Baby Steps Toward Quantum Computers 308
Mz6 writes "In a step toward making ultra-powerful computers, scientists have transferred physical characteristics between atoms by using a phenomenon called entanglement, which Einstein derided as 'spooky action at a distance' before experiments showed it was real. Such 'quantum teleportation' of characteristics had been demonstrated before between beams of light. Teleportation between atoms could someday lie at the heart of powerful quantum computers, which are probably at least a decade away from development. Researchers using lab techniques can create a weird relationship between pairs of tiny particles. After that, the fate of one particle instantly affects the other; if one particle is made to take on a certain set of properties, the other immediately takes on identical or opposite properties, no matter how far away it is and without any apparent physical connection to the first particle." Reader starannihilator adds: "Physics Web provides a good graphic summary of the phenomenon, as well as a good technical article."
Analogue vs Digital (Score:3, Interesting)
can someone qualified answer this question (Score:5, Interesting)
i.e. will my computer crash when there is a solar flare?
will the new "heatsinks" be lead shields?
will we need to rotate the shield harmonics? (j/k)
please... inquiring minds want to know.
How to choose? (Score:2, Interesting)
This... (Score:3, Interesting)
Not quantum computing, but (Score:3, Interesting)
Can someone explain why this can't be used for FTL communication? The folks at Cornell [cornell.edu] seem pretty convinced [cornell.edu] that FTL communication is impossible, but from my reading of the article, in this experiment the first particle is forced into a known state, so (IANANuclearPhysicist but) it seems to me that if the state of the second particle can be measured (even if that measurement causes the state to change), communication has been accomplished. What am I missing?
Need 3 particles (Score:3, Interesting)
Re:Not quantum computing, but (Score:5, Interesting)
It's not intuitive, but the "collapse of the wave function" metaphor fits observation.
Ultimate Long Distance Communications (Score:2, Interesting)
We hope to be able to use this for computing, but we know it could be used for communication even better. All we have to do is develop better, cheaper tools for manipulating & reading the particals.
Unfortunatly, so far it only seems to work with pairs, we can't seem to get multiples going, so use is limited. but let's try this from the military point of view: In theory, we could build 'ansibles' (to steal from Orson Scott Card) that operate in pairs. Every ship and command unit could have one, the other one would be connected to a complex of normal computers that woudl determine which other ansibles to send the message to.
No static or bad connections, and no need for encryption as there is no way to intercept the communications!
I'm still confused by this. (Score:3, Interesting)
Does this mean they might finally break that 7-qubit barrier that quantum computers up until this point had seemed to have been limited to?
I really don't get exactly what's going on. I ASSUME the news doesn't mean that they've find a way to transmit information instantaneously using QE.
Electrogravity (Score:2, Interesting)
FTL is not an impossibility; it just stands in relation to relativistic physics as it stands in relation to classic physics.
As many know, around a black hole there is a very strong gravitational field. This field has the property of bending the dimension of time itself. We can therefore state that time is not linear, and that a hypothetical theory of electrogravity would be entirely four-dimensional. This would mean that as far as the theory is concerned, there is no difference between cause and effect (as you can from our 3D perspective look at it backwards and forwards; wine filling a shattered glass that reassembles and hops up on the table), and time would be something that only stood in relation to us. The actual EG math, formulas et al., would be like the math familiar from school. - No time variable. - The formulas simply show how things stand in relation to each other, and if one thing is the cause or the other is effect; that is entirely up to us to determine.
Re:Ultimate Long Distance Communications (Score:1, Interesting)
You mean, "steal from Ursula LeGuin"? That's where Card got it from, and he does mention that the name/idea was taken from an old SciFi book in the Ender series.
A method to break Quantum Encryption? (Score:3, Interesting)
Okay, so this is probably incorrect, but it is a train of thought. With the state of quantum encryption being that if a third party observes the key in transit, it is apparent, and the key is useless, would this have a potential application to break this encryption.
Using this method, the duplicated particles could be observed, leaving the original particles in the encryption stream relatively unmolested. Yes, it would be impractical and the equipment needed would be very distinctive and difficult to hide, but it raises the possibility.
Re:Analogue vs Digital (Score:4, Interesting)
How do you measure spin? (Score:3, Interesting)
IANAP, and in the high level articles I've read, I've never seen spin discussed to anymore depth beyond just that it's a property of fundamental particles. I know that force particles have integer spin (and thus ignore the exclusion principal), and matter particles have half integer spin (and have to obey the exclusion principal), but I don't know what that means physically, or how you measure it. Does it have to do with angular momentum? From a macro world of physics, to measure the angular momentum of something, you can apply a torque and see how quickly it accelerates. I also know that you can measure the charge and mass of a particle by seeing what sort of spiral it makes in a cloud chamber. Is measuring spin related to either of these techniques at all? Thanks for the help!
Komi
Re:Not quantum computing, but (Score:3, Interesting)
What you're thinking of doing is creating an entangled pair, and keeping one particle on Earth, and keepting the other on a spaceship. Then by changing the state of the Earth particle, you could affect the state of the spaceship particle. Right?
Yup, exactly.
The problem is, we have no way to choose what state the particles will go into when we observe one. Its a random outcome, and you can't acheive any communication if the output is just random noise.
But I thought that's exactly what this experiment accomplished. The Physics Web article and diagram certainly suggest that they're teleporting a known state, via the use of a third particle to influence one side of the pair; am I reading them wrong?
Furthermore, from the spaceship's viewpoint, how do you tell if your particle's state has changed due to an incoming transmission?
I'd assume you just repeatedly observe it at fixed intervals to generate a bitstream (or whatever-stream) of incoming information. Even if your clocks shift a bit, you can include periodic timing bits to calibrate--sort of like the Atari 400/800 did with programs recorded on cassette, where stretching of the tape would change the lengths of the recorded bits. This eliminates the need for a subchannel to say "we just made an observation"; just observe all the time and ignore anything that looks like static.
change in properties of other determinable?? (Score:2, Interesting)
Hate to spoil your fantasies (Score:3, Interesting)
However, it seems that every time somebody mentions something about 'quantum' people around here go into Batman and Star Trek Mode.
1. This whole thing is still very much in the early days of fundamental research. Think Babbage or Archimedes or something similar. I suspect that much of the hype about 'quantum computing' is simply a magical mantra that produces funding.
2. There still is no such thing as teleportation, not even theoretically. Entaglement only means that you can get two objects to behave 'in step' even at a distance, but so far it has always involved that they start out together, ie. physically close to each other. Teleportation on the other hand is normally thought of as transporting mass from one point of space to another, sort of magically, without passing through the space and time that seperate the two points. There really isn't much chance of that ever making even theoretical sense.
Re:Analogue vs Digital (Score:2, Interesting)
As a former experimentalist, I realize that qc is very hard to DO. I am not close enough to the field to say whether is "fundamentally not practical" hard to do, or just "takes a lot of hard work" hard to do. It is still worth researching in any case.
I am cynical enough about academic research and the way that researchers follow the grant money to be unsure about this particular researcher's motives. It may be that he decided that qc was exiting enough to do on its own. I saw a guy stick to his work even though he did not get any grants for many years because he was deeply interested. I was rather pleased to hear recently that he got a couple of nice grants and has a couple of RAs now. I saw a guy move to biophysics (another fashionable field) because there were problems that intrigued him.
I have also seen researchers whore for grants in a big way. Academia is about ego. Publish or perish! Without grant money, it is difficult for an experimentalist to do anything worthwhile. It is not so bad for theorists, but grant money still pays for graduate students (RAs that are on the project full time as opposed to TAs that also have teaching duties) and postdocs. Money simply makes it easier (in terms of manpower and equipment) to do the kinds of things that get you published. The number of publications, and even more so how often you are referenced determines you stature in academia. An unpublished professor is a nobody out in the wilderness. Science has its fads, just like the worlds of business, music and fashion. These fads tend to manifest themselves in the form of where the grant money is.
Re:Analogue vs Digital (Score:4, Interesting)
I mentioned this yesterday as well, but for an idea of what qubits are you can take a look at my currently unfinished Java Quantum Computation applet [jhu.edu]. As of now one can only do single-qubit operations, but eventually I hope to have a demo of quantum teleportation (teleportation of a single qubit, or spinor, that is).
This applet will give you an idea of what qubits are. Essentially they're a 'spinor' which in quantum-mechanical terms is a 2-element discrete wavefunction. In lay terms, this just means a set of two complex numbers (properly normalized). They are also displayed in a more visible representation, called the 'Bloch Sphere'.
This applet will let you take any input qubit, and operate on it with 6 different single-qubit quantum gates, and see the resulting qubit.
Look at the two qubits represented on the Bloch sphere. The yellow vector represents the qubits. The red dot indicates a classical 'zero' and the blue dot indicates classical 'one'. In classical computing any bit can only point exactly to the red or blue dots. In quantum computation a qubit can point anywhere on that sphere.
[For the mathematically curious, a qubit is 2 complex numbers, which would be 4 independent parameters. However, the sum of the modulus squared of each complex number must be unity, so that constraint leaves only 3 free parameters. Secondly, the entire qubit can be multiplied by any arbitrary phase constant (e^i*gamma) which changes the spinor but not its relative values. Hence, there are only two parameters for each qubit that really matter, so it can be expressed in 2D, mapped nicely to the sphere.]
In classical computing there are only 2 single-bit gates - Not and Buffer (actually, I never formally studied computer science, so someone please correct me if this isn't true). 'not' flips the bit, 'buffer' keeps the bit unchanged. In quantum computing there are infinitely many single-bit gates, some of the common ones are demonstrated in the applet. Basically, these gates can control how relatively 'one' or 'zero' the bit is by the superposition, as well as change the relative phase.
Anyway, I should be adding in two-qubit operations soon (like the infamous controlled-not) and hopefully get to something worthwhile.
So this applet isn't very useful for actual simulation of quantum computation yet, but it will you give an idea of what qubits are and how they can be represented.
Re:Analogue vs Digital (Score:2, Interesting)
It actually has more potential than this. It is not random, if 'measured' properly. This is the whole philosophy of Quantum communication, which in my opinion is actally the most interesting theoretical aplication of Entanglement.
Whats preventing development is the ability to reliably and measure and remeasurean entangled pair without affecting certain properties of it s twin. Hence the whole paradox of changing the nature of a photon by measuring some of that photon -hence removing any value from the measurement in the first place. But it is possible to measure a photon without altering it. You may remember experiments in Paris using iridium to actually measure the change in phase of the iridium that is fired into the photon, not measuring the photon itself, but rather a relatively acceptable phase change in the iridium. Now as I understand it, the main problems are that scientists can slow down and even completely freeze a photon of light, as demonstrated by a Dutch scientist. Then the technology is there to measure this 'frozen' photon repeatedly. As far as I know, its just not reliable enough yet...
As for Quantum teleportation (which really is more quantum replication than teleportation) many many years await before it can be applied usefully on a large scale. Same almost for Quantum computers - though they may arrive sooner than we think)
How come it can't be used for communication ? (Score:3, Interesting)
After all, transmission of information in a computer circuit is no different than communication.
Would this analogy work? (Score:2, Interesting)
It would be like I had two coins that I could flip. Two classical coins could come up as both heads, both tails or one head and the other tails. Normal statistical behavior.
An entangled version of these coins when I flipped them would always come up either both heads or both tails for example. (It could also always be if one is heads, the other must be tails as well)
If this happened with classical coins we would say that something about the coins or environment was rigged. This is what Einstein thought.
However with quantum entangled coins this would be perfectly acceptable behavior.
Re:Analogue vs Digital (Score:2, Interesting)
One idea I've heard is that they're actually just ONE photon, showing its face in two different points in spacetime simultaneously.
And regarding the challenge of getting information out of quantum-entangled particles, if we could get the Dutch freezing process down, we could: alter-freeze-read-alter-freeze-read
Re:Help me understand this! (Score:3, Interesting)
No. Once you've measured them, the entanglement is destroyed. Actually, it's not quite right to say you change the state of the one or the other particle, because in an entangled state, the entangled particles do not have a defined state on their own. They only have a joint state, the entangled state. Now measuring them causes that state to "collapse" into one where the particles have a well defined state. However, which state they have is mostly random. The only thing which is fixed is (a) that this state corresponds to whatever you've measured (e.g. if you measured the z-Spin, you'll get a state with defined z-Spin, while if you measured the x-Spin, you'll get a state with defined x-Spin instead), and (b) that the other particle will be in a state which is determined by both the original entangled state and the state the measured particle has after your measurement, even if at the time of the measurement the other particle is lightyears away and has no physical interaction with the measured particle or the measuring device.
So basically, you cannot really change the state of a far-away particle, but you can force a far-away particle which had no well-defined state into one that has, if you have the particle it is entangled with.
Should be clear by now: You can detangle them by measuring them.
You have three entangled particles.
Well, that's the complicated one. Does this [wikipedia.org] help you?