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Making Quantum Crypto Actually Work
There's a piece on the newest breakthrough in quantum crypto on Feed. It goes over some of the background that we've all read before, but the implication of actually making it somethig useful beyond the current short distance is pretty darn cool.
Um, are you trying to initiate a flame?
Not to say that your points aren't generally valid, but, I hate to tell you, but Big Brother is already sending private messages back and forth that we can't read. Remember, the gov't (well, the military) has the patience and need to use OTPs. OTPs always win.
The more serious problem is that a prerequisite for quantum crypto is stronger quantum computers. Stronger quantum computers can breeze through current crypto mechanisms with startling ease, dramatically raising the bar of what is a 'secure' keylength. Start thinking 4096bit, buddy. Gov't needs privacy, they have it and will keep it. Citizens also need privacy, we can have it sometimes, but might be about to loose all possibility of it.
All that being said, c'mon. We know we're not gonna stop technology, so we need to start thinking seriously about how to address the implications. We're not the RIAA here, we know the light is a train, and we're smart enough to find a side-tunnel to not get killed.
Returned Peace Corps IT Volunteer
The article focuses on one piddly part of quantum crypto. It's more powerful than it reveals, as this /. thread and it's related article discuss.
Returned Peace Corps IT Volunteer
"Because entanglement degrades over time, it's necessary for long-distance communication that a pair be as perfectly entangled as possible to begin with. Zeilinger's proposed mechanism, a polarized beam splitter (PBS), is a little cube of gas that generates highly entangled states in photons that are only weakly entangled, making them robust for long-distance communication."
Whah??? So if you "cook up" an entangled pair of photons, don't you have to then send one to the recipient? How does the recipient get their photon? Or does, through the magic of QM, the entangled photon just "appear" at the other side?
Forget encryption: if we can transmit information simultaneously (in the very physical definition of the term), that itself is an AMAZING feat. You could basically have all the information in the world replicated *instantaneously* everywhere. What's stopping me from generating billions and billions of "bits" of entangled photons, and just using them for massive storage and "free" simultaneous communication?
It's 10 PM. Do you know if you're un-American?
Einstein hated the way people talk about this stuff, because he believed in the hidden variable explanation. This makes perfect sense without action at a distance if you imagine that the information exposed by reading was set at the time the particles became quantum entangled and carried by both particles all along.
The common explanation (the one taught in universities) is that the data of quantum state is created (purely randomly) at the moment it is read. Hence spooky action at a distance when you read one entangled particle, because it creates the same data in its partner, no matter how far away. The Einstein/hidden variable explanation is that the data is read from hidden variables (which are changed by the reading, in chaotic ways we don't have a model of, and so can't predict, thus creating apparently random new settings for the variables); there's no spooky action at a distance because "quantum entanglement" simply means that they somehow have the same hidden variable settings.
The justification for going with the spooky explanation is that it is "simpler" and thus preferable by Occam's Razor. To me, this is just bad philosophy, and a misunderstanding of the uses of Occam's Razor. For one thing, it throws out determinism, saying that not only are the reasons for things we can't predict hidden, but there are no reasons for them at all! For another, it tells people to stop looking for the hidden variables and the rules that create the apparently random values, because there are no hidden variables and quantum state is truly random.
It's not a difference in actual predicted results, it's a difference in philosophy. Einstein preferred the theory which admitted its gaps over the one that pretends things don't exists whenever you can't see them.
It's a common theme in his work: his theories suggest things beyond those fully predictable by his theories (such as black holes), thus spurring new research. If relativity was dominated by the same bad philosophy as quantum mechanics, it would claim that the interior (beyond the event horizon) of a black hole does not exist because we apparently can't observe it, just as it claims that the internal state of a quantum particle doesn't exist.
Favoring a "complete theory" is pure hubris, and has contributed to the stagnation of quantum theory.
Respect Einstein, give hidden variables a chance!
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"How is this useful in transporting a message? Could someone either explain what I'm missing"
You're not missing anything, that was my first thought as well. The current analysis of "spooky action at a distance" implies that while there is a statistical correlation, it is insufficient for transmitting data. That fact is something sorely lacking from this Science, and I would like to have it addressed. Spooky interaction of electron spin is not sufficient for communicating a message, though it may be useful for verifying a message. What gives?
Check out the heading "Putting Entangled Photons to Work" here for more info. There's a lot missing in this quantum encryption proposal mentioned in the article...
Did you guys catch the really cool part about this proposal? Entangled photon pairs react in such a way that when the state of one photon is changed, the other is changed instantly. Therefore this is not just quantum encrypted communication, but quantum encrypted communication faster than the speed of light.
If you want to read a to read a far less pseudo-science description of this phenomenon, may I suggest the unisci article. There's a good article on the whole entanglement phenomenon at Daily Insight here.
p.s. "spooky action at a distance" was Einstein's phrase for it...
First, I'd like to point out that quantum computation and quantum encryption are two almost completely separate concepts. Quantum encryption is based on the fact that quantum states cannot be measured without altering. The most common example is the polarization of a photon, but it will work for any quantum state, so long as there exist, effectively, two unique states that can transmit the data.
Quantum computation, however, is much more complex and much more interesting. Quantum computers are based on the concept of quantum entanglement, the ability of a quantum state to exist in a superposition of all of its mutually exclusive states: It's a 1 and a 0. However, this is not as easy to use as one might think. While it's true that if you have n quantum logic gates you have the ability to input 2^n data values simultaneously (as opposed to only 1 piece of data if you have n digital logic gates), this is not going to be the end of classical computing for a few reasons. First, quantum computers have to be perfectly reversible. That means for every output there's an input and vice versa. And there has to be no way of knowing the initial states of the data. You don't process data, you process probabilities in a quantum computer; if you know exactly what any one value is throughout the computation, you can find out all of the values: the superposition ends and you're stuck with a useless chunk of machinery. This means YOU CAN ONLY GET ONE RESULT FROM ANY QUANTUM COMPUTATION, THE END RESULT. You can't see what the data in the middle is or the computer becomes useless. (Landauer's principle makes heat loss data loss. When your processor gets hot, it's losing data. If the same thing happened to a quantum computer, it wouldn't be quantum anymore.) Decoherence is what happens when you randomly lose data to the environment by design, not by choice, and the superposition ends. This is bad for Q.C. Oh, and quantum computers can only do *some* things faster, like prime factorization and discrete logarithms. Not multiplication or addition. Plus, the circuits that would do basic arithmetic would be bigger and slower than what you've currently got.
So what does this all mean? It means that quantum computers are going to provide some advantages (real quick big number factorization), and some disadvantages (that whole RSA standard). The most realistic initial use of quantum computers will be as add-ons to existing super-computers to resolve certain types of NP-Complete headaches that regular math can't simplify yet. At best they will someday be an add-on to your PC; but they will never replace the digital computer.~
If you want more info, check out http://www.qubit.org, it's got some decent tutorials.