Making Cesium Atoms Do a Quantum Walk

An anonymous reader recommends an Ars Technica account of a breakthrough in efforts toward quantum computing. German scientists have managed to get cesium atoms in a state called a "quantum walk": basically a superposition of all the possible states of a particle. "Quantum walks were first proposed by physicist Richard Feynman and are, in terms of probability, the opposite of a random walk. A random walk might be modeled by a person flipping a coin, and for each flip he steps left for heads and right for tails. In this case, his most probable location is the center, with the probability distribution tapering off in either direction. A quantum walk involves the use of internal states and superpositions, and results in the hypothetical person 'exploring' every possible position simultaneously." In the abstract of the paper from Science (subscription needed for full-text access), the researchers say: "Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata."

Comments

Encryption plan

[XanC]

Do we have a plan for when one day, our current methods of encryption all become breakable at once?

Re:Encryption plan

[Killer Orca]

Do we have a plan for when one day, our current methods of encryption all become breakable at once?

What a wasted opportunity, your first post is supposed to say "First post, or is it?"; well I suppose you can always wait for the next quantum computing breakthrough.

Re:Encryption plan

[slickwillie]

It could just be a lot of quantum talk.

Re:Encryption plan

[Anonymous Coward]

Do we have a plan for when one day, our current methods of encryption all become breakable at once?

What a wasted opportunity, your first post is supposed to say "First post, or is it?"; well I suppose you can always wait for the next quantum computing breakthrough.

"3very p0st" would have been an acceptable alternative, in my opinion.

Re:Encryption plan

[fuzzyfuzzyfungus]

A massive cash advance drawn against every credit card in the world, and a castle made of pure unicorns in a country with flimsy extradition treaties?

That's my plan, anyway.

Re:Encryption plan

[TiberSeptm]

A working AND cost effective quantum computer capable of decrypting your pr0n is still a ways off.

If someone wants to spend that kind of money and resources to get you, then it doesn't matter what kind of decryption they have. If they can't ruin you by decrypting your secrets then they can just make something up. Fake compromising information is going to be the easier way to go for long enough that you shouldn't have to worry about it. I mean a planted local news story or thorough facebook+myspace+blogspot+whatever campaign calling someone a "pedophile" will drive them to hang themselves faster than stealing their identity anyways.

Re:Encryption plan

[XanC]

What about all the backup tapes or laptops that are stolen, but we don't worry about it because the data was encrypted?

Today we don't consider that data to have been compromised. But in the not-too-distant future it could be cracked instantly.

Re:Encryption plan

[fuzzyfuzzyfungus]

It's probably less about people "getting you"(I suspect that, today, relatively few people are actually being protected from a hostile superior power by the strength of their crypto) and more with things like the breakdown of electronic commerce security, the spoofability of cryptographic signatures(Goodbye SSL) and new difficulties in secure authentication(SSH would be about as useful as telnet).

If a superior power simply wishes to ruin you that is, as you say, typically easy without any codebreaking. People who don't have that kind of power, but would love to compromise your secrets, are markedly more common and crypto is pretty much what keeps them at bay right now.

Re:Encryption plan

[buchner.johannes]

I wouldn't worry. As long as there are NP problems that take extremely long unless you have a hint, we have encryption methods.

Re:Encryption plan

[TiberSeptm]

That, and the degree of effort required and risk is enough to make it worth their while. It's kind of like locking your car. If someone were to create a device that allowed someone to pass effortlessly through car windshields that doesn't automatically mean the device would be all that practically useful to a car thief. If it cost millions of dollars and required liquid nitrogen or helium cooling, then you're not going to see a rash of car thefts using said device, despite it appearance as a perfect tool for the job.

This is my point. Even the advent of a functioning and powerful quantum computer is not enough to warrant panic. The advent of a relatively cheap (even a couple million) and low-profile quantum computer would be something to worry about. Until these things can be used effectively by a couple guys in a warehouse, don't require a power footprint the size of a grow-house, don't require compressed low temperature gasses to run, and don't require quantum physicists to operate, you will not have to worry about people destroying the effectiveness of all encryption.

The impetus to actually use this sort of encryption cracking technology would have to either come from the desire to crack a particular high value encryption, or a scheme to pull in vast amounts of encrypted information to fish for things of value. Drudging through old laptops would be a ridiculous way to make something like that cost effective and you would have to be paranoid and delusional to think that someone would bother to bring such a complex and expensive apparatus to bear against YOUR old laptop.

So, like I said - until this is something that people sniffing wifi traffic at Panera bread or that Nigerian prince who emailed me the other day can afford and hide - it's not worth getting freaked out about.

Re:Encryption plan

[SEWilco]

His first post state would have collapsed had he observed it. It was only by acting without looking that he preserved the state.

Re:Encryption plan

[XanC]

But all the encrypted data in the wild that's considered safe because it's encrypted would have to be considered compromised, right?

Re:Encryption plan

[civilizedINTENSITY]

So we have another ten years left?

Re:Encryption plan

[trum4n]

Once they build, then harness this computer, we still have to wait 10 years for them to figure out how to program it without reading data, after all, once you observe the data, it will be changed. It's kinda like Safely Remove in Windows XP.

By using Plug and Play technology as it was intended, you have corrupted all your data. They invented a technology, and couldn't implement it, so it's useless, and some day they will figure it out.

Re:Encryption plan

[buchner.johannes]

Well, if you have used encryption for several years now, you probably made a move from 128bit over 512bit to 2064bit key size. For some encryption methods quantum computing will just be another step, but a really big one.
For others, quantum computing may "solve the decryption" directly by the different approach (superposition, probabilistic calculations).

Re:Encryption plan

[smaddox]

First of all, only public key encryption algorithms based on factoring would be broken. Others would still be strong (until a quantum algorithm was written to break them).

More importantly, properly applied one-time-pad encryption would still be unbreakable. I wouldn't be surprised if certain military/intelligence organisations were already using one-time-pad's that were distributed before missions (on DVD or HDD).

It is also worth noting that public-key encryption is already breakable at typical bit-strengths, if you are given enough time (all of the RSA challenge numbers have been factored). For certain applications timing is everything. For others, it is imperative that the code never be broken (thus one-time-pad's must be used). Quantum computers will still take a while to break a code, and it will be a looooong time (on a technological scale) before the average person has access to one. Thus, decrypting with a quantum computer will still take a significant amount of time, so for certain applications public key encryption will still rule. As quantum computers begin to permeate, more secure encryption will be necessary.

Finally, lets remember that quantum encryption is likely to be possible in the future. It is reasonable to assume that it will advance at a similar speed to quantum computers. Thus, strong encryption will still exist.

Re:Encryption plan

[bzipitidoo]

Don't start measuring your quantum particles just yet.

We don't know that a quantum computer will be able to break every encryption scheme we have. We have the famous open problem of whether P=NP. (I'd bet against.) For those who don't know, P is the set of all problems solvable in polynomial (that is, relatively quick) time, and NP is the set of all the problems solvable in polynomial time if only it was practical to try every possible solution in parallel, or there was some fast (polynomial time or better) means of eliminating a significant portion of any subset of less than optimal solutions. (Yes, P is a subset of NP.) Somewhere between P and NP is QP, the set of all problems solvable by a quantum computer in polynomial time. Maybe QP is equal to NP, we don't know. We know that some problems apparently not in P are in QP. The most notable of these problems is factoring. We also have problems we know are in NP but which we don't know are in QP, mainly because no one has come up with a quantum algorithm to do them. And also, no one has come up with a quantum algorithm for any of the problems known to be "NP-complete", that is, all the way at the NP end of the range between P and NP. If just one of those problems is solvable by a quantum computer in polynomial time, then they are all solvable and QP=NP, because NP-complete problems can all be converted to one another with algorithms in P. As far as we know, factoring is easier than NP-complete and harder than P. It's somewhere between P and NP, not all the way at one end. Of course if P=NP, then QP would be equal to both, and a quantum computer wouldn't be worth fooling with as it would be no more powerful than a classical computer. Even if a quantum computer can solve them all, because QP=NP and never mind P, almost certainly we can come up with new encryption schemes. Perhaps new methods will all have to exploit quantum effects to be secure, and perhaps not.

Even supposing QP=NP, we don't know that we can do quantum computing on a large enough scale to exploit this. I keep thinking it could prove impossible to manage more than a very few quantum bits. It may be physically impossible to handle more than a dozen for long enough to do useful work, and that could conceivably be a necessary consequence of QP=NP, which could be closely related to whatever reasons physicists may discover.

Or maybe it is possible to do significant quantum computation. And maybe a necessary consequence of that is that QP!=NP, in which case we would also know that P!=NP, and the exploratory users of the first significant quantum computer can collect the million dollar prize for solving that problem among the many other things they could do. Unless we should find a way to scale quantum computing up to arbitrarily large numbers of quantum bits for indefinite lengths of time, it will remain very difficult to explore these questions.

Re:Encryption plan

[sconeu]

If someone wants to spend that kind of money and resources to get you, then it doesn't matter what kind of decryption they have. If they can't ruin you by decrypting your secrets then they can just make something up.

Ob XKCD

Re:Encryption plan

[Kagura]

such as in an aluminum pole

Another festivus miracle!!

Re:Encryption plan

[plnix0]

One important consideration you omit is that the superior power can't destroy everyone because their power depends on most people either supporting them or being rather apathetic about them. So the super power desires the means to acquire enough information about everyone to decide whom to eliminate. Now that information is (effectively) not encrypted? Their job just got easier.

Re:Encryption plan

[CarpetShark]

I think the general plan is to lament how this could be possible, despite the fact that everyone ignored the possibility.

Re:Encryption plan

[Big Hairy Ian]

Simple Re-encrypt using the same quantum tech you are worried about.
The issue is more of a case of which country gets there 1st.
My money is on China

Re:Encryption plan

[tenco]

Quantum talk? Is this like small talk, only more fuzzy?

Re:Encryption plan

[Engine]

Not really. You set the initial state and read out the final results, which is in principle straightforward. The only thing you are not allowed to do is to try to measure the state in between.

Re:Encryption plan

[Engine]

Quantum key distribution is already available commercially, see for example:

http://www.idquantique.com/

Quantum computers do still have a very long way to go before they are useful for anything else than factorizing very small numbers. The last record I heard of was 15, which was already quite a while ago, but I find it unlikely that they have managed to do any significant improvements since then.

Re:Encryption plan

[Linker3000]

ROT13

Re:Encryption plan

[Linker3000]

No, you say every possible permutation of your sentences simultaneously and then when the other person hears this they instantly forget what they have heard.

Re:Encryption plan

[Hurricane78]

In a country? With that much cash, you can have your own country. With blackjack. And hookers!

Aaand a huge military, making the country survive in the first place.

Just be sure to transform all that money into gold. Because it will be worth shit, when nobody accepts it anymore.

Re:Encryption plan

[JSlope]

Try to google for Post-Quantum Cryptography, only Public Key Cryptography is in danger, traditional symmetric algorithms are not affected much by quantum computers. There are public key algorithms which might be resistant to quantum computing, but only time will tell for sure :)

Re:Encryption plan

[sjames]

What are you talking about, that's exactly what it said here. I guess you lost the quantum coin toss.

I'm not drunk, offischer. I'm doing a quantum walk

[wdef]

"And that geodesic is not shtraight either. Sho's I'll just superimpose my states back in da car and be on my way ..."

Reliability of Cesium

[BadAnalogyGuy]

Cesium is an interesting element in that it is perfectly reliable. While some elements will differ in atomic weight due to random changes in their electron sphere radii and the number of neutrons in the nucleus, Cesium has a perfect vibration rate independent of external stimuli. It is so regular and reliable, in fact, that we base our entire measurement of time on clocks composed purely of Cesium.

If, as is demonstrated here, Cesium can be used to explore multiple quantum states in a regular and reliable fashion, the possibility to build quantum computers and automata based on Cesium goes way up. Not only would these "computers" function better than our current computers, they would always be 100% perfect (unless Intel manufactures them, lol) and not prone to error or breakage.

Re:Reliability of Cesium

[Engine]

I do hope that no one is suggesting that the clocks are made purely by cesium. What they do is that they measure the hyperfine splitting frequency of cesium and calibrate their clocks to make sure that the frequency they measure is exactly the value they should get according to the definition of a second in terms of the cesium hyperfine splitting frequency.

Re:Reliability of Cesium

[Engine]

There is different isotopes of Cesium too, it is just that they have chosen one specific isotope for the measurements. In that regard Cesium isn't unique at all. I don't know what you mean with the "random changes in their electron sphere radii", but I don't see how Cesium would be different from other alkali elements in that regard.

The last part of your comment is just false. There are problems that quantum computers would be able to solve that you can't solve with any practical classical computer, but the a quantum computer would be a very sensitive device, and finding ways to get around the very high error rate in a QC is a very active area of research.

Re:Reliability of Cesium

[Hognoxious]

There is different isotopes of Cesium

Is one of them spelled with an a?

Re:Reliability of Cesium

[Engine]

If you are Brit, they all are.

Re:Reliability of Cesium

[Hognoxious]

Nope, we don't fill balloons with haelium.

Re:Reliability of Cesium

[Engine]

I know that you are deliberately obtuse, but helium isn't an isotope of cesium.

Quantum CPU extensions?

[DigiShaman]

As far as I know it, we have three main instruction sets. Integer, Floating Point, and Vector (SSE, MMX..etc). Would it more likely be that we would end up with the forth set being Quantum? Or, would it be possible to have an entire CPU quantum based?

Re:Quantum CPU extensions?

[BadAnalogyGuy]

To your first question: Yes. There would be a new instruction set called "Eigen". It would contain all possible values simultaneously. The interesting thing about such a value is that it could be used to determine the correct value of any problem simply by casting it to the appropriate data type. Since the other instruction sets can only contain a single value at any time, the correct value (for our universe) is automatically saved in the other data type.

For your other question: Yes and no.

Re:Quantum CPU extensions?

[DigiShaman]

For your other question: Yes and no

*smacks my forehead*

Of course!!! I would need a quantum CPU to get the correct answer because "Yes and No" are still undetermined. Now I understand ;)

Re:Quantum CPU extensions?

[Vectronic]

...the correct value (for our universe) is automat... 42

Re:Quantum CPU extensions?

[TiberSeptm]

It might be inaccurate to call any quantum computer an "entire CPU" even when it is the processor of interest in a given system. While they are currently more of an experiment that is being observed and manipulated with the aid of traditional computing devices and lasers, even when they are more refined they are more likely to fill the roll of a sort of co-processor. This is because although they theoretically do certain tasks far better than a traditional processor (or, in the case of integer factorization, they can actually do it at all) they don't look like they could be all that great at many of the tasks we normally expect a processor to do.

Even in a system where the primary task was something that quantum computers should be inherently great at, you would still need a powerful traditional processor to actually do anything. This really isn't all that different from the idea of the GPU based desktop supercomputers out there. The GPU's do what they're specialized to do much better than a traditional processor, but you still need to scale the computing power of the traditional CPUs in the system to actually utilize all that power. Similarly, a quantum computer would require a non-insignificant ammount of traditional processor power to keep the quantum processor busy and handle the output. It seems reasonable that specialized supercomputers will eventually consist of a mix of GPUs, traditional CPUs, some quantum processors, and possibly neuron-based processors in a mix determined by the task the supercomputer is designed to do. A super computer designed to model molecular interactions would clearly benefit from both high powered GPUs and quantum processors handling the tasks that they can handle far better than whatever traditional processors the computer used.

Re:Quantum CPU extensions?

[TiberSeptm]

With a quantum computer, the answer can be a superposition of yes and no states.

Re:Quantum CPU extensions?

[SEWilco]

answer = (string) ( eigen(! NULL) )

Re:Quantum CPU extensions?

[intx13]

As far as I know it, we have three main instruction sets. Integer, Floating Point, and Vector (SSE, MMX..etc). Would it more likely be that we would end up with the forth set being Quantum? Or, would it be possible to have an entire CPU quantum based?

Quantum computation is unlikely to replace classical computation. There are certain problems at which quantum computation excels (problems that involve period-finding in some way, shape, or form) and many problems that it doesn't excel at (anything else).

A quantum encryption co-processor is most likely the first way in which quantum computation will reach the classical computing world, and for physical reasons (you need an actual quantum communication channel to attach to) I wouldn't expect it on your Dell any time soon.

Re:Quantum CPU extensions?

[SEWilco]

Oops, I had the answer and forgot to print it. I should have created a debugged program. program = (debug) ( eigen( ! NULL ) ) run( program )

Re:Quantum CPU extensions?

[weicco]

What about maybe?

Re:Quantum CPU extensions?

[WarJolt]

Nope.

Re:Quantum CPU extensions?

[WarJolt]

It's yes and no. We determined it to be both simultaneously.

Re:Quantum CPU extensions?

[CarpetShark]

See here:

http://en.wikipedia.org/wiki/Quantum_computer

Re:Quantum CPU extensions?

[szo]

That's what superposition means, just less fancy :)

Re:Quantum CPU extensions?

[Skrynkelberg]

No. Maybe corresponds to a 1 probability of something being true. In a quantum computer, the probability of true is 1 and the probability of false is 1, thus the "yes and no".

Re:Quantum CPU extensions?

[szo]

No. Quantum or not, the sum of the probabilities of all possibilities still 1.

Re:Quantum CPU extensions?

[ioshhdflwuegfh]

As far as I know it, we have three main instruction sets. Integer, Floating Point, and Vector (SSE, MMX..etc). Would it more likely be that we would end up with the forth set being Quantum? Or, would it be possible to have an entire CPU quantum based?

Sure it would. Modern processors do things with several bits at once (like 32 or 64 bits integers, floats that you mention). Quantum computer calculates with several quantum-bits (so-called q-bits) at once, using their entanglement together with quantum evolution and a measurement on the evolved q-bits. This has nothing to do with some word Eigen that other posts are mentioning, because we can simulate quantum computers classically, so Eigen is not necessarily operation that only quantum computer does--we can actually calculate eigen values using classical computers just fine. Once built, quantum computer could be an additional unit in the computer system, like QPU (Quantum Processing Unit), similar to today's GPUs.

Re:Quantum CPU extensions?

[radtea]

That's what superposition means, just less fancy :)

Nope, and this is a good straight line for my futile quest to explain something about quantum weirdness, because it is precisely the difference between "maybe" and "superposition" that makes life interesting for a quantum mechanic.

"Maybe" is a classical concept. If we see a cat get into a box, and then there is a sudden yowling and howling from the box, and you ask me, "Is the cat ok?" and I reply, "Maybe" we are talking about a classical situation, in which the cat "really is" either OK or !OK. There are two possible states and classically they are mutually exclusive and jointly exhaustive, regardless of anything else we do to the system. We don't have to look at the cat or measure the cat, we know that it can only be "OK" or "!OK" (for some sufficiently crisp definition of "OK").

"Superposition" is a quantum concept. If a photon interacts with a double slit apparatus and you ask me, "Did it go through the left slit?" and I say, "Maybe" I've said something incoherent unless I quickly stick an apparatus for measuring which slit it went through into the photon's path, because until a measurement is made that distinguishes a photon that passed through the left slit from one that passed through the right, the photon is in a superposition of both states, which are still jointly exhaustive but no longer mutually exclusive, and there is no "fact of the matter" about which slit the photon "really" went through until we ask it with an appropriate apparatus.

The big question to me, which no one from Copenhagen to Consistent Histories or Decoherence answers, is why the classical world--that is, the world of human experience--arises from the quantum world at all. Which is to say, no one has ever answered Max Born's question, "WHY must I treat the measuring apparatus as classical? What will happen to me if I don't!?"

The standard interpretations all take for granted that there is a classical world in which superposition is unobservable, but this papers over the enormous ontological gap between the classical and quantum worlds. The classical world obeys Aristotelian limits on contradiction and causality and locality: a thing cannot both be and not be the same thing at the same time and in the same respect. The quantum world does not obey these limits: the photon can both be and not be a photon that has passed through the left slit, but the wavefunction pulls off some nonlocal legerdemain to clean up after itself when we try to catch it out.

Various interpretations make arguments about HOW this cleanup happens, but no one says anything about why the classical world exists at all: why we are unaware of all the "extra" components of wavefunctions floating around loose after a measurement has been made. Decoherence comes closest to an answer by simply declaring that interference phenomena are the only means by which we can be aware of these other components, but it still says nothing about why we are privileged to observe the effects of one component and not all the others, when the natural expectation would be that we would be that after a measurement had taken place we would be aware of the measurement apparatus as being in an incoherent superposition of orthogonal states.

Re:Quantum CPU extensions?

[radtea]

Man, I sure screwed up that last sentence, which should read:

"...when the natural expectation would be that after a measurement had taken place we would be aware of the measurement apparatus as being in an incoherent superposition of orthogonal states."

Re:Quantum CPU extensions?

[Octavarius]

"The interesting thing about such a value is that it could be used to determine the correct value of any problem simply by casting it to the appropriate data type."

This is incorrect. Determining the superposition's state won't give you the correct answer. It will give you a random answer from all of its possible states -- weighted by the chance of that being the right answer. This makes quantum computing much trickier.

http://scottaaronson.com/blog/?p=208 is a great article if you want to understand how some Quantum algorithms we know of will work using this "probable-answer" property.

Re:Quantum CPU extensions?

[Engine]

Your answer will never be a superposition. You prepare the initial steps into superpositions and then use superpositions in the calculations, but you will get a definite answer in the end. Of course you can run your computer many times and if your programming is such then you can get different answer for each run even if the inputs were the same.

Re:Quantum CPU extensions?

[AshtangiMan]

That's better. I was about to flame your ass for being such a moron.

Re:Quantum CPU extensions?

[radtea]

Nope. This is exactly the point I was trying to make: extremely short wavelengths explain why we don't observe interference phenomena. But they don't explain why we don't observe the cat as being in a superposition of ALIVE and DEAD.

That is, they don't explain why the world of experience differs from the quantum world, and this is the central question.

GIVEN that the only way we can detect the quantum world is via interference phenomena, then the really short wavelength of macroscopic objects explains why we don't detect interference phenomena.

But WHY is interference the only way we can detect quantum phenomena? Why don't we just perceive the damned wavefunction? I think there is an essentially anthropic answer to this, in the form of an anthropic metaphysics: we experience a limited aspect of reality because the very fact of having such experience requires such a limitation. Kant would approve.

Misunderstanding this, most likely

[gcnaddict]

Theoretically speaking, if we could get, say, an entire ship and all of its inhabitants to do this "quantum walk"...

wouldn't we be well on our way towards creating an improbability drive?


I'm probably hugely stretching this beyond what it means.

Re:Misunderstanding this, most likely

[intx13]

Theoretically speaking, if we could get, say, an entire ship and all of its inhabitants to do this "quantum walk"...

Ah, but you can't. Quantum mechanics applies only to quantum particles, not big honking spaceships. Of course nobody has integrated quantum mechanics with classical mechanics yet, so you never know ;)

The thing is, quantum mechanics is just a mathematical system that seems to work pretty well. As in, it predicts what really tiny things will do extremely well. When a quantum particle takes on different states at a time, that is a mathematical concept that, when applied, produces a result that agrees with what we actually see. It involves complex numbers, high dimensions (that don't necessarily agree with what we consider "dimensions") and other mathematical constructions. The math works great... but you have to be careful about extrapolating too far and assuming the math is the reality.

Re:Misunderstanding this, most likely

[shermo]

Quantum mechanics applies to large particles. Classical mechanics are merely an approximation of quantum mechanics when applied to large particles.

Wikipedia to the rescue

http://en.wikipedia.org/wiki/Correspondence_principle

Re:Misunderstanding this, most likely

[Anpheus]

From what I've read on the issue, such as Feynman's books and other novels targeted toward those of us who do not have a complete grasp of quantum mechanics, you are wrong.

Caveat emptor, this is merely what I've read:

Classical mechanics as explained by Feynman were the result of the sum of all possible histories, among other interpretations. Regardless of one's interpretation, Feynman and others found that as you crunch the math for larger and larger quantities of particles, the results closer and closer approximate what we think of as classical physics. As a result, classical physics is an approximation of quantum mechanics, which is a theory of how the universe really works.

Re:Misunderstanding this, most likely

[mindbrane]

If you download the 2009 intro to General Principles of Chemistry from the mit OpenCourseWare offerings you'll get some pretty good stuff on the relationship of Quantum Mechanics and Classical Physics. IIRC the wave descriptions of big league fast balls are used (lectures 4 & 5). I'll leave it there as any attempt by me to go into the particulars will go high and outside.

Re:Misunderstanding this, most likely

[3waygeek]

Quantum mechanics applies only to quantum particles, not big honking spaceships.

Not quite a spaceship, but quantum tunnelling has been shown to apply to non-quantum particles.

Re:Misunderstanding this, most likely

[Anonymous Coward]

Actually Quantum Mechanics applies to individual atoms regardless of size. Classical mechanics corresponds to the mathematical limits when the number of particles becomes large, i.e. you take the mathematical constructs of quantum mechanics and extrapolate to the number of particles being infinity and you come up with the mathematical construct for Classical Mechanics.

So Anpheus is correct. How do I know this? Well, I have a Ph.D. in Physical Chemistry.

Re:Misunderstanding this, most likely

[mea37]

...well, yes, until you get to really large (or fast) scales, at which point QM comes into direct conflict with relativity.

I've heard of recent attempts to reconcile the two, but last I heard there was still no universally accepted answer.

Re:Misunderstanding this, most likely

[JoCat]

If we had a quantum pirate ship we could make people walk the Planck.

Re:Most probable location is the center?

[buchner.johannes]

A random walk might be modeled by a person flipping a coin, and for each flip he steps left for heads and right for tails. In this case, his most probable location is the center

IIRC, the center is only a solution in 1D and 2D, not in higher dimensions (esp. 3D).

How to make German cesium atoms walk

[SEWilco]

"Left! Right! Left a wife and seventeen children in starving condition with nothing but gingerbread left! Left!"

Hey, babe, come for a quantum walk...

[vjoel]

with me? We can explore every possible position simultaneously.

Note to Ars Technica

[blueg3]

Please do not write further articles about quantum computing. This one was both factually inaccurate and unreadable. :P

Would P and NP

[gorrepati]

fall in the same class of problems w.r.t to Quantum computing now?

Re:Would P and NP

[JTeutenberg]

The answer is: we don't know. Noone's devised a polynomial time quantum algorithm for any of the NP-complete problems yet.

Re:Would P and NP

[AllureFX]

Whether polynomial time or not, "hypothetical person 'exploring' every possible position simultaneously" does sound a lot like nondeterministic TM.

In other news

[4D6963]

In other news, two Cesium atoms were shot dead by Crip gang members in East Los Angeles. Eyewitnesses report the two Cesium atoms were seen performing the Quantum Walk at the time of the shooting.

Note to Self:

[billybob_jcv]

Reading /. stories that include the phrase "...first proposed by physicist Richard Feynman..." make my head explode.

another awful, awful analogy

[plnix0]

Encryption is nothing like a car lock. The cheap thieves aren't the ones we have to worry about with regard to encryption. Governments in fact are quite willing to spend many billions of dollars for a device which can crack citizens' encryption in order to help them control those citizens more effectively. Combine that with the internet -- a vast system of networks through which large amounts of data are piped and which, unfortunately, happens to contain large several bottlenecks at which governments can strategically position scanners/loggers/databases, and you've got an internet with no privacy, no security, whatsoever. When the first organization acquires fully functional quantum computers, the rest of us lose encryption. At that point we're back to whispering in the woods and hiding secret messages in holes in the ground... if we can evade their surveillance networks.

or a scheme to pull in vast amounts of encrypted information to fish for things of value.

Too bad the entities with large sums of money are the same ones who want to sort through encrypted data looking for things of interest.

Quantum walk?

[Runaway1956]

At my age, I have trouble with a duck walk.

Who composed the music?

[Mogster]

I'm imagining something like a cross between Mancini's Baby Elephant Walk and the Hamster Dance

Full article

[Dice Fivefold]

You can get the full article from the arxive:

http://arxiv1.library.cornell.edu/abs/0907.1565

It is really a beautiful experiment. I have never seen such a demonstration of how deterministic the propagation of the wavefunction is. By simply running the experiment backwards they manage to get the atom to go back to it's initial position in the walk.

Obligatory Python

[jspenguin1]

lt's not particularly quantum, is it? I'm afraid that the Ministry of Quantum Walks is no longer getting the kind of support it needs. You see there's Defence, Social Security, Health, Housing, Education, Quantum Walks ... they're all supposed to get the same. But last year, the Government spent less on the Ministry of Quantum Walks than it did on National Defence!

Re:oi... stats 101 ftw

[Engine]

You are wrong. The most probably location is in the center. Sqrt(n) describes the spread of the end locations, i.e. corresponding to the standard deviation of the end locations.

Do people even need to take a stats course any more?

Re:Quantum Computing Crackpottery Marches On

[Engine]

D-Wave Systems is quite suspect and doesn't have much respect in the scientific community. On the other hand, Quantum Computing (QC) does rest on sound scientific principles, and in the quest for it we learn a lot. The gain if we succeed would be enormous. It is easy to get the impression that quantum computing only can be used for factorizing numbers, but the big gain would rather be in other fields of science, such as medicine and biology where we would use QC to simulate e.g. proteins.

The link you provided questions quantum physics in general. Quantum physics is extremely well tested theory that is the foundation for all modern science och technology. It's not that no one has tried, but it has proved very difficult to provide a competing theory that explains even a small part of the known phenomena on the atomic level.

Re:Quantum Computing Crackpottery Marches On

[Louis Savain]

The link you provided questions quantum physics in general.

Not it doesn't. It only questions the Copenhagen interpretation of quantum physics, especially the concept of superposition of quantum states. An interpretation is not a theory. It is just a guess. In this case, it is a very lame guess and silly on the face of it.

Re:Quantum Computing Crackpottery Marches On

[Engine]

Superposition of states is fundamental to quantum physics and not a part of the Copenhagen interpretation. The principle of superposition has been tested over and over again and is as far from a guess as you can come. Especially since it is counter-intuitive it has been scrutinized and tested more than most fundamental principles in science.

Quantum programming

[sjames]

When talking about quantum computing, don't forget that someone has to write the programs. If you thing programming in a SIMD (Single Instruction Multiple Data) is difficult, try SIID (Single Instruction, Infinite Data).

Also remember that there are a few REALLY hard problems to solve before we can have a quantum computer compute anything. For example, to factor a key, you have to have two 'registers' and somehow get them to be the superposition of all primes less than the key value. That is, all non-primes are illegal states. Now you have to have them entangled in such a way that they collapse down to the two particular large prime factors of the key. That is, you have to somehow make all non-solutions to the problem illegal quantum states.

Anyone care to even pie-in-the-sky speculate how you make a quantum state contingent on primeness? Or on being a factor of a specific value? How about entangling 32 qbits together such that their legal states correspond to binary representations of all of the values the register might be and no others?

One day, we may know the answers to those questions, but it's going to be quite a while. There's every reason to believe that quantum computation won't be a matter of writing a program for a quantum computer, but rather a breakthrough in research that allows you to purpose build a quantum computer that solves a particular problem set. IF (and it's a big IF) we don't run into a necessary computational operation that the universe won't support in the quantum world or that it won't support in a way that's easier or faster than computing sequentially on a conventional computer, then we may one day have general purpose quantum computing.

If that works out, whatever language we program them in, list comprehensions will be the most important construct.

Re:Quantum Computing Crackpottery Marches On

[Louis Savain]

This is BS and you know it. Superposition is certainly part of the Copenhagen interpretation. The hard irrefutable truth is that nobody has ever observed superposed states. The only thing that is tested is the probabilistic nature of quantum interactions. The entire concept of a wave function collapse is just silly guess work. One could just as easily say that the property has a given state but the state can instantly change when the particle interacts with another (during observation) in order to obey conservation principles.

The problem with quantum physicists is that they don't have a clue as to why nature is probabilistic and they don't even care to know why. And yet, in spite of this glaring lack of understanding, they feel free to come up with all sorts of cockamamie fairy tales to explain phenomena that they obviously don't understand. Worse, they believe in their own fairy tales. But not everybody is fooled. See you around.

Re:Quantum Computing Crackpottery Marches On

[Engine]

You are plain wrong on all accounts:

Superposition is not a part of only the Copenhagen interpretation. You obviously didn't read the Wikipedia article you link to. It doesn't even contain the word superposition.

No, no one has ever observed superposed states, since the wavefunction collapses as soon as anyone try to observe it. It is not silly guesswork. The brightest minds in modern time have failed to come up with any other explanation. Einstein was only one of them. You can not say "the property has a given state but the state can instantly change when the particle interacts with another". Read about Bell's inequality[wikipedia.org]. It has been experimentally proven that you are wrong.

We, quantum physicists, don't have a clue why the nature is not probabilistic, but we certainly care to know why. I was on a seminar yesterday were the discussion came up again for the nth time. We are though very good at describing and predicting the phenomena. It is amusing how everyone that have read or heard some popular account of quantum physics thinks they happen to be smarter than the best scientists of the last century.

The same people who think they aren't fooled still enjoy the fruits of quantum mechanics. They use the power from nuclear power plants, they use semiconductors and lasers every day. Things that wouldn't be possible without quantum mechanics.

Quantum mechanics is very counter intuitive. Quantum mechanics is hard. It takes effort and time to understand even for the gifted. It is just sad how many people that haven't put in the effort or don't have the capacity to understand it feel free ridicule it. As Bob Dylan sang: Don't criticize what you can't understand

Re:Quantum Computing Crackpottery Marches On

[Louis Savain]

What are you, a wise guy? Bell's inequality is about entangled particles and nonlocality. That has nothing to do with superposition of states. The Copenhagen interpretation has to do with the Schrodinger wave function, which is about superposition. You don't even understand the very theory you're arguing about.

The only reason that quantum mechanics is counterintuitive and hard is that physicists are clueless as to what is really going on. This should be a clue that current interpretations are wrong and should be either revised or abandoned.

Fuck off, you dumb lardass

[Hognoxious]

Instead of driving your 98 litre Hummer thirty yards to the burger joint three times today, why don't you stay in your trailer and learn to spell Aluminium properly?

Re:Fuck off, you dumb lardass

[Engine]

You don't even know how to troll properly. I'm Scandinavian, and the only reason I chose to spell Cesium in this way was because that was how it was spelled in the post. When in Rome... You have now not only proved your illiteracy, but also your stupidity by your unfounded conclusions, you inbred gnome.

Re:Quantum Computing Crackpottery Marches On

[Engine]

Bell's inequality says, loosely speaking, that if you have locality then you must have superpositions. Most physicists thinks we have locality, otherwise information could be transmitted faster than light. If information can be transmitted faster than light, we get problems with causality and then we are in a real mess. Then you could really start to talk about counterintuitive.

The Schroedinger equation is central to quantum physics. It describes how a state evolves with time. Again, it is not something that is specific to the Copenhagen interpretation.

What do you suggest that we should replace the principle of superposition with? How do you explain the double slit experiments without superpositions?

Re:Quantum Computing Crackpottery Marches On

[Louis Savain]

Sorry, nonlocality does not imply fater than light communication. Those who worry about faster than light travel simply do not understand the science of nonlocality. Nonlocality means nonspatiality, i.e., distance is an illusion. There is no transmission of information between two entangled particles. They are facets of the same coin. Nonspatiality should be a wake-up call to physicists, IMO. The paradigm shifting implications threaten to revolutionize physics. Thomas Kuhn comes to mind.

My entire point is that one does not do science by insisting that we abandon logic. Science is the result of applying logic to our observations and correcting the false assumptions in our models of nature until things make sense. Anything else is voodoo and superstition.

Voodoo science is what quantum computing scientists are doing, IMO. It's really chicken shit, on the face of it. Why? Because they have no understanding of the foundational issues.

(By the way, one does not need Bell's inequality to figure out that space is an illusion. Simple logic tells us that.)

Re:Quantum Computing Crackpottery Marches On

[Engine]

Quantum physics is very logical. It is just counter intuitive. That you can't understand it, doesn't mean it is illogical. Kuhn's paradigm shift did already happen. It happened with the advent of Quantum Physics. It's just that you haven't caught up yet.

I'm sorry, but you are obviously just a tiresome crackpot. You don't understand quantum physics. You don't understand superpositions or entanglement. You give up relativity and the notion of space to save what you call logic. Your post doesn't contain any substance, and I will not answer any more of them until they do.

How about the question I stated earlier: How do you explain the double slit experiments without superpositions? Your ideas aren't worth dust before you explain this simple experiment.

Re:Quantum Computing Crackpottery Marches On

[Louis Savain]

I'll tell you what. As soon as you explain why bodies fall I'll provide an explanation for the double slit experiment.

In the meantime, keep on kissing ass and see if I care. And since I seem to have been drawn into a pissing contest, fuck you too. How about that?