## Baby Steps Toward Quantum Computers 308 308

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)

## Re:Analogue vs Digital (Score:3, Funny)

## Re:Analogue vs Digital (Score:5, Informative)

Regards,

Steve

## Re:Analogue vs Digital (Score:4, Interesting)

## Re:Analogue vs Digital (Score:4, Informative)

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 (Score:3, Informative)

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 entangle

## Re:Analogue vs Digital (Score:3, Insightful)

You cannot measure

anythingwithout 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 me

## Re:Analogue vs Digital (Score:5, Informative)

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 (Score:2)

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.How do you know this for certain? If the NSA's scientists have cracked this you can be damn sure that they're not going to tell anyone about it.

HH

--

## Re:Analogue vs Digital (Score:5, Informative)

verifysome 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 [itworld.com] 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 [paperin.org]

## Re:Communication! (Score:3, Informative)

## 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 abo

## Re:Analogue vs Digital (Score:2, Insightful)

## Re:Analogue vs Digital (Score:3, Informative)

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 (Score:5, Insightful)

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 (Score:3, Informative)

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

## Re:Analogue vs Digital (Score:3, Insightful)

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 com

## Re:Analogue vs Digital (Score:5, Informative)

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: Pipe Dream vs Reality (Score:2, Funny)

## 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.

## Prime Intellect? (Score:4, Funny)

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

## Headline (Score:3, Funny)

## 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.

## Re:can someone qualified answer this question (Score:3, Insightful)

## Re:can someone qualified answer this question (Score:4, Funny)

"HELLO COMPUTER"

## Re:can someone qualified answer this question (Score:2)

What might this be a reference to?

## Re:can someone qualified answer this question (Score:4, Informative)

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 (Score:5, Informative)

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. [qubit.org] 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 (Score:2, Informative)

## Re:can someone qualified answer this question (Score:5, Informative)

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....

## ECC baby (Score:2)

## Teleportation (Score:2, Funny)

## Re:Teleportation (Score:2)

I see you, and raise you the videocassette, the home video camera, the DVD, e-commerce, and online payment systems.

The videocassette and DVD? Both were driven by porn (VCR more than DVD, as DVD was more an evolution - but porn is one of the few users of "multiple angles"). Seriously, look at the history. Home video cameras? Amateur porn!

E-commerce was basically invented by porn. Online payment systems, the same.

Like it or not, porn does tend to drive some technology's adoption into the

## Yes, fast (Score:3, Insightful)

## Re:Yes, fast (Score:5, Funny)

## Re:Yes, fast (Score:2, Funny)

"640k of memory is enough for anyone" - Gates

## Re:Yes, fast (Score:2, Funny)

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

wantit? Hell, I'dwantit! Who wouldn't?## Re:Yes, fast (Score:2)

## Re:Yes, fast (Score:5, Informative)

someapplications 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 [wikipedia.org] in time O(sqrt(N)). The best known classical algorithm for factoring a number is exponential, while Shor's algorithm [wikipedia.org] does it in time O((log N)^3) (allowing polynomial-time breaking of RSA).

## Right on (Score:2)

Still, this is good. A few more angstroms out of electronics means a few more decades of potential improvement.

## How to choose? (Score:2, Interesting)

## Re:How to choose? (Score:3, Informative)

## Re:How to choose? (Score:2)

## Re:How to choose? (Score:3, Informative)

## Re:How to choose? (Score:2)

Like all quantum computer algorithms, Shor's algorithm is probabilistic: it gives the correct answer with high probability, and the probability of failure can be decreased by repeating the algorithm.

And it runs in O((logN)^3) time. So not linear but sublinear unless my math is rustier than i thought.

## Re:How to choose? (Score:2)

And it runs in O((logN)^3) time. So not linear but sublinear unless my math is rustier than i thought.I'm not sure just what you mean by "sublinear," but a few experiments show me that f(x)=ln(x)^3 goes up slightly faster than x does. Could you explain what you mean?

## A QM foray into the private lives of Alice and Bob (Score:5, Funny)

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?

## Re:A QM foray into the private lives of Alice and (Score:3, Funny)

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

The particle replies,

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

Ba-dah-

bing!## This... (Score:3, Interesting)

## Re:This... (Score:2)

## Re:This... (Score:2, Funny)

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... (Score:3, Informative)

[*] for 2-state particles (simplest case), the measurement of th

## Re:This... (Score:3, Informative)

## 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

ifthe 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 (Score:5, Interesting)

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

## Re:Not quantum computing, but (Score:2)

## Re:Not quantum computing, but (Score:5, Informative)

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 (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

## Re:Not quantum computing, but (Score:2)

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?IANAQP, but my interpretation of their diagram: Basically, you start with your source particle S and destination particle D. The goal in quantum teleportation is to copy the exact state from S to D. They suggest doing this by creating an entangled pair, A and

## Re:Not quantum computing, but (Score:2)

We could take these with us somewhere, then perform the collisions and teleport states, thus communicating a limited amount of data back at FTL speeds.

Now I realise there are current limits on how long entanglement lasts, plus all sort of quantum error correction required, but essentially it *could* be feasible, no?

## Re:Answers anyone?? (Score:3, Informative)

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

## Re:Answers anyone?? (Score:3, Insightful)

this is the information carrying process, as you carry the information about the state of the total systemat this step you disentangled the system, so any further attempts to guess states at the other end are meaningless from either side## Re:Not quantum computing, but (Score:3, Informative)

## Need 3 particles (Score:3, Interesting)

## Re:Need 3 particles (Score:2)

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?Disclaimer: I'm a physics student, not a physicist. However, it is my poorly educated thought is that they could use a Fredkin Gate [arxiv.org].

Dogg

## 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 oth

## Re:Ultimate Long Distance Communications (Score:2)

However, it can be used in conjunction with a classical communication channel to provide uneavesdroppable encryption.

## 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.

## Re:I'm still confused by this. (Score:2)

If this whole thing of instantaneous communication seems odd, it should. While we are yet to find anything near a economica

## Spooky Action at a Distance (Score:3, Funny)

"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:Spooky Action at a Distance (Score:2)

## Re:Spooky Action at a Distance (Score:2)

## 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

## That means destiny would be a proven fact (Score:2)

## Faster Than Light Communication (EPR) (Score:2, Informative)

## Re:Faster Than Light Communication (EPR) (Score:3, Informative)

No "information" is exchanged in the "teleportation" it is just that one can "copy" a quantum mechanical state from one place to anotherNot 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? (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:A method to break Quantum Encryption? (Score:2, Informative)

This follows from 2 facts

1. Quantum measurements can be replaced by quantum gates

2. Quantum gates preserve the inner product of two states.

## Re:A method to break Quantum Encryption? (Score:2)

Within special relativity, causality can be preserved by forbidding information from travelling faster than the speed of light this does not mean A cannot communicate w/ B FTL but that no useful information to an outside party can be passed (i.e., 1 cannot

## I want this stuff... (Score:2, Funny)

kindest regards,

mo

## Re:I want this stuff... (Score:2, Funny)

## Thank you Einstein... (Score:2)

## 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:How do you measure spin? (Score:2, Insightful)

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 c

## Re:How do you measure spin? (Score:2)

## Re:How do you measure spin? (Score:2, Informative)

## Argh!!! NOT teleport, NOT affects. (Score:5, Informative)

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.

## I love stuff about quantum computing! (Score:3, Funny)

## change in properties of other determinable?? (Score:2, Interesting)

## This is not good... (Score:3, Funny)

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

## 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.

## Quantum Window PCs? (Score:4, Funny)

I'm kidding...well, sorta.

## 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.

## Re:Umm...this is old news. (Score:2)

## Re:Umm...this is old news. (Score:5, Informative)

atoms(as opposed to photons) in quantum teleportation.## Re:Umm...this is old news. (Score:2, Informative)

Stupid 2 minute rule.

## Re:Thanks! (Score:2, Funny)

karma whoreoccurr. If you can troll, then I can whore.## 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## Re:Help me understand this! (Score:4, Informative)

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

didenter 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! (Score:3, Funny)

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.