# Quantum crypto still not proven, claim Cambridge experts

Two killjoy researchers from the University of Cambridge have cast doubt on whether quantum cryptography can be regarded as ‘provably secure’ – and are asking whether today’s quantum computing experimentation is demonstrating classical rather than quantum effects. Computer scientists Ross Anderson and Robert Brady have published …

#### Is the error message only for me?

Is anyone else seeing "Task Queue failed at step 5: Playlist could not be loaded due to crossdomain policy restrictions."?

#### Re: Is the error message only for me?

Yes, I did wonder if it was the work firewall but obviously not

#### Re: Is the error message only for me?

We can't tell - the answer would change if we observed it!

#### Re: Is the error message only for me?

Don't c where the uncertainty is :(

Did it for me too.

For the Physicists on El Reg - How does the single/double-slit diffraction experiment relate to Maxwells Aether theory of electromagnetic wave propagation?

#### Maxwells theory of electromagnetic wave propagation

The point is, Maxwell equations it is a theory about light as waves. The single/double-slit diffraction experiment shows that light can be thought of not only as waves but also particles (the photon). You can send individule quanta of light one at a time through the slits, but still have a difrraction pattern created like waves. This also works for electrons and other 'particles'. Hence, the wave-particle duality.

#### Re: Maxwells theory of electromagnetic wave propagation

That duality relies upon the dubious Copenhagen school of Quantum Physics being correct, and that is not proven; worse still, some clever experiments have already shed doubt on quantum particle entanglement theory. I suspect that QE data transmission is not secure from sniffing, so doomed to fail. I also suspect that Quantum Computing will be proven to be wishful thinking.

The reality is probably that light is not a limited human concept like a particle or a wave, but rather something like a cloud, vortex, or string of energy, or a hyper dimensional 'intrusion' into 3D space time, which only seems to have particle and wave like properties.

We are probably limited by our current concepts and senses of a 3D reality as to how reality actually works; reality probably works in a lot more dimensions than.we are currently aware of, and this probably explains why weird stuff appears to happen from a limited 3D view of reality.

#### Carver Mead's been saying it for years

http://freespace.virgin.net/ch.thompson1/People/CarverMead.htm

#### Re: Carver Mead's been saying it for years

Even if you could fire a "single" photon would it not hit the Aether particles and cause them to propagate as a wave?

#### Re: Carver Mead's been saying it for years

"Even if you could fire a "single" photon would it not hit the Aether particles and cause them to propagate as a wave?"

You can 'fire' a single photon.

What Aether particles ? Suggest you need to do some reading around this whole area

#### Re: Carver Mead's been saying it for years

>Suggest you need to do some reading around this whole area

Nah he's fine - engineers can stop at Newton - and developers safely draw a line after Euclid.

>What Aether particles ?

æther is kind of like Dark Matter, but easier to type.

#### Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

*“Despite the investment of tremendous funding resources worldwide, we don't have working testbeds; we're still stuck at factoring 15 using a three-qubit algorithm”*

This may well be due to engineering problems. Maybe a detailed study of the current state of the art will show that....

*[The paper] suggests that current experiments have not yet proven that “local realism” (that is, classical behaviour without the “spooky action at a distance” that so bothered Einstein) is violated.*

O'Really?

It's good to know that Helium II does not actually exist. Or the Quantum Hall Effect. Maybe we are imagining that we understand the mathematics at the foundation of QM. Could be that the LHC is just an empty hole in the ground. Who knows.

The paper intro states:

*Second, we consider a recent soliton model of the electron, in which the quantum-mechanical wave function is a phase modulation of a carrier wave.*

Yes. Models that "explain" quantum mechanics using the tools of classical physics have been a dime a dozen for the last hundred years or so (and there are actually interesting approaches) Most have been written up by people who haven't yet learned about the things the players have already forgotten, who are either very young or very old and not yet/no longer "believe in QM" or maybe who are computer science researchers or auto mechanics who have a cool idea and want to write something up. Okay.

It's a bad sign when someone hasn't yet sussed that the "wavefunction" (What an ancient pre-WWII term, really. Smells of undead cats.) that he wants to replace is not necessarily a "thing in space" but is actually the state of a quantum-mechanical system in general, not necessarily extended in space. So it's pretty pointless to try to replace it by "another thing in space" ("Modulation of a carrier wave"? Really, now. What is this, fixing of radios? Frack off.)

Let's randomly search for a paper with Anton Zeilinger in the author list....

Bell violation with entangled photons, free of the fair-sampling assumption.

*The violation of a Bell inequality is an experimental observation that forces one to abandon a local realistic worldview, namely, one in which physical properties are (probabilistically) defined prior to and independent of measurement and no physical influence can propagate faster than the speed of light. All such experimental violations require additional assumptions depending on their specific construction making them vulnerable to so-called "loopholes." Here, we use photons and high-efficiency superconducting detectors to violate a Bell inequality closing the fair-sampling loophole, i.e. without assuming that the sample of measured photons accurately represents the entire ensemble. Additionally, we demonstrate that our setup can realize one-sided device-independent quantum key distribution on both sides. This represents a significant advance relevant to both fundamental tests and promising quantum applications.*

I must be imagining things. Where is my Ubik vaporiser?

#### Re: Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

Regardless of your opinion of Ross Anderson's writings, there's a fundamental unresolved issue.

Consider the dependencies that are often overlooked.

Quantum cryptography -> Properties of the theory of quantum mechanics -> Physical implementation exploiting modelled quantum mechanical behaviour of photons -> Mother nature

Classical cryptography -> Properties of an artificial mathematical system -> Physical implementation of that mathematical system.

Spot the difference?

The theory of quantum mechanics is backed by experimental results pretty well and we've been able to exploit that in many areas of human endeavour. But there are two problems:

- Quantum mechanics doesn't match experimental results
*exactly*, and worse still is doesn't agree other theories (e.g. Einstein's relativity). One can only conclude that something somewhere is wrong in one or other or both of them. The correct solution to this conundrum may or may not invalidate the properties of quantum mechanics on which quantum cryptography is based. Physicists are fairly confident in that it won't, but that's not exactly a proof. Worse still you can't prove that no one else has a theory which better describes nature (so that would be more correct) where you can violate quantum entanglement - To paraphrase the great (and rarely wrong) Richard Feynman, it's only a
*theory*. That doesn't mean that's how mother nature really works. In contrast classical cryptography is based purely on the system of mathematics where the rules are all ours. There's still room for mistakes, oversights and different solutions, but mother nature doesn't come into it.

#### Re: Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

I read the paper as saying that current experiments *in Quantum Cryptography* have not yet proven that "local realism" is violated. In other words, quantum computers have not yet unequivocally demonstrated that they rely on quantum effects for their results. They aren't claiming that quantum effects do not exist.

#### Re: Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

Actually, both systems are on similar shaky ground. In the mathematical case assumptions have to be made about the difficulty (or computational effort) required to reverse the encryption. For example, the difficulty of factoring the product of two large primes. I don't believe that has ever been proven to be difficult. It's just an empirically observed fact, not unlike the fact that QM seems to be a good description of nature.

Anyone could come along at any time with a better method/theory and break the encryption in either system.

#### Re: Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

I haven't read the paper, but I think it's pretty obvious that they are suggesting that *current qubit experiments have not yet proven that "local realism" etc.* (my emphasis), not that quantum mechanics doesn't exist. They're not physicists (I think), but they're not stupid either. And I didn't downvote you.

And I don't know what your beef with wavefunctions is. Maybe you did it another way, but it's a perfectly good model.

#### Re: Some dude writes something up. Next: animal entrails, bone casting and the I Ching.

Okay, so.... Suppose this is World Quantum Wrestling Championship and some skinny guy duo in gaily colored dresses enter the ring....

MAD SMACKDOWNS then happen at Scott Aaronson's blog (IMAO well deserved). We read:

*First thought: it’s ironic that I’m increasingly seeing eye-to-eye with Lubos Motl—who once called me “the most corrupt piece of moral trash”—in his rantings against the world’s “anti-quantum-mechanical crackpots.” Let me put it this way: David Deutsch, Chris Fuchs, Sheldon Goldstein, and Roger Penrose hold views about quantum mechanics that are diametrically opposed to one another’s. Yet each of these very different physicists has earned my admiration, because each, in his own way, is trying to listen to whatever quantum mechanics is saying about how the world works. However, there are also people all of whose “thoughts” about quantum mechanics are motivated by the urge to plug their ears and shut out whatever quantum mechanics is saying—to show how whatever naïve ideas they had before learning QM might still be right, and how all the experiments of the last century that seem to indicate otherwise might still be wiggled around. Like monarchists or segregationists, these people have been consistently on the losing side of history for generations—so it’s surprising, to someone like me, that they continue to show up totally unfazed and itching for battle, like the knight from Monty Python and the Holy Grail with his arms and legs hacked off. (“Bell’s Theorem? Just a flesh wound!”)*

Like any physical theory, of course quantum mechanics might someday be superseded by an even deeper theory. If and when that happens, it will rank alongside Newton’s apple, Einstein’s elevator, and the discovery of QM itself among the great turning points in the history of physics. But it’s crucial to understand that that’s not what we’re discussing here. Here we’re discussing the possibility that quantum mechanics is wrong, not for some deep reason, but for a trivial reason that was somehow overlooked since the 1920s—that there’s some simple classical model that would make everyone exclaim, “oh! well, I guess that whole framework of exponentially-large Hilbert space was completely superfluous, then. why did anyone ever imagine it was needed?” And the probability of that is comparable to the probability that the Moon is made of Gruyère. If you’re a Bayesian with a sane prior, stuff like this shouldn’t even register.

.... Iit’s worth noting that, if (for example) you found Michel Dyakonov’s arguments against QC (discussed on this blog a month ago) persuasive, then you shouldn’t find Anderson’s and Brady’s persuasive. Dyakonov agrees that scalable QC will never work, but he ridicules the idea that we’d need to modify quantum mechanics itself to explain why. Anderson and Brady, by contrast, are so eager to modify QM that they don’t mind contradicting a mountain of existing experiments. Indeed, the question occurs to me of whether there’s any pair of quantum computing skeptics whose arguments for why QC can’t work are compatible with one another’s. (Maybe Alicki and Dyakonov?)

#### Good old Dr Anderson

Makes me smile & inspires me to think *every* time I read anything he & his colleagues publish.

Raising a pint behind my mask.

#### Prof. Morgan Freeman

That's a neat video. But do Americans generally mispronounce silicone as silicon?

#### Quantum computing and Physics

I've long been telling people that I expect the on-going failure to construct a significant quantum computer (hundreds of qubits) will be a gateway to new physics. There's some reason, that we don't yet have an inkling of, why it can't be done. In cosmology, there are similar thoughts surrounding rapidly rotating black holes, naked and ring singularities, and time travel ... but (perhaps fortunately) experimental cosmology is a long way off.

The alternative is that the universe is even wierder than I'm currently prepared to contemplate. Also, mundanely, that all cryprography is toast!

#### SciFi Toasts BRAIN

I think that is a given fact, after reading posts like yours.

All the quantum priest claim they can do in the future is to quickly factor numbers, thereby making asymmetric crypto unsecure. Almost all other popular ciphers (such as 3DES or AES) appear to be not affected.

#### Re: SciFi Toasts BRAIN

I don't know the details of AES etc, but it's a much greater class of problems than just asymmetric crypto-breaking that could in theory be tackled by a quantum computer.

Basically, N qubits can represent all numbers from 0 to 2^N-1 **at once!**. You then perform a series of operations on those qubits that if performed on a single number, would tell you (yes or no) that it was a member of the set of solutions smaller than 2^N-1 of your problem. For asymmetric cryptography, that search is for one of two large prime factors. You then observe your system in the quantum sense. It must collapse into one of the possible solutions of your problem - one of the two prime factors, which is all you need to break a code. For other problems there might be a known or unknown number of solutions larger than two, but provided the number of solutions is smallish, repeated runs of your quantum computer would statistically guarantee you'd find all of them ... even if knowing just one isn't all you need to get the rest conventionally.

As the number N of coupled qubits becomes large, quantum computing becomes exponentially closer to magic. That, or we discover a reason why it stops working when N is bigger than some new-physics-determined number.

The observable universe contains something of the order of (only!) 2^84 hadrons. This is why I find it inconcievable that a quantum computer could work with N qubits in the hundreds or thousands, let alone all the way up to millions, trillions and beyond. (Avogadro's number is ~10^26! )

Greg Egan writes the hardest of hard SF. For one possible implication of a working quantum computer for large N, read "Luminous"!

#### Re: Quantum computing and Physics

>There's some reason, that we don't yet have an inkling of, why it can't be done.

It's bloody difficult to "read" quantum states.

#### nonlocality and entanglement

"Entanglement says that two entangled particles (say, photons) will reflect each others’ state at a rate greater than the speed of light."

"at a rate greater than the speed of light." - This is nonlocality not entanglement - see last paragraph of...

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

#### This is an OUTRAGE!

Consentual entanglements between quantums are nobody's business !!!! ... oh wait, didn't see "boffins" in the link, the subtitle says "experts". Good thing I speak "Reg". Never mind.

#### I'm sure I speak for all quantum physicists

when I admit that, yes, it was all just a big hoax all along. The cat is out of the bag.

#### Einstein

Einstein was right: God does not play dice with the universe. Maybe.

Or did someone already say this?

#### God does not play dice with the universe

It's more like Keno, where the dealer is smiling *all the time*.