# Quantum Computing Not an Imminent Threat To Public Encryption

#### Soulskill posted more than 6 years ago | from the in-search-of-schrodinger's-catastrophe dept.

119Bruce Schneier's latest blog entry points out an interesting analysis of how quantum computing will affect public encryption. The author takes a look at some of the mathematics involved with using a quantum computer to run a factoring algorithm, and makes some reasonable assumptions about the technological constraints faced by the developers of the technology. He concludes that while quantum computing could be a threat to modern encryption, it is not the dire emergency some researchers suggest.

## Eat my shorts slashdot !! (-1, Offtopic)

## Anonymous Coward | more than 6 years ago | (#22836332)

Eat my shorts slashdot !!

## Re:Eat my shorts slashdot !! (0)

## Anonymous Coward | more than 6 years ago | (#22836590)

## Re:Eat my shorts slashdot !! (0)

## Anonymous Coward | more than 6 years ago | (#22838574)

http://imagecache2.allposters.com/images/pic/145/HR0148~The-Simpsons-Eat-My-Shorts-Posters.jpg [allposters.com]

hardeeharhar

## So how do you encript ..... (-1, Offtopic)

## 3seas (184403) | more than 6 years ago | (#22836336)

## Re:So how do you encript ..... (0)

## Anonymous Coward | more than 6 years ago | (#22836378)

## And the answer is ..Re:So how do you encript ..... (2, Funny)

## 3seas (184403) | more than 6 years ago | (#22836548)

The best encryption is disconnection. Its unbreakable even by quantum computers to the nth power.

the next best is perhaps a sequence of seemingly unrelated actions followed by a false positive... or other such use of seemingly unrelated data/actions.

A few might remember the seemingly different things you could do to cause a developer hidden message to come up on the Amiga.

## Re:And the answer is ..Re:So how do you encript .. (1)

## bh_doc (930270) | more than 6 years ago | (#22840900)

(No need to point out technicalities. I'm joking.)

## Schneier knows his stuff (3, Informative)

## CRCulver (715279) | more than 6 years ago | (#22836360)

Practical Cryptography [amazon.com]is a friendly guide to encryption that doesn't assume too much knowledge of the heady math involved. Plus, the man invented Blowfish, one of the most popular and fast algorithms around.## Re:Schneier knows his stuff (5, Informative)

## CRCulver (715279) | more than 6 years ago | (#22836376)

Applied Cryptography [amazon.com].Practical Cryptographyis a bit too basic an overview written with a co-author.## Re:Schneier knows his stuff (0)

## smilindog2000 (907665) | more than 6 years ago | (#22836656)

Quantum computers being able to factor big numbers is the best proof I've seen that factoring is not NP complete. If it were, we could just use these futuristic quantum computers (I'm talking far-future, many thousands or even millions of qbits) to solve just about anything. The darned things would be like oracles, just ask them any super hard question, like how to prove Fermat's Last Theorem, and they'd just spit out the answer. The things would be like talking directly to God. Is that even remotely possible? I don't think so. Factoring numbers is just not as hard as any NP complete problem.

## Re:Schneier knows his stuff (4, Insightful)

## gomiam (587421) | more than 6 years ago | (#22837264)

by using a deterministic algorithm. Quantum computers are non-deterministic, and that's why they can be used to factor large integers. "Check all periods of r so a^r=1 (mod N) at the same time" certainly isn't deterministic.The darned things would be like oracles, just ask them any super hard question, like how to prove Fermat's Last Theorem, and they'd just spit out the answer. The things would be like talking directly to God. Is that even remotely possible? I don't think so. Factoring numbers is just not as hard as any NP complete problem.You might as well conclude that grass is purple, for all the sense that paragraph makes.

## Re:Schneier knows his stuff (1)

## cnettel (836611) | more than 6 years ago | (#22837544)

## Re:Schneier knows his stuff (2, Informative)

## smilindog2000 (907665) | more than 6 years ago | (#22837724)

There's no evidence to date that a quantum computer can be used to solve NP complete problems, and most people seem to think factoring is in a class in between P and NP-complete [scottaaronson.com] .

## Re:Schneier knows his stuff (2, Interesting)

## CreateWindowEx (630955) | more than 6 years ago | (#22838240)

While solving NP-complete in P time with a quantum computer sounds insane, I still find even the factoring thing pretty creepy. I suppose it is really just bringing in all the crazy truths of quantum mechanics up into the human scale where we are forced to deal with their strangeness directly. I still harbor a secret hope that someone will discover that a quantum computer needs energy proportional to 2^N (N = number of qubits), thus making them no more powerful than a classical computer. I have zero evidence to back this up, though!

## Re:Schneier knows his stuff (3, Informative)

## Fëanáro (130986) | more than 6 years ago | (#22837784)

What you described is the property "NP-hard".

For a problem to qualify as NP-complete, it is also neccessary that an algorithm that can solve this problem can also be used to solve every other NP-hard problem, with only an additional transformation of its input and output in polynomial time.

Prime factoring is not NP-complete. There is as far as I know no transformation for the input and output of a prime-factoring algorithm, that would allow it to solve other np-hard problems as well.

If prime factoring was np-complete, then since a quantum algorithm is known for it, it would be certain that a quantum computer could also solve all other np-hard problems.

As far as I know, no quantum algorithm with polynomial time has been found for any NP-complete problem. So we do not know whether a quantum computer could do this

## Factoring IS NOT NP COMPLETE (2, Informative)

## wurp (51446) | more than 6 years ago | (#22839202)

Read Scott Aaronson's blog [scottaaronson.com] to get a clue about quantum computing.

Also read about Schor's algorithm, which is the known algorithm to factor large numbers in log(n) time *if your quantum computer has enough entangled qubits to represent the number*. Again, though, remember that FACTORING IS NOT NP COMPLETE, only NP hard. Other NP hard problems are harder than factoring (for example, any NP complete problem

Also read about Grover's algorithm, which is a general algorithm to solve NP complete problems, and which HAS BEEN PROVEN TO BE THE FASTEST way to solve the NP complete problem of lookup in an unordered dictionary. Grover's algorithm finds the answer in n^1/2. Obviously if the fastest algorithm to solve a specific NP complete problem is n^1/2, you cannot have a way to solve all NP complete problems in log(n).

Look up Grover's algorithm and Schor's algorithm on Wikipedia, and you'll see that the GP is speaking beyond his knowledge.

I should say that I also made the mistake of thinking factoring was NP complete, and made a fool of myself on Scott's blog before the many, many people more knowledgeable than I about QC on that forum corrected me.

## Re:Schneier knows his stuff (1)

## smilindog2000 (907665) | more than 6 years ago | (#22837838)

## Re:Schneier knows his stuff (1)

## gomiam (587421) | more than 6 years ago | (#22838430)

I consider this a contradiction... the existence of such a computer violates my sense of reality. It is far easier to believe that factoring is indeed not quite as hard as NP-complete problems.Got a couple of spares? Your sense of reality seems to be a bit faulty. We already have seen those cut-offs before: they happened when someone decided a computer was a good enough tool to design chips instead of using pencil and paper. Anyway, this is not a mater of something being "easier to believe": either factoring is NP-complete, it isn't, or we can't decide. There is no belief required. Deciding that, because we don't yet know whether quantum computers can factor integers in polynomial time or not, they can't and quantum computer doesn't work is ... let's say illogical. Then again, if you are happy jumping to conclusions like that...

Turning back to the debate at hand, if factoring isn't NP-complete, does that mean it is harder than NP-complete problems, easier or what?

## Re:Schneier knows his stuff (1)

## smallfries (601545) | more than 6 years ago | (#22838438)

It's not hard to follow; if I give you a candidate solution to a problem and you can check quickly if it is true or not (i.e. in poly-time) then the problem lies in NP. So all problems in P lie within NP. Some problems in NP are harder than others i.e. 3-sat. If any single one of those problems can be reduced to the problem being discussed (in poly-time) then then it is complete within NP - it is one of the hardest problems in NP.

The poster than you were replying to was pointing out (in terms that you didn't understand, I suggest you read his reply closely) to you that the difficulty of factoring is not related to the difficulty of NPc problems. He is entirely correct, not only is there no theoretical basis for relating the hardness of the problem but if you have spent any time working on NPc problems, and on factoring you would understand that they are very different.

Finally, quantum computers cannot (currently) factor large integers. It is silly for you to make that claim in a discussion about whether or not QC would ever scale to large problems at all. Also the integers that they can factor are not a direct result of their non-determinism. Don't forget that we can run non-deterministic algorithms on devices other than QCs.

## Re:Schneier knows his stuff (1)

## randomplanck (1069958) | more than 6 years ago | (#22839052)

## Re:Schneier knows his stuff (0)

## Anonymous Coward | more than 6 years ago | (#22839208)

Why? Simple. If factoring were NP-Hard that would imply that NP is in BQP. This is strongly suspected to not be the case. I could point you to a whole myriad of sources. It just happens that there is a really easy to read article on this subject in this month's Sci Am. I suggest you pick it up.

Sincerely,

An Anonymous Coward with a PhD in Quantum Computation

## Re:Schneier knows his stuff (0)

## Anonymous Coward | more than 6 years ago | (#22836492)

Thank goodness for laments terms.

## Re:Schneier knows his stuff (2, Insightful)

## insertwackynamehere (891357) | more than 6 years ago | (#22836568)

## Re:Schneier knows his stuff (5, Funny)

## rucs_hack (784150) | more than 6 years ago | (#22836618)

Hope that clears it up for you...

## Re:Schneier knows his stuff (1)

## gomiam (587421) | more than 6 years ago | (#22837292)

There is also, if I read this correctly, some chance that it will turn into a cat.But you won't know if it's alive or not until you look inside the box.

## Brilliant explanation... (1)

## Joce640k (829181) | more than 6 years ago | (#22838492)

## Re:Schneier knows his stuff (1)

## insertwackynamehere (891357) | more than 6 years ago | (#22838506)

## Re:Schneier knows his stuff (1)

## db32 (862117) | more than 6 years ago | (#22839264)

## Re:Schneier knows his stuff (0)

## Anonymous Coward | more than 6 years ago | (#22838340)

## Re:Schneier knows his stuff (1)

## insertwackynamehere (891357) | more than 6 years ago | (#22838516)

## Re:Schneier knows his stuff (1, Informative)

## Anonymous Coward | more than 6 years ago | (#22836610)

The name "quantum gate" is quite misleading, and that may be the reason why Mordakus mis-understand. It's actually means quantum operation. Shor's algorithm requires 72k^3 quantum gates, is means requites 72k^3 quantum operations. It does not means we need to build a quantum computer with 72k^3 gates. That more like you build one CPU, one piece of hardware that execute one program.

What is the technical challenge is, we need to be able to build quantum computer that big enough to hold "k" qubits, and technique that apply "quantum gate" (meaning executing quantum operation). So, the Quantum CPU may

not need 5 Trillion qubits, but much be able to apply quantum operation on k qubits. The important point is Shor's algorithm is O(k^3), a polinomail time algorithm, which is fast!

## Re:Schneier knows his stuff (5, Interesting)

## letsief (1053922) | more than 6 years ago | (#22836614)

Right now a lot of people working in the field say quantum computers are about 40 years off. The scary thing though is how its likely to play out. For a few decades quantum computers will likely remain "40 years off" (in the fusion sense), but then someone is going to figure out how to get the error rates below threshold, and then quantum computers will be only 10 years away. That doesn't give us much time to stop using our favorite public key algorithms. That's too bad for nTru; (they have a public key system that is likely resistant to quantum computers), their patents will be long expired.

## Re:Schneier knows his stuff (0)

## Anonymous Coward | more than 6 years ago | (#22837150)

Ignore CRCulver, he's a keyword-triggered state machine that fires Amazon links without thought or judgement.

## Re:Schneier knows his stuff (2, Informative)

## SiliconEntity (448450) | more than 6 years ago | (#22838342)

## Re:Schneier knows his stuff (1)

## wwphx (225607) | more than 6 years ago | (#22836992)

## Re:Schneier knows his stuff (1)

## DaleGlass (1068434) | more than 6 years ago | (#22837020)

## Re:Schneier knows his stuff (0)

## Anonymous Coward | more than 6 years ago | (#22837736)

Out of curiosity, what do you think is wrong with Schneier's work in security consulting?If he doesn't reply it's most likely because he couldn't figure out how to work an Amazon referrer link into it.

## Re:Schneier knows his stuff (1)

## 0ptix (649734) | more than 6 years ago | (#22839664)

The main exception are his FSE (fast software encryption) papers but here too it's since 2000 he's only produced 1 paper and that had 6 authors. However before that (in the 90s) he did have a string of relatively interesting papers related to cryptanalysis mainly together with John Kelsey who is a much more prolific cryptanalyst even in recent years. For example he has 5 papers in Eurocrypt and Crypto. As a measure of FSE vs. Crypto also for the field of cryptanalysis, the major MD4, MD5, SHA-0 and SHA-1 breaks were all published at Crypto.

Basically what i'm saying is that he is great in security and even respectable in the most practical corner of crypto. But there are many many people out there who have a much more impressive track record as far as pure crypto is concerned.

I guess the best testament to how seriously some people out there take him would be this collection of useful facts [geekz.co.uk] .

## not exactly a "threat" (3, Insightful)

## pedantic bore (740196) | more than 6 years ago | (#22836362)

The day PKIs that use factoring or discrete logs become easy to crack is the day when there's going to be a lot of tremendous amount of money spent on stop-gap security measures until someone figures out something new...

## Re:not exactly a "threat" (1)

## CRCulver (715279) | more than 6 years ago | (#22836410)

I imagine one-time pads will come back in style.

## Re:not exactly a "threat" (3, Insightful)

## owlstead (636356) | more than 6 years ago | (#22836530)

I imagine one-time pads will come back in style.

## Re:not exactly a "threat" (3, Informative)

## letsief (1053922) | more than 6 years ago | (#22836740)

## Re:not exactly a "threat" (1)

## MMC Monster (602931) | more than 6 years ago | (#22836918)

## Re:not exactly a "threat" (1)

## kalidasa (577403) | more than 6 years ago | (#22836962)

## Re:not exactly a "threat" (2, Insightful)

## owlstead (636356) | more than 6 years ago | (#22837056)

Well, just think about all the SSL enabled sites out there, and remember you will now have a N * N (client * server) number of relations that need to setup a symmetric key (which is what a one time pad is, basically). Also note that you don't have a certificate infrastructure, so you cannot just go to VeriSign or any other trusted third party and buy a certificate from there. You cannot download one time pads, because you don't know who it is coming from.

Say you have 200.000.000 client computers in the US alone, and some 5.000 sites with one time pad SSL. Then you have 1.000.000.000.000 relations to set up. Not quite as much as the number of dollars wasted on the war in Iraq, but it's getting there. Of course, you won't use each and every service, but it would take a little bit too much time to exchange disks by post when you are trying to connect to a new service.

## Re:not exactly a "threat" (2, Informative)

## blueg3 (192743) | more than 6 years ago | (#22837546)

One-time pads cannot be reused. That's why they're called one-time. This means that the quantity of one-time pad data that needs to be securely shared between two communicating parties is equal to the quantity of data they want to exchange. Say a bank transaction involves communicating 1k of data. The bank needs to give you -- securely -- 1k of OTP data per transaction you'll want to make. Generally, this is inconvenient. Since real one-time pads are unbreakable, this just means the vector for attack is moved somewhere else -- most likely, how you communicate the OTP data. (If someone can impersonate you to a bank employee and get a few transactions' worth of pad, the security of a one-time pad is irrelevant. If you use an existing cryptographic algorithm to exchange one-time pad data, you're wasting time -- simply transmit your real data using this algorithm.) One-time pads have been used, but only in a few specific situations. In general, they're not useful.

Applying a one-time pad to data is generally done by simply XORing the pad (which consists of random bits) with the data. Reusing a one-time pad is incredibly easy to break.

## Re:not exactly a "threat" (1, Informative)

## TheRaven64 (641858) | more than 6 years ago | (#22837502)

## Re:not exactly a "threat" (1)

## pyite (140350) | more than 6 years ago | (#22837598)

## You have to get the pads to the other person... (2, Insightful)

## Joce640k (829181) | more than 6 years ago | (#22838522)

If you have a way of transmitting the pads securely then you could just use the same system to transmit the messages - no encryption needed!

## Secure asymmetric encryption? (1)

## wurp (51446) | more than 6 years ago | (#22839254)

## Re:not exactly a "threat" (2, Informative)

## mattpalmer1086 (707360) | more than 6 years ago | (#22836696)

## Re:not exactly a "threat" (1, Informative)

## Anonymous Coward | more than 6 years ago | (#22836682)

http://www.iacr.org/archive/crypto2000/18800147/18800147.pdf [iacr.org]

## Well, lucky for us (1)

## Watson Ladd (955755) | more than 6 years ago | (#22836436)

## Re:Well, lucky for us (2, Interesting)

## snarkh (118018) | more than 6 years ago | (#22836726)

As far as I know, it is not known whether quantum computers can solve NP-hard problems in polynomial time. To say that they fail at NP-problems may be premature.

## Re:Well, lucky for us (5, Informative)

## russotto (537200) | more than 6 years ago | (#22836786)

## Re:Well, lucky for us (1)

## snarkh (118018) | more than 6 years ago | (#22837458)

Well, it is conceivable that NP is solvable using quantum computers in poly time, while pretty much everyone believes that P!=NP for ordinary computors.

## Re:Well, lucky for us (1)

## Yossarian45793 (617611) | more than 6 years ago | (#22837642)

## Re:Well, lucky for us (1)

## 26199 (577806) | more than 6 years ago | (#22838006)

If quantum computers can't solve problems in NP quickly, then presumably it follows that normal computers can't either.

This would prove P != NP, which hasn't been done. Ergo, it can't have been proven that quantum computers can't solve problems in NP quickly.

## Re:Well, lucky for us (0)

## Anonymous Coward | more than 6 years ago | (#22837842)

## Re:Well, lucky for us (1)

## Agar (105254) | more than 6 years ago | (#22841146)

Thanks for the laugh.

## Re:Well, lucky for us (2, Interesting)

## Watson Ladd (955755) | more than 6 years ago | (#22836892)

## Re:Well, lucky for us (1)

## snarkh (118018) | more than 6 years ago | (#22837486)

I am not sure what you mean by a non-linear operator. Certainly there are numerous ways of approximating various NP problems with different degree of accuracy.

## There's nTru (1)

## letsief (1053922) | more than 6 years ago | (#22836774)

## dongs (0)

## Anonymous Coward | more than 6 years ago | (#22836466)

Those based around multivariate systems of equations (e.g. HFE), coding theory (McEliece and friends) and other esoteric stuff like lattice based systems (NTRU) are pretty much immune to quantum approaches.

It's just a matter of changing the whole infrastructure to something else, not the end of the world.

## Re:dongs (2, Insightful)

## CRCulver (715279) | more than 6 years ago | (#22836534)

Yes, but what about the matters you've already encrypted? Early buzz around PGP was that it would supposedly take millions of years to decrypt even a single message. For those people who encrypted their private communications, it is a big shock that all your secrets may be read in just a few years.

## the fear (1, Insightful)

## Deanalator (806515) | more than 6 years ago | (#22836494)

## "polynomial time" (3, Informative)

## l2718 (514756) | more than 6 years ago | (#22836566)

## Re:"polynomial time" (1, Informative)

## Anonymous Coward | more than 6 years ago | (#22836932)

## Quantum computing is no threat because... (2, Informative)

## SIGFPE (97527) | more than 6 years ago | (#22836586)

## Re:Quantum computing is no threat because... (2, Interesting)

## letsief (1053922) | more than 6 years ago | (#22836676)

## Re:Quantum computing is no threat because... (2, Informative)

## ScrewMaster (602015) | more than 6 years ago | (#22837222)

We may never have either nuclear fusion or quantum computing, as currently envisioned. As you say, it's impossible to predict. All we can say with some assurance is that we'll probably figure out

somethingthat will achieve the same ends.## Re:Quantum computing is no threat because... (1)

## SIGFPE (97527) | more than 6 years ago | (#22838864)

## What's all this k**3 stuff? (0)

## Anonymous Coward | more than 6 years ago | (#22836624)

If you built a multiplyer out of magic logic,

and constrained the output and enough of the input bits so that there is only one solution,

then maybe the magic logic could figure out the rest of the multiplyer state.

I don't know if quantum logic can/will be the magic logic,

but as long as we choose algorithms with only one solution,

we will have to worry about it.

## Encryption will move on, too (2, Interesting)

## tringtring (1227356) | more than 6 years ago | (#22836692)

## Re:Encryption will move on, too (1)

## sempernoctis (1229258) | more than 6 years ago | (#22837192)

## Does exist any quantum computer proven to work? (1)

## a_claudiu (814111) | more than 6 years ago | (#22836858)

1. Entanglement. Is this a fact or a theory? Looking on web I found only few experiments with some possible loopholes. I found the principle hard to grok.

2. Heisenberg principle. It mainly states that observing an object you are changing the state of the object. The Heisenberg example from wikipedia is using a photon for measuring the position of an electron and the photon is changing the position of electron. What is happening if you are using a smaller particle that is not impacting the electron so much? Are you going to change the constant? Looks mostly like a limitation from a time when the atom was considered to be indivisible.

I have some background in learning physics in university but I was not very good at it. My personal opinion is that superposition is just a nice way of doing statistics using discrete values for covering the not so discrete results of experiments.

## Re:Does exist any quantum computer proven to work? (1)

## blueg3 (192743) | more than 6 years ago | (#22837602)

To make a long story short, though, both quantum entanglement and the collapse of wavefunctions due to measurement are experimentally-confirmed fact, and small quantum computers have been built.

Your personal opinion sounds like the opinion of many physicists in early quantum mechanics days. It's called a hidden variable, meaning that there is a deterministic process going on that we can't get at, and that superposition is just a convenient statistical model. This was more or less summarized in the EPR paradox. The answer to this was Bell's theorem, which showed, again cutting a long story short, that whether superposition and entanglement were "real" or just hiding some deterministic hidden variable could be determined experimentally. This has been done, and superposition and entanglement are real. (Though Einstein would be happy to know that despite the apparent "spooky action-at-a-distance" of entangled particles, they cannot be used to transmit information faster than lightspeed.)

## Re:Does exist any quantum computer proven to work? (4, Informative)

## slew (2918) | more than 6 years ago | (#22837628)

Although QM computers do use basic entanglement for creating superpositions, understanding Shor's algorithm (the one everyone is concerned about since it's factoring in polynomial time) is mostly just understanding QM superposition. Entanglement gives generic QM computers great parallel processing power by superposition by explaining how QM probability wave combine under superposition, but Heisenberg limits the computing power of a QM computer in a non-trivial way as well because after you collapse the wave functions by measurement you give up the parallel processing enabled by Entanglement (e.g., if you peek inside the oven, it stops working, if some of the heat leaks out of the oven even with the door closed, it doesn't work as efficiently, the oven being the QM computer).

FWIW, Shor's algorithm essentially converts factoring into a sequence period finding exercise. You might imagine that that's something easy to do if you had a machine which given a bunch of superimposed waves with a certain modulo structure could tell you the period (hint the ones that don't modulo with a specific period with self interfere and measure as zero, where the one with that period with self-reinforce). With a QM computer you do this all in parallel with superimposed probability waves and when you measure it, the highest probability one you measure is the one that doesn't self-interfere (the ones that self-interfere has probability near zero). Basically this measurement is wave function collapse which doesn't actually depend on entanglement or heisenberg to understand (although it does require you to believe in QM wave functions and measurement operators).

Entanglement is really a strange artifact of QM that explains probability correlations that you see in QM experiments that can't be explained classically. It's really more of an artifact of the existance of probability amplitude waves (the QM wave function) rather than an effect that directly enables the QM computer. Of course if you didn't have QM wave functions you wouldn't have a QM computer so I guess that's a chicken and egg scenario. Entanglement is like the "carburator" function of the QM computer. The QM computer uses superposition of QM wave functions to work and when you have more than one QM wave function, they get entangled when you start superimposing wave functions and the way the waves entangle helps you compute in parallel so it's important to understand how these waves entangle.

Heisenberg's principle is a consequence of wave function collapse (measurement) which also limits the QM computer (this limiting effect is often called QM de-coherence). Heisenberg isn't required by a QM computer when it's computing, but you need to see the result somehow so when you measure the result, one of the side effects is the Heisenberg principle (although that's also a chicken-egg problem, since HP is a consequence of QM wave function collapse and w/o QM there's no superposition computing). The closest explanation I can think of is that Heisenberg's principle is the "heat" caused by friction of the QM computer. You need friction to stop the computer to read out the result, but at the same time you can't get rid of a little friction while it's running either (causing de-coherence). The side effect of this friction is heat.

You may have a personal opinion that superposition is a "nice way of doing statistics using discrete values for covering the not so discrete results of experiments", but there is experimental evidence that your personal opinions is at odds with physical reality. As QM computers that do QM computing (including IBM's NMR experiment which implemented shor's algorithm) have already been implemented it's hard to refute that something non-classical is going on.

It may be that in the end, QM is total malarky and there's some other weird unexpected thing going on, but there are mountains of evidence that whatever is going on, it isn't as simple as "hidden variables" or "continuous vs discrete".

As with most physics, Entanglement and Heisenberg' principles were originally artifacts of the new QM theory that people thought would be novel (i.e., not able to be explained with the current classical theory). They have however both been experimentally verified to some extent, but generally the verification is statistical in nature and is definitly shown to be non-classical behavior. Like I said earlier, it's possible that something weird and not QM is going on, but it's definitely something.

Today we are interpreting the experimental evidence as being consistant with QM theory which is not consistant with classical theory. Tommorrow, who knows. Science is always about new theories, but it's also a fact that the classical theory doesn't predict what we observe today, so we start with QM as the new starting point.

Don't feel bad about not being able to grok QM. Even Einstien and Feynman basically gave up trying to actually understand or find it intuitive at all and just treated QM as a mathematical theory that happened to explain experimental results.

## Re:Does exist any quantum computer proven to work? (1)

## a_claudiu (814111) | more than 6 years ago | (#22838434)

My main problem stays in "continuous vs discrete" problem as you mention it. I still believe that the discrete values are just "good values" in a wave equation like "Schrödinger equation" and not really discrete. The main problem as I see it is that "Schrödinger equation" is practically unsolvable using current mathematics and finding a continuous equation is like finding an inverse function of an unknown distribution.

Of course as somebody mention it I'm just a "layperson" but I'm trying to improve.

Thanks again for your responses.

## Re:Does exist any quantum computer proven to work? (3, Informative)

## Phroon (820247) | more than 6 years ago | (#22837690)

1. Entanglement: It is fact. If you send a photon through a certain type of non-linear crystal, two photons will emerge that are entangled quantum mechanically. To truly understand this requires some knowledge of quantum mechanics, a basic introduction to QM and entanglement can be found here [ipod.org.uk] and here [davidjarvis.ca] if you care to learn more.

2. Heisenberg principle: You inadvertently stumbled onto the problem yourself, kinda. When trying to measure the position of the electron, you use a high energy photon and this photon. When this high energy photon interacts with the electron it alters the velocity of the electron, so you know less about the velocity of the electron. When trying to measure the velocity of the electron, you use a low energy photon. This low energy photon measures the velocity well, but it moves the electron a little bit, so you don't know its position. This issue is the essence of the Heisenberg uncertainty principle.

## Re:Does exist any quantum computer proven to work? (1)

## Davey McDave (926282) | more than 6 years ago | (#22837818)

2. The problem with talking about quantum physics is that you deal with principles quite unlike real life. Every particle is a wave (see de broglie). It can be represented by a wavefunction, the square of which is the probability of detecting it at any given point. Different energies represent different waves, but not every wavefunction is possible. Hence, only certain energies can exist, meaning we have discretely based quanta.

Heisenberg's principle is not a technical limitation, but an artefact of the maths.

It's impossible to seperate some properties from others. The function of momentum, for example, is linked to the function of position (if I remember correctly, it's ih d/dx). The more you narrow down the probability of one function, the other gets wider and wider. It's like playdoh - the more you squish it, the more it spreads out in every other direction. If you're mathematically inclined, the function of space is actually a fourier transform of the function of momentum, and vice versa.

- a physics student who's fed up of fourier

## Re:Does exist any quantum computer proven to work? (1)

## ortholattice (175065) | more than 6 years ago | (#22838034)

You may be interested in Bohm's interpretation of quantum mechanics, which is much more intuitive because it deals with realistic particles travelling in a "quantum field," very analogous to classical particles in say an electromagnetic field, and there is none of this mysterious "collapse" or things not existing until you observe them. Standard nonrelativistic QM falls out as the statistics of Bohm's theory. The two are mathematically equivalent. The problems with Bohm's theory are (1) there are faster than light influences (that you can't signal with, though), but I don't see that as philosophically different from the nonlocal correlations of standard QM; (2) while very intuitive, it is somewhat impractical to compute with since you have to work out the statistics of many particle trajectories whereas standard QM essentially already is the statistics; and (3) (probably the most serious) no one has figured out how to incorporate relativity in a clean way. Google: Bohm quantum mechanics [google.com] .

## Shor's Algorithm (3, Interesting)

## debrain (29228) | more than 6 years ago | (#22836972)

If that's the case, it's probably worthwhile to discuss Pollard's Rho algorithm [wikipedia.org] , which has a poorly understood worst-case complexity (as a Monte Carlo method), but has a potential average case complexity that is comparable to the quantum.

## Re:Shor's Algorithm (1, Informative)

## Anonymous Coward | more than 6 years ago | (#22837406)

Besides, if you believe that some method is faster in practice than what the theory says, there are some RSA challenges out there with big money prizes for you to win.

## Re:Shor's Algorithm (1, Troll)

## profplump (309017) | more than 6 years ago | (#22837540)

## Re:Shor's Algorithm (0)

## Anonymous Coward | more than 6 years ago | (#22838966)

If that's the case, it's probably worthwhile to discuss Pollard's Rho algorithm, which has a poorly understood worst-case complexity (as a Monte Carlo method), but has a potential average case complexity that is comparable to the quantum.

When stuff like that gets modded +4 Interesting, you know you are reading Slashdot!

## well duh (1)

## ILuvRamen (1026668) | more than 6 years ago | (#22837084)

## Am I missing something? (1)

## mark-t (151149) | more than 6 years ago | (#22837090)

## Incorrect interpretation? (1, Informative)

## blueg3 (192743) | more than 6 years ago | (#22837130)

While it's clear a quantum computer won't be breaking your RSA keys any time soon, there's a big difference between remarking that a 4096-bit key will require "billions" of qbits and the more correct claim that it will require thousands of qbits (at least 20k qbits).

## Botnets are more of a threat (1)

## BountyX (1227176) | more than 6 years ago | (#22837184)

## Use quantum computers to ENCRYPT (1)

## presidenteloco (659168) | more than 6 years ago | (#22837306)

Why can't we use them to come up with an equally mind-blowing way of encrypting?

I don't mean single photon secure fibre channel stuff. That seems fairly impractical to deploy to the whole internet.

I mean, why can't some mathematical genius come up with a new encryption algorithm that you

can only implement on a quantum computer, and which produces a cipher text so random that it

can't be decrypted even by another quantum computer unless it knows the secret.

Does anyone have any ideas how such an algorithm might work?

## Re:Use quantum computers to ENCRYPT (0)

## Anonymous Coward | more than 6 years ago | (#22839232)

authentication. Proving you are who you say you are.but yeah, you would think there is some way to do this with a quantum computer so that it can't be easily broken by a quantum computer.

## Complete article (without subscription) (2, Informative)

## Muhammar (659468) | more than 6 years ago | (#22837308)

http://www.scottaaronson.com/writings/limitsqc-draft.pdf [scottaaronson.com]

Scott posted it on his blog on 2/18, see http://www.scottaaronson.com/blog/ [scottaaronson.com]

(The blog is often quite technical as you can expect but funny and worth following just for its non-techical bits. Circumcision and Australian models are also discussed on frequent basis.)

## Part of a long lineage of naysayers (2)

## cleduc (146288) | more than 6 years ago | (#22837332)

Heavier-than-air flight is impossible; a train will never go faster than a person can run or the passengers will asphyxiate; there is no reason why anyone would want a computer in the home; etc. [wilk4.com]It's a bad idea to discount future technological advances wholesale.

## In what ways is this a problem? (1)

## failedlogic (627314) | more than 6 years ago | (#22837430)

I would consider that if Quantum computers exist, it would pose a serious threat to security and military applications as your enemy would always be listening. I don't know if E-Commerce would grind to a halt, since governments would initially be the only ones to afford it. I would think that instead of hitting e-commerce the better thing to go after -for a government- would be banks and securities exchanges, military and state secrets.

Personally speaking, I don't have a great deal of information that needs unbreakable encryption (neglecting the computer were stolen, etc). Were it financial data, I would keep it on a PC off-line. Of course, if banks are vulnerable, that's another consideration altogether. If I lived in a communist state or a dictatorship, I think the issue would change yet again. There would be information you would want to keep away from the government such as attempts for a coup or rebellion. Or plans to destroy the Death Star!

## There is a problem with this (4, Interesting)

## damburger (981828) | more than 6 years ago | (#22837584)

He makes an extremely cogent argument, but it is hampered by the lack of information we have about the state of the art in quantum computers.

Domestic spying is massively popular with western governments right now, and if you think that the NSA and GCHQ aren't doing secret research into quantum computers you are out of your mind. Furthermore, it is a commandment of signals intelligence that you do not let the enemy know you have broken his code - and in this case the enemy is us. We have no idea how far along they are. We have no idea what the generational length is for the quantum computers that are certainly being developed in secret.

Basically, this essay could be published and make just as much sense either before or after a critical breakthrough had been made by one of the aforementioned agencies and they hadn't told anyone. Thus, we have no way of knowing if we are already past that point or not.

Given that it has already been shown that quantum computers are not infallible, would it not make sense now to start working on encryption methods designed to flummox them?

## Re:There is a problem with this (1)

## BountyX (1227176) | more than 6 years ago | (#22837732)

## Re:There is a problem with this (1)

## Gazzonyx (982402) | more than 6 years ago | (#22838452)

That is, so far as research goes. I think implementation lags the private sector by several years due to bureaucracy.

## Re:There is a problem with this (1)

## damburger (981828) | more than 6 years ago | (#22838728)

Yeah, but who do you think made up the rule of thumb ;)

Seriously though, you can't really make that kind of prediction with such a new field of technology. It would be like trying to guess how far along the Manhattan project was as a civilian in 1942.

Whilst interesting, this guys piece is the crypto equivalent of the Drake equation; sound maths, but it doesn't tell us anything because we have no way of knowing any of the variables.

## Not Schneier's analysys but some schmuck's (1)

## internet hate machin (1134055) | more than 6 years ago | (#22838436)