Baby Steps Toward Quantum Computers

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."

Comments

Analogue vs Digital

[Nermal6693]

I think (although I'm not certain) I read somewhere that a quantum computer is like an analogue computer - where you're not restricted by 0 and 1. Is that correct?

Re:Analogue vs Digital

[Anonymous Coward]

It was just a dream, Bender. There's no such thing as two.

Re:Analogue vs Digital

[LnxAddct]

A quantum computer is completely different. The only thing in common in the binary number system. In a classical computer you have bits, either a 1 or a 0. In quantum computers you have qubits which can be a 1 or a 0 or actaully both values at the same time! This can manifest tself in amazing ways. You can try every solution to a problem instantaneously because instead of having to count throught all of the possible inputs, i.e. going from 0 to 255 with 8 classical bits, in a quantum computer 8 qubits actualy are the values of 0 through 255 all at the same time. The answer is then decomposed or observed forcing the quantum state into a final and complete solution. Some quick info for those who have no idea what qunatum anything is... an observation is essentially defined as any force that forces a quantum state to be amplified into a definitive state. Quantum entanglement occurs when two paritcles intereact for a short period of time (i.e. two photons crossing) and then go off on their own, they can travel to oppisite sides of the universe and whatever happens to one, instantaneously happens to the other. Literally, no moment of time occurs between the change, its quite amazing. If you polarize one photon, the other will instantly be affected. Also if particles A & B are entangled and C & D are entangled then if B entangles with D then A automatically becomes entangled with C. This allows for some truly amazing things. One final note, although quantum entanglement was first observed with laser light(photons), it has since been reproduced with much larger particles including ruby atoms and even bucky balls (google it if you dont know what one is)
Regards,
Steve

Re:Analogue vs Digital

[jfern]

With n classical bits, they can be of 2^n possible states.
With n quantum qubits, they can be any normalized (overall phase doesn't matter) complex vector in 2^n dimensions.
However, when you measure them, the wave-function will collapse (unless you believe in the many world's multiverse), and you'll get n classical bits.

Classical information is simply a subset of quantum information.

Re:Analogue vs Digital

[natmsincome.com]

Not so much Analogue vs Digital but rather Serial vs Parallel.

In searial you do one instruction per peice of data. In parallel you try EVERY piece of data in one instruction.

Some problems are trivial in serial but hard in a parallel and other problems are trivial in parallel but hard in serial.

Simple Example:
Iterative calculation are great in serial but aren't that good in parallel as you can calcualte the second value till you have the previous value.

The Famous example:
The big thing that quantum computers will do is make parallel problems trvial. The big two being simulations and cryptology. Cryptology is only hard because you have to try so many different combinations. Quantum would allow you to try EVERY combination at a single time. This make encryption almost useless at any key length.

It's also usefull for simulations like ray tracing and vector maths where you have a complex eqation where you just have to run for every possible variable.

So ever is a single iteration takes 1 hour for a quantum computer instead of 100th of a second for normal computers it will change the world. Breaking a key 2048 bit key will take exactly 1 hour instead of million+ years. Rendering a frame will take 1 hour on a single computer instead of 4 hours on 1000+ computers.

That being said it would be useless for Word, Excel or Firefox :-)

Imagine a quantum computer that does 5 Hz out perform a cluster that does 5 TeraHz.

Re:Analogue vs Digital

[Medevo]

Somewhat, but you are a little off.

The best way I have found to think about quantum computations is that digital computers think in 1's and 0's

Quantum computers allow you to ad "decimal places" to this traditional logic (0.1, 0.2, 0.9, 1.0). As you increase the number of quantum bits, instead of just increasing the number of calculations a second you can do (like with our processors today) you are in fact adding new more "decimal places" by simply looking at the qubits in terms of accuracy. Even a simple quantum computer of 30 or 40 qubits could theoretically out power any single processor today depending on the quantum accuracy involved.

Medevo

Re:Analogue vs Digital

[jfern]

Quantum computers aren't quite as powerful as you make them out to be. At the end of your algorithm, you have to perform a measurement, and each qubit when measured only gives you 1 classical bit.

It's been proven that quantum computers are no better than classical computers at sorting (both O(n log n), although they are better at finding something in an unsorted database (Grover's algorithm does O(sqrt(N)), instead of O(N) classically).

No one has proven that quantum computers are faster than classical computers for factoring. We just know of a fast algorithm for a quantum computer and not a classical computer. It's likely that quantum computers are much faster there, though.

Re:Analogue vs Digital

[Medevo]

The limit of computing is, as you say, on the developer's side, no argument here. It its at least partially reasonable that when quantum computers become more available, that ingenious developers will find ways to squeeze out more power.

Moreover, at the end of the day, you still extract bits from qubits. While one day in the distant future we may be able to interact computers entirely in a quantum environment, but it's a long way off.

The real potential in quantum computers is the problems of density, power, and heating, that have plagued development of faster CPU's seem to apply on a lesser scale to quantum circuits (not that they don't have there unique problems). At the same time, quantum computers could/would suffer a lot less problems with bandwidth/time delay (light/QE info transfer).

Traditional MOSFET based transistors, while powerful (look at today's advanced chips) have been around for a while; there is no harm in looking for something new and better.

Even if quantum computers provided a liner growth curve in processing power to qubits, we could expect a greater throughput in it (due to above stated factors).

Medevo

Re:Analogue vs Digital

[spacecowboy420]

OK, maybe I'll sound like a jackass, but I gotta ask anyway. It seems to me that if you can reproduce entangled particles reliably, and you have, lets say two hosts, both with one half of the set of the entangled particles. If you were to manipulate the state of one set, and that immediately affects the state of the entangled partner on the other host, wouldn't that be the effectively TRUE wireless communication. One where the rate of communication is limited only by how fast you could read and process the set of particles that are local? Wouldn't that be as secure as it gets - media to intercept? Sure, there would need to be software to interface with the states based on the input from the hosts - but if you could do this, you could control the mars rover in realtime. Is this where this is headed, or am I confused?

Re:Analogue vs Digital

[Scorillo47]

Note that entanglement is just one approach in building quantum computers, and it is not really the ONLY approach.

Generally, a quantum computer consists in several quantum systems (for example captured particles, etc). The (quantum) state of these systems varies according to a well-known equation, called the Schrodringer equation. This is a very simple equation that describes the evolution of the system (the derivative of the current vector state) in respect to the current current state & time.

The nice thing about quantum computers is that they operate with multiple simultaneous states, therefore achieving some sort of parallelism. Basically a quantum system can be considered to have a superposition of states - it has two states at once if you want. Some of these states might converge to the same state depending on the hamiltonian or on the external interactions.

The hard part is that you never know when such a computer stops its calculation since the transformation state is fully reversible and goes on ad infinitum. If you want simply to test if the computer reached the end of the calculation, you will affect the current state. Anywyay, this challenge plus many others (for example the precision of the measurement, etc) makes quantum computing very challenging.

Still, there is a theoretical possibility that you can get a high degree of parallelism in certain configuration. A classical result from Shor (you can search on Google) shows that one of the classic problems in arithmetic - integer factorization - can be done in a polynomial time on a quantum computer. This simply means that RSA encryption can be potentially broken, irrespective to the length of the key. But we are still safe - so far nobody built a working quantum computer that would carry on simple calculations like factorizing the number 15.

On the other side, entanglement is an interesting quantum fenomenon which works like this:
1) First, you have to have a way to build pairs of entangled particles. There are several ways to do this, for example by having any quantum process that generates a pair of photons.
2) Second, if you modify the vector state of one particle, the vector state of the other one will be equally affected, regardless of the distance between these two particles!

What's interesting is that entanglement guarantees instantaneous quantum state change therefore contradicting somehow the theory of special relativity. This theory says that events cannot be 100% simultaneous if they occur in different points in space - there is a timing separation based on the particular reference chosen. Practically, no standard matter interaction can be faster than the speed of light.

But there is an exception here - "collapsing the vector state". If you measure the state of a particle, its state will collapse along one of the measured dimensions (according to certain probabilities). The corresponding entangled particle will suffer a similar change, so if you measure now the state of the this second particle you will see that its vector state has already changed - and you can even perform a partial correlation between the results of the two measurements.

In conclusion, enanglement guarantees instantaneous "interaction" regardless of the distance between these paired particles (this is why Einstein called it "spooky action at a distance" - because technically it is propagated with infinite speed). Anyway, it has be proven a while back that this does NOT contradict the special theory of relativity since this is not a standard matter interaction, like gravity, etc.

Going back to computers, entanglement is an interesting approach which might enable new algorithms or new ways to build such computers. But keep in mind that we are in the stone age of quantum computing right now...

Re:Analogue vs Digital

[cicadia]

Well, you're not a jackass, but it is a bit more complicated than that. Unfortunately, there doesn't seem to be any way to actually transmit information instantaneously with entangled particles. It's true that two entangled particles will undergo the same transitions at the same time, but since you can't predict in advance or control what transition will occur, it doesn't help you send any information to a person looking at the particle at the other end.

You're right, though, that it's about as secure a communication channel as you can get. It's actually the basis for quantum cryptography -- two people share a set of entangled photons, and they can guarantee that the measurements they make on them will be identical, giving them a shared secret key that no one can intercept. They still have to communicate over regular channels to actually send any real information, though.

Re:Analogue vs Digital

[dave1791]

I am not saying that I think qc is BS. Not at all; in fact, when theorists gave (not at uni anymore) talks on the subject, I rarely missed them.

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

[WoodenRobot]

Check out this on theEPR paradox.

Re:Analogue vs Digital

[wass]

Wow, 2 quantum computation articles on /. within two days.

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. 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

[Scorillo47]

Correct - there is no way to transmit pure information through photon entanglement for example. But it is possible to use this technique to verify some information transmitted in conjunction with a separate (classic) channel.

This has two consequences:
1) First, it is practically possible to use entanglement to build networks that are 100% guranteed to transmit either correct information or error.
2) Second, since measuring any particle will necessarily change it state gives an interesting conclusion: it is impossible to tamper the communication channel that transmits entangled photons. As soon as you attempted to measure what's on the channel, the verification mentioned above (i.e. the correlation between the final measurement of the two entangled particles at the two ends) will fail!

Therefore you have a bullet proof method that will prevent active/passive attacks on the entangled channel. The technique was actually employed in practice - see this link for example.

NB - this technique still doesn't prevent attacks that fully substitute one of the ends with a completely identical device so the other end still thinks it is talking to the right person. But in combination with standard cryptography techniques for the insecure channel, this techniue is almost impossible to break. A nice overview is presented here

Re:Analogue vs Digital

[essreenim]

If you have an entangled EPR pair, measuring one collapses the other into the same state. You can share random data, not useful for transmitting information.

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)

Re:Analogue vs Digital

[maxwell demon]

Whats preventing development is the ability to reliably and measure and remeasurean entangled pair without affecting certain properties of it s twin.

You cannot measure anything without affecting it. That's one of the basic properties of quantum mechanics. Especially, if you have an entangled pair, and measure one parner, then you destroy the entanglement. Always, and inevitable.

Even if you try to circumvent it by first having it interacting with something else and then measuring that other thing: If by measuring that other thing, you get information about the partner (more exactly, about the value of one observable of that partner), then it means the "circumvention system" was included into the entanglement, and your entanglement will be gone as soon as you measure that.

Worse, measuring the original entangled pair without the "circumvention system" will have them behave as if they already had been measured, that is, you cannot make use of that entanglement without having access to all systems which take part in it, which now includes the "circumvention system". Actually, that's how decoherence works: The interaction with the environment causes the environment carrying information about the system ("measuring" the system), and therefore destroying coherence since you cannot completely know/control that environment.

Re:Analogue vs Digital

[shambalagoon]

How can two photons clear across the universe communicate instantaneously?

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:Analogue vs Digital

[ganhawk]

Let me try to explain quantum teleportation whith what I know of it.

There is no equvvalent macro phnenomenon for quantum teleportation. But let me try this example.
Quantum teleportation is something like this..

If you have a metal box that can be broken into two metal boxes. Initially there are two colored balls in the metal box. You cannot see the balls. When you break the box into two, each ball stays in one box. You can now seperate the box by a large distance. This pair of boxes is similar to entangled pair. By obeserving the first box, you can determine the color of ball in the second box.

At the quantum level, without knowing the color of the ball, it is assumed to be in a state of superposition. so Observing the first photon forces the state on the second photon.

Prime Intellect?

[Anonymous Coward]

Isn't this the correlation effect mentioned in the prime intellect story?

In the PI universe, a Beowulf cluster of these imagines YOU!

can someone qualified answer this question

[Anonymous Coward]

Just say 20 years from now I am on my quantum fandangle computer that does sub-atomic calculations, what happens when background radiation hits the processor and flips a few 1s and 0s?

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.

Re:can someone qualified answer this question

[phoenix.bam!]

I'd have to say (not that I actually know) that there would be equal danger now from a solar flare crashing your computer as there will be on a quantum computer. But what the hell do I know? You should go ask Scotty.

Re:can someone qualified answer this question

[Anonymous Coward]

Scotty's busy trying to talk to the mouse.

"HELLO COMPUTER"

Re:can someone qualified answer this question

[jfern]

The problem with quantum information is that you can't clone (copy) an arbitrary quantum state, and you can't measure an arbitrary state without destroying the quantum information.

However, there still exist quantum error correcting codes that can correct an arbitrary error. Classically, one only gets bit flip errors. In quantum computation, you have to worry about phase flip errors, for instance instead of a|0>+b|1> you have a|0>-b|1>.

The smallest quantum code that can correct an arbitrary non located (located errors are easier) error on 1 qubit requires 5 qubits. There's a 7 qubit "CSS" code that is important for fault tolerance.

For fault tolerance, you concatenate a code with itself many times, and if your errors are independent of each other, then by doing all sorts of complicated fault tolerant techniques, you can get fault tolerance. What happens is you get a fault tolerance threshold. If your rate of errors are less than that, you can do arbitrary quantum computation with O(M) qubits in O(N polylog N) time, where O(M) is the qubits required on an error free quantum computer, and O(N) is the time required on an error free quantum computer.

Re:can someone qualified answer this question

[nihilogos]

Just say 20 years from now I am on my quantum fandangle computer that does sub-atomic calculations, what happens when background radiation hits the processor and flips a few 1s and 0s?

Quantum error correction. is a sub-field of quantum computing concerned with just that, how to effectively perform a quantum computation in the presence of background radiation and other stuff which sub-atomic thingies tend to be quite sensitive to.

The likelyhood of flipping a few zeros and ones ( and other errors which can afflict quantum bits) is very high, and in reality is more a continuously decay than an instant flip.

It has been shown, however, that this continuous decay is equivalent to flip errors and phase errors (the other sort of quantum error) occuring with some probability. That probability is 1 in 10 for most of the current experiments, compared to your box in front of you which is more like 1 in 10 billion.

Fault-tolerant quantum computing is a theory field of research concerned with how good quantum computers have to be before quantum error correction can work. The best results at the moment suggest a probability of error of 1 in 1000 is good enough. The experimenters have a fair ways to go yet.

Re:can someone qualified answer this question

[jfern]

Some of us are working on getting a better result than 1 in 1000. ;) Actually, the important thing is, it depends on what sort of noise you get from your gates.

Re:can someone qualified answer this question

[mcrbids]

That probability is 1 in 10 for most of the current experiments, compared to your box in front of you which is more like 1 in 10 billion.

Would you really think even a e-machine is that error prone?

Think about it...

2.5 Ghz * 32 bits/cycle = 80,000,000,000 - that's 80 BILLION bits per second...

Of course, that's theoretical, there's buffering delays, cache, noops, etc. But, given the theory, there'd be 8 random errors every single second.

Something doesn't sound quite right, here, especially when you figure the vast majority of computer are sold with no error correction at all on the system memory ?

I think that 1 in 10 billion is probably quite a few orders of magnitude off....

Teleportation

[Naffer]

So we've got the one atom thing down now. The trick is getting a whole lot of atoms to do it at the same time. If we can convince the porn industry that it would be beneficial to them, We'll be teleporting around the world in less then 5 years. Maybe I should patent teleporting prostitutes.

Yes, fast

[Milo of Kroton]

But what cost? Only government would want new technology this fast, maybe your NSA, that around codebreaking.

Re:Yes, fast

[tachyonmkg]

Only the five richest kings of Europe will be able to afford them.

Re:Yes, fast

[jfern]

"We see a worldwide demand for maybe a couple of computers" - IBM
"640k of memory is enough for anyone" - Gates

Re:Yes, fast

[Bl33d4merican]

Only government would want new technology this fast, maybe your NSA, that around codebreaking.

Only the government would want it? Hell, I'd want it! Who wouldn't?

Re:Yes, fast

[plaa]

Comparing the speed of a quantum computer and classical computer is comparing apples and oranges. Quantum computers work with a totally new set of rules, which allows some applications to make use of quantum properties.

The main property that classical computers lack is that of superposition of states. One can understand this as calculating some result starting with all possible numbers at once, instead of testing each starting value as its own. (In reality it's more complicated than this, of course.)

Some applications, eg. codebreaking, number crunching and database applications could get a vast boost out of quantum computing. Other applications may not. The most probable places for quantum computers (at first) will probably be number crunching, networking applications (quantum cryptography etc) and database applications.

For a comparison, searching an unsorted database is classically an O(N) operation, but a quantum computer can do this in time O(sqrt(N)). The best known classical algorithm for factoring a number is exponential, while Shor's algorithm does it in time O((log N)^3) (allowing polynomial-time breaking of RSA).

Re:Umm...this is old news.

[beeplet]

This is the first time anyone has been able to use atoms (as opposed to photons) in quantum teleportation.

How to choose?

[Shambhu]

This wiki looks good, and if it isn't too technical, maybe I can find the answer. However, every other article, paper, or discussion that I have seen skips this one question of mine: How is the choice made between all the superimpositions to select ther 'right' answer? Everyone goes to great lengths to explain the superimposition part and its implications for massively parallel computation, but no one ever says how you choose the result! Does anyone have a clue about this?

Re:How to choose?

[ajayg]

Good question. In fact, this is one of the trickier problems to solve when coming up with a QC algorithm. The trick is, to use the phenomenon of coherent interference to yield the result that you are looking for. Interference here is basically the same as wave interference. So, after our QC executes an algorithm and finds the solution to a problem for all N inputs simultaneously, we then have to interfere our output result state (which now exists as a coherent superposition of N different outcomes) in such a way as to obtain the result we are looking for. A good example you might want to look up is the Deustch-Josza algorthm, which though useless for most practical purposes (in my opinion :-)), shows how we can use intereference in a smart way to obtain the desired result.

Re:How to choose?

[jfern]

A typical quantum algorithm puts most of the wavefunction into the state(s) that you want. By applying various quantum unitary gates repeatedly one can do this. It's kind of hard to explain exactly "why". One then measures the state, and with with probability p gets a correct answer. If p> 50%, one can repeat the algorithm a bunch of times to make sure one has the right answer.

A QM foray into the private lives of Alice and Bob

[wwest4]

Alice, instantaneously transfers information about the quantum state of a particle to a receiver called Bob. The uncertainty principle means that Alice cannot know the exact state of her particle. However, another feature of quantum mechanics called "entanglement" means that she can teleport the state to Bob.


Alice: Bob, now that our qubits are entangled, I don't know if mine's spin up down.

Bob: How 'bout I observe yours for you. How about there?

Alice: Nope.

Bob: Here?

Alice: Closer to this side of the gaussian, Bobby.

Bob: How about here?

Alice: OOOOOHHH! You collapsed my wave function DeBroglie!

Bob: Your qubit is now spin up, in case you were wondering... who's DeBroglie?

This...

[Cyno01]

Sounds more like the basis for instantanious comunication (read too much OSC). If we ever invented non reltivistic FTL or spread far enough that we'd need instantanious communication it would probably be based on this.

Re:This...

[Mr. Roadkill]

Sounds more like the basis for instantanious comunication (read too much OSC). If we ever invented non reltivistic FTL or spread far enough that we'd need instantanious communication it would probably be based on this.

Actually, even if we never develop FTL transportation, FTL communication could be very, very useful.

Find stars with earth-like planets, send probes containing quantum-entangled data comms gear and pretty well documented interfaces, and invite them to offworld their call centres to India.

Re:This...

[Too Much Noise]

Actually no. There are 2 steps to the process: the 'teleportation' one (collapsing the remote state) and the 'turtle' one (tell the other party what result you measured so that he can rotate his collapsed state to the right one). The second phase is the actual information transmission and it's slower-than-light. Also, you can't skip it by 'guessing', as the possible values for the collapsed state do not even form an orthogonal state[*]. Sorry.

[*] for 2-state particles (simplest case), the measurement of the unknown+transmitter system has 4 possible outcomes, so the receiver can be in one of 4 states in a 2d space => non orthogonal, there's no measurement that will preserve all of them simultaneously, hence there's no knowing whether you destroyed the state or not by only measuring the receiver.

Re:This...

[NonSequor]

Nope, doesn't work that way. In order to make this work you also need a classical (ie slower than light) communication channel. In quantum teleportation, one person interacts a qubit with one half of an entangled pair of qubits, performs a measurement, and then sends that measurement to the other person. The other person then performs an action on their half of the entangled pair that transforms it into the same quantum state as the original qubit. The original qubit is altered in this process and each entangled pair can only be used once.

A lot of people misunderstand the nature of entangled pairs as a result of the fact that many reporters do not understand how they work. An entangled pair is just a pair of qubits set up in a quantum state so that there is a 50% chance they will both be 0 and a 50% chance they will both be 1 (this is an oversimplification, but it's still better than how they are usually explained). If you measure one half of the pair, you automatically know what will happen when you measure the other one.

Not quantum computing, but

[achurch]

Can someone explain why this can't be used for FTL communication? The folks at Cornell seem pretty convinced 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?

Re:Not quantum computing, but

[wwest4]

Because Alice can't know the state of the information she's sending. If she does, then the superposition collapses.

It's not intuitive, but the "collapse of the wave function" metaphor fits observation.

Re:Not quantum computing, but

[jettoblack]

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?

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.

Furthermore, from the spaceship's viewpoint, how do you tell if your particle's state has changed due to an incoming transmission? The only way to know would be to observe it. But, we don't know if that particle had been observed by Earth yet. If it had, then we just disturbed the state that Earth had set. If it hadn't, then we just forced it (and Earth's particle) to a random state. True, the Earth's particle will now be set to the same random value, but random values are still uselss for communication.

For it to work, you'd need a second channel of information, which could transmit some kind of key to decoding the random states into data. Of course, this channel of information would have to go FTL too, so its a Catch-22...

Re:Not quantum computing, but

[achurch]

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.

Re:Not quantum computing, but

[jettoblack]

Once you observe either particle of an entangled pair, the entanglement ends and the state is fixed to a single possibility. You can't flip a particle back and forth and still observe changes to its former mate.

Need 3 particles

[miyako]

I am not a physicist, or a physics student, or even an arm chair physicist, but from what I understand, creating a quantum gate requires (at least?) 3 particle entanglement, which is quite a bit more difficult than 2 particle enganglement. Can anyone better versed in the subject confirm or refute this?

Ultimate Long Distance Communications

[Strenoth]

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.

[mcc]

Is the idea here basically just that this means that they'll be able to transmit information between qubits without the qubits having to be right next to each other?

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.

Spooky Action at a Distance

[www.fuckingdie.com]

What happens when quantum computers, which are able to use quantum teleportation, start to exert influence directly over the matter that makes up say a Human Brain for example. Or to make matters worse the brain accidentally starts to exert control over the computer.

"We are sorry - the application you were running has crashed because you were thinking unhappy thoughts."

or

"You have 60 seconds to close and save all thoughts before your brain will be automatically restarted"

Can we say sasser-"cranial edition"

Re:Umm...this is old news.

[jfern]

NMR quantum computing techniques have been done a few years ago, but most people think that they don't scale very well. The biggest experiment involved using 7 qubits to find the answer to the age old question: what are the factors of 15?

Stupid 2 minute rule.

Electrogravity

[Ceriel Nosforit]

If it is FTL communication, then we've stumbled into the area of electrogravity.
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.

Faster Than Light Communication (EPR)

[tal_mud]

This can not be used for faster than light communication. No "information" is exchanged in the "teleportation" it is just that one can "copy" a quantum mechanical state from one place to another, which of course is crucial for building quantum computers. For more explanation on the difference between entangelment and FTL communication see for example see a discussion of the EPR Paradox.

Re:Faster Than Light Communication (EPR)

[wass]

No "information" is exchanged in the "teleportation" it is just that one can "copy" a quantum mechanical state from one place to another

Not quite.

You're correct that quantum teleportation will transfer a quantum wavefunction from one point to another. But it cannot 'copy' the wavefunction. In order to send the wavefunction, the original wavefunction must be destroyed during the process.

Sorry, fanout is strictly prohibited in quantum computing.

A method to break Quantum Encryption?

[SkiifGeek]

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:A method to break Quantum Encryption?

[jfern]

Nope, there's a theorem called the no-cloning theorem that says that you can not copy an arbitrary quantum state. There's no way to start with a state |v> and get |v> |v>, which would mean I could perform destructive measurements on one |v> and be left with |v>.

This follows from 2 facts
1. Quantum measurements can be replaced by quantum gates
2. Quantum gates preserve the inner product of two states.

I want this stuff...

[Anonymous Coward]

Can you imagine playing Unreal Tournament at a ping of 0? and having a Inernetlink with and unlimited speed? [well depends on the put and get on the link ion] You could probably syncronize what ever you want in just a few s.

kindest regards,
mo

Re:I want this stuff...

[DigiShaman]

But what happens if you frag someone before you know it? Oh wait...that would be a good thing ;)

How do you measure spin?

[Komi]

I know this is slightly off topic, but what physically is spin, and how do you measure it? These experiments always talk about how this property called spin can be entangled with other particles.

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:How do you measure spin?

[jfern]

The following 3 things are equivalent
A qubit
The spin of an electron
The polarization of a photon

They are equivalent that they can each be representated by a 2 dimensional complex vector, where you don't care about the overall phase (and 0 isn't allowed).

Every played around with polarized lens filters? You have a horizontally polarized lens followed by a vertically polarized lengs, and no light goes through.

You add one that is polarized at 45 degrees, and suddenly 1/8th of your orginal light is going

You can think of your lenses as measuring your qubits (polarization of each of the photons), in different basises, and only letting the ones that were measured as a |0> through.

Re:How do you measure spin?

[jfern]

You want the Stern-Gerlach experiment You send the particle through a magnetic field, and then detect it on a photographic plate.

Argh!!! NOT teleport, NOT affects.

[elhedran]

Normally I am not so pedantic but the poster repeatedly misrepresented what is happening in entanglement.

4 times in the post it was said that the particles teleport or communicate, they don't.

Its more like the particles are using the same day planner to decide what to do next.

Think of it like to processes running the same code. if they have the same inputs, they will have the same outputs. It doesn't mean they communicate or teleport.

The reason it bugs me so much when people talk as if the particles interact after they have been entangled is it leads someone sooner or later to start asking why we can't use that to beat the speed of light for communication, or a dozen other things that have nothing to do with entanglement.

Re:Communication!

[Kiryat Malachi]

Except that because you can't control the transition that occurs, you still need a classical communications channel to communicate any actual information. Which is limited by lightspeed.

I love stuff about quantum computing!

[Anonymous Writer]

Too bad I can't bloody understand any of it!

change in properties of other determinable??

[little_prince]

"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." ---- Can it always be told beforehand (whether true for all cases) if the other will take identical or opposite properties? Is is controllable/determinable by us what properties the other will take? If A&B entangled and later C entangles with either of them, will it be considered that all three are entangled with each other? and if any property of A changes it will cause some change in properties of B and C to maintain the harmony b/w A,B&C to a state that was? (ought to be?) before the property of A changed? On the wondering scifi side, does all the discussion here, seem to point that parallel universes are possible?? One of the Futurama episodes deals with lots of weird parallel universe stuff (entire universe in a box stuff).

This is not good...

[Barkmullz]

Having scientist using words like "spooky" and "weird" cannot be a good thing...

Hate to spoil your fantasies

[jandersen]

Well, actually I don't, but that's another matter.

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:Answers anyone??

[Zaak]

Can someone solve our quarrel? Is he right and the only thing stopping FTL comms is they ability to consistently change spin? Or am I right in thinking quantum teleportation is just quantum entanglement over distance (seperate 2 particles, check one and infer the other's spin, nothing more)?

When two particles are in an entangled state, it means that an observation of one counts as an observation of the other as well. That can be interpreted as information traveling instantaneously from one particle to the other. Lots of people have gotten wacky ideas because of this. However, the information that "travels" between the particles is random, and cannot be used to send information. Bear in mind that it's not the change of spin that is communicated between the two. It's the measurement of the spin, and it only works once, and only if you've managed to maintain the entangled state while you separate the particles.

The unfortunate fact of the matter is that no known phenomenon can be used to transfer information from one place to another faster than light can travel between them. It's not a matter of technical hurdles that must be overcome. It's a matter of fundamental limitations in the way the universe works.

TTFN

Re:Thanks!

[g-to-the-o-to-the-g]

Rarely does such a great opportunity to karma whore occurr. If you can troll, then I can whore.

Re:Answers anyone??

[Too Much Noise]

1. make the entangled state
   
2. move your particle as far as you like - this is the information carrying process, as you carry the information about the state of the total system
   
3. make a measurement on your particle and 'know' the state of the other - this is just your prior knowledge of the total system; also, note that at this step you disentangled the system, so any further attempts to guess states at the other end are meaningless from either side
   

Re: Pipe Dream vs Reality

[Anonymous Coward]

I stand by my prediction that there will never be a quantum computer. It's just a pipe dream of being able to compute all possible combinations simultaneously. It's one of those things that's just not going to happen. Other examples of things that will never happen, no matter how bad we want them:

• Natalie Portman + grits
• Cold fusion
• Time travel
• Warp drive
• Linux on the desktop
• World peace
• SCO execs get jailtime
• Quantum computers
• Teleportation
• Viable "step 2" in the three step business plan

The Improbability Theorem states that all of the above statements can be expressed as "step 1" in the three step business plan. I'll leave the proof as an exercise for the reader.

Headline

[commodoresloat]

Am I the only one who read the headline and imagined this giant baby walking slowly toward a rack of computers?

Re:Headline

[yecrom2]

Yes.
Yes you were.

Quantum Window PCs?

[dcw3]

So does this mean that all the future Windows Quanta PCs will go blue screen at the same time?

I'm kidding...well, sorta.

How come it can't be used for communication ?

[master_p]

Others said that measurement of an entangled particle will make it loose its state (collapse of superposition), but how are we going to get information out of the quantum computer ? can we use the same way to successfully read the quantum state for communication ?

After all, transmission of information in a computer circuit is no different than communication.

Would this analogy work?

[mpn14tech]

I was trying to think in everyday terms why quantum entanglement seems so strange and came up with this. I am not sure if this is accurate so correct me if I am wrong.

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.

Voodoo

[Sparky66]

I can't wait for this.. Imagine, once scaled up, this will allow real Voodoo dolls to work! I can't wait to get one, and teleport jabs to my ex-wife.

Re:A QM foray into the private lives of Alice and

[EReidJ]

A quantum particle is speeding down the road in its car. A policeman pulls the particle over, gets out of his vehicle, and walks over to the quantum particle. The policeman asks:

"Do you have any idea how fast you were going?"

The particle replies,

"No, but I know exactly where I am!"

Ba-dah-bing!

Re:Help me understand this!

[maxwell demon]

1. If two particles are entangled, and you measure one... the other one instantly changes it's state. Once you have done this, can you measure either one of them AGAIN and produce another state change in both? Can you keep doing this without re-entangling them?

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.

2. If the answer to 1 is yes, then has anyone found a way to DE-ENTANGLE the particles?

Should be clear by now: You can detangle them by measuring them.

3. What happens if you take a particle that has been entangled with another particle, and try to entangle it with a third? Is this first entangement broken, or do you now have three entangled particles?

You have three entangled particles.

4. This is what's really been bugging me...

Let's say you entangle particle A and particle B.

If you cannot measure it without changing it's state, then how do you know that particle B's state changes when you change particle A's state?

In other words...

If have have two boxes... A and B, which have lids on them which are shut, and if I look in box A, and either a rubber duck, or a pineapple appears, how do I know that the contents of box B have changed? I cannot open box B to look at the contents beforehand to know when they change, because that would set the state of box A.

Furthermore...

If I cannot look in box B until after I have looked in box A, then how do I know that box B's contents have changed at all?

In other words...

If have have two boxes... A and B, which have lids on them which are shut, and if I look in box A, and either a rubber duck, or a pineapple appears, how do I know that the contents of box B have changed? I cannot open box B to look at the contents beforehand to know when they change, because that would set the state of box A.

Well, that's the complicated one. Does this help you?

Re:Help me understand this!

[pclminion]

If have have two boxes... A and B, which have lids on them which are shut, and if I look in box A, and either a rubber duck, or a pineapple appears, how do I know that the contents of box B have changed? I cannot open box B to look at the contents beforehand to know when they change, because that would set the state of box A.

This is confusing. You talk about things "changing" and looking in the box to see the "contents" beforehand. In the entangled state, the boxes have no "contents" to speak of, only superposed wavefunctions. By observing what is inside the box you collapse both the superposition and the entanglement.

You are asking, how can you know definitively that, before you open one of the boxes, there indeed exists an entangled superposition inside the boxes. You cannot know this. If you open a box to observe the contents, you will never observe a quantum superposition (that would be an absurdity -- it would cause your brain to enter a superposition as well. What the heck would that feel like?), you instead cause the objects to collapse to a well-defined state.

It makes no sense.

Quite right :-) But in some way, it's all connected with consciousness and observation. It seems like our consciousness is always in a well-defined state, and this "rubs off" on whatever we observe, causing any superpositions to collapse. And even if our brains did enter some kind of superposition, would we know it? Would we perceive the superposition, or would we be two superposed people, each observing what he thinks is a well-defined state?

These are questions we probably won't have answers for for a long, long time.

Re:Help me understand this!

[pclminion]

Conciousness has been proved by experiment to be UNNECESSARY in causing objects to collapse to a well-defined state.

I didn't mean to imply otherwise. It is the most obvious and well-known way of causing states to collapse.

I don't want ..

[cfuse]

I don't want a quantum computer as much as a quantum network card.

If the transmission distance is unlimited, I would set up a access point at home (connected to the net) and carry around my quantum networked device.

Even better would be to use this technique to communicate with space probes (ie. Mars rovers). No more waiting for data.