because of gravity holding him there. One difference between the two is because one is in an accelerated reference frame and the other is not, their clocks are moving at different rates.
The fix for this is to have a third entangled photon in another reference frame. As long as the motion relative between all reference frames are known, the person on the ground and the satellite can use the third entangled photon as a reference point for them to make measurements between the two with their own entangled photons.
Sorry if I'm off about this but if someone corrects me then, hey, it's a learning process for us both.
Re:You lost me at hello... (Score:3, Informative)
The satellite is in free fall and, according to general relativity, not in an accelerated reference frame.
If you're not moving in a straight line at a constant speed, you're in an accelerated reference frame. Satellites are in orbit; there's no such thing as a straight-line orbit.