Physicists are enabling increasingly large quantum systems to go into

marky3

Physicists are enabling increasingly large quantum systems to go into a superposition state, now reaching a stage involving many thousands of particles. From what i understand they seem confident they can carry on this trend indefinitely. However doesn't decoherence impose some limitation on how many particles can be in a superposition state. If they were ultimately able to successfully carry out an experiment that took a macro sized object into superposition would that not violate the idea of decoherence? I have been under the impression that decoherence, although not strictly proved, is generally accepted by most physicists.

tom.stoer

The idea of decoherence relies on the interaction (entanglement) with the environment; the larger a qm system becomes, the more difficult it becomes to isolate it from the environment.

unusualname

There is no theoretical limit on the number of particles, but obviously it becomes more difficult to isolate environmental effects for larger systems.

Decoherence is a statistical effect, which in theory can be made arbitrarily small but in practice this is difficult, eg the main difficulty with producing Bose-Einstein condensates is isolating these unwanted statistical (environmental) effects which will "heat" the gas sufficiently to destroy the macroscopic quantum state.

There are indications that entanglement can help protect macroscopic quantum states from decoherence, eg recently it has been discovered that photosynthesis may use entangled quantum effects for a time interval beyond what would normally be considered possible by decoherence

Quantum Photosynthesis

The implications of this are huge, since the previous arguments against quantum processes being significant in the brain and consciousness were that body-temperature decoherence would destroy any possible quantum states.

Quantum entanglement in photosynthetic light-harvesting complexes

Quantum biology has come in from the cold

alxm

Sorry but this is pure BS.
There has never been any discussion about the fact that radiation-matter interaction, the kinetics of photons and phonons and electrons is purely quantum-mechanical. To my knowledge, nobody ever said (for instance) that electrons could not exist in a superposition in a biomolecule. Or any other molecule. We know for a fact that they do.

And this does nothing to invalidate those arguments. Which did not say that "any" possible quantum superpositions would be impossible, merely that these entanglements and superpositions were insignificant to chemistry. This remains the case. Electrons, photons and phonons act on an altogether different timescale than atoms and molecules.

Other than the average time it takes for, say, an electron to get from point A to point B, it does not matter one whit to chemistry whether it goes back and forth or exists in a superposition or passes through 2 or 5 or 100 intermediate states. Such a transfer is reversible anyway! But once you actually form a molecule, or your enzyme undergoes a long-scale conformation change, or such, you have definitely decohered, and the process is irreversible.

This 'quantum-brain' nonsense remains nonsense without any mainstream support, including among people like myself who actually do use quantum mechanics to study biochemical systems. In fact, I think many of them are just as annoyed as I am about these hokey theories getting conflated with the legitimate science. You're comparing apples and oranges.

There is little reason to doubt that the brain works by the same biochemical processes as everything else in the body. And there is no reason whatsoever to believe that there are any significant quantum superpositions going on there. Chemistry ultimately means moving atoms around, and you do not see any superpositions of atomic positions, except for a few cases of hydrogen tunneling. (And decoherence is only part of the reason for not having atomic superpositions in chemistry)

Quantum chemistry has been an established field for 80 years now. It's time to stop pretending that quantum mechanics is something unique and distinct from chemistry that could somehow explain things that chemistry cannot. They are not separate disciplines.

Hurkyl

Keep in mind there's a difference between a quantum superposition and a "useful" quantum superposition.

If two photons become entangled and one of them starts flying off towards Alpha Centauri, then the fact the pair of photons are in a superposition is irrelevant, since we won't be able to perform any experiments involving both of them.


Decoherence is the situation where the superposition has become too unwieldy to have any practical relevance, so that the statistics of the reduced state are all that matter.

unusualname

There's no need to get emotional.

The results on entanglement robustness in photosynthesis are at the very least surprising, if not spectacular (you only avoid decoherence for a very short time, ~500 fs), and deserve a better response than yours

If you can't get the original article, you can read an interview and overview of the results here:

http://www.scienceagogo.com/news/201...runc_sys.shtml


Also google "entanglement robustness", researchers have found ways to maintain partial entanglement in multi-qubit systems transferred over fibre-optics, so that some of the qubit states fall victim to decoherence but some others survive.

This sort of "noisy" entanglement scenario sounds just about right for consciousness, and the difficulty of "focusing" for long periods, but I realise this is pure speculation, the experimental results on photosynthesis are real science. You might dismiss the relevance of 500 femtosecond quantum coherence in algae and plants, but at least dimiss it politely.

alxm

Yeah, well astronomers get pissed off when people ask them about astrology as well.

I don't dismiss the result at all, nor its real significance. The research group I'm in has about a dozen papers published on quantum-chemical studies of photosystem II.
But when you use it to support woo theories of quantum consciousness, then you're using legitimate research to justify things it does not lend support to.
Things which the people who performed the research in question likely do not support either.

Not only is it speculation, it's unwarranted speculation, and a fundamentally unscientific mindset. One should not ad-hoc decide that the human brain works quantum-mechanically, despite no reason whatsoever to assume that it does, and then go looking for ways to 'make it be true'!

I haven't been impolite to the Whaley group or anyone in it (even if I've never met them, I am however acquainted with their neighbors lead by William Lester). I do not dismiss the relevance of their work. I dismiss the relevance of their results to 'quantum-brain' theories, and I dismiss the idea that these superpositions affect the overall chemistry. I doubt they would disagree.

The electron transfer ends up producing a chemical reaction. Either that reaction occurs or not. You do not have a superposition of reactant and product states. And the mechanisms of the brain certainly do not depend on single molecules either. You're the one jumping to conclusions across 6 orders of magnitude here, not them.

unusualname

You first said the possibility of non-trivial quantum effects in photosynthesis was BS, now you're pretending your entire aggressive response was directed to my passing mention that quantum effects in the brain have previously been dismissed because of decoherence arguments (tegmark etc)

In my second reply I thought I'd share a fun speculative idea, just to upset you really.

Dealing with decoherence problems may become one of the greatest technological challenges in the near future if quantum computing bears fruit, and it's useful to know that we can learn some tricks from the superb nano-engineer that is evolution, unless of course that research I mentioned is BS.

Have you got anything interesting to say about decoherence in large systems?

alxm

I did not say that. I said that it's BS to suggest that "entanglement can protect macroscopic states from decoherence". I don't even know where you got that idea.

At the same time I pointed out that quantum effects are expected when it comes to photon absorption and electron/phonon-transfer kinetics, which are wholly quantum-mechanical phenomena to begin with. Entangled states related to phonon transfer have not been observed in biochemical systems. AFAIK, nobody's looked for them either. This result was interesting because it indicates such a state (of some mechanistic significance), and makes for something which could be experimentally verified.

What would a 'trivial' quantum effect be in this case?

Anyway the main point of interest in that result is not that you have these entanglements, but rather the resulting effect that the excitations are preferentially directed to the reaction site. Some of the efficiency of the enzyme may be attributable to its utilization of non-classical dynamics for energy transfer. And that's plenty interesting in itself if you ask me.

No my aggressive response was to you implying these results lend crediblity to 'quantum-conciousness' ideas.
If you're now implying that it's at odds with the Tegmark's results, that's wrong as well. Tegmark's estimates of decoherence times were on the order of 10^-13 s.

But the more important reason to dismiss large-scale quantum effects in the brain is that there is simply no reason to assume that there are any.

So you admit you're trolling?

I agree. Which is why I do quantum chemical studies of biochemical systems. And that research is not BS, and my research is not BS.
Quantum-consciousness stuff however, is BS. And I know I speak for more people in the field than just myself when I say I'm fed up with having legitimate research that happens to involve quantum mechanics and biochemical systems conflated with ideas perpetuated by a few fringe scientists and guys like Deepak Chopra.

Keep it cold and isolated and make sure the substance has as few degrees of freedom as possible. I.e. 'conventional wisdom' on decoherence.

unusualname

If you have a multi-particle entangled qubits you can prepare them in states such that breaking the entanglement with a subset of the qubits doesn't destroy the state of the others, this has trivially been demonstrated with 3-qubit W-states, use your imagination for what that implies for larger systems. If you had googled "entanglement robustness" like I suggested you would have known "where I got that idea"


The usual ones that you are talking about involved in chemical processes, and the effects that get washed out by decoherence at a macroscopic level.

You seem to be arguing that the results from photosynthesis are not that remarkable, even though by your naive classical reasoning the entangled states should be destroyed by decoherence before they could effect the energy transfers.

The research on photosynthesis shows that the conventional wisdom is BS.

I'm pretty tired of people who think QM is a finished subject, pretty much fully explained by current theory, evolution is far cleverer than you and is demonstrating just how wrong your mid-twentieth century mind-set is.

alxm

"Classical reasoning"? How the do you arrive at the conclusion that I'm reasoning 'classically' when from my first post in this thread I've repeatedly pointed out that these processes are wholly quantum-mechanical?

I simply did not say any such thing.

No, it doesn't. A theoretical study that predicts a decoherence can persist for around 10^-13 s in an enzyme, does not invalidate (for instance) Tegmark's calculations which estimated a decoherence time in a cell to be on the same order of magnitude.
(One would also expect the decoherences in an enzyme to be able to persist a bit longer than that, since those calculations were for things in solution, which is a very different environment to a more rigid protein structure)

If you had an experimental result showing something dramatically different, that would be another matter.

Nobody said QM is a 'finished subject'. But chemistry is in fact explained well by current theory. The fact that QM is 'unfinished' does not mean one has carte blanche to go making stuff up without any evidence to support it.

Again with the personal attacks, eh?
Evolution is neither clever nor dumb, it just is. Some of the things which have evolved are quite elegant. Others are equally inelegant. There are hundreds of enzymes devoted to nothing but transporting electrons around. That's the opposite of exploiting quantum mechanical effects, that's taking a quantum mechanical object and putting it in a gigantic and slow classical vehicle.

unusualname

This discussion is becoming pointless, I realise you took offense to my suggestion that the photosynthesis research might have wider reaching implications in neural functions, fair enough, but claiming the opposite is pretty silly, an honest scientist would say it's unlikely but admit they could not rule it out because of our current ignorance.

You seem pretty certain about decoherence limitations in biological systems so there's no point arguing with you. But other less assuming people are involved in very active research in decoherence and are obtaining results that Tegmark et al had no clue about. (By "classical reasoning" I mean pre quantum-computing era ~1980s say, it amazes me that generation after generation of scientists never get the fact that we are pretty ignorant about what is and is not possible.)

A quick search or arxiv.org for "entanglement decoherence" returns dozens of recent relevant papers: eg

Experimental multiparticle entanglement dynamics induced by decoherence


Storing Small Photonic Cluster States in a Dephasing Environment
Noisy entanglement evolution for graph states
Entanglement preservation for multilevel systems under non-ideal pulse control
Experimental Quantum Error-Free Transmission
Geometric measure of quantum discord under decoherence

look, someone's even managed to get entanglement between calcium ions lasting for 20 seconds, although in rather artifical conditions http://www.springerlink.com/content/m58075444qur9704/

It seems pretty obvious that we will soon be constructing robust (against decoherence) room temperature quantum computers, and if we can do it I wouldn't bet against evolution managing something similar.

alxm

People could demand that concession about anything and everything. You could take that gap in our knowledge of how the brain (or anything else) works and insert the Flying Spaghetti Monster or whatever you like into it. After all, if I don't know how it works, how could I possibly know it's not the Flying Spaghetti Monster? Tell me what exact physiological/biochemical process is going on in the brain which cannot be understood with our current theories without invoking macroscopic quantum entanglement? There isn't one.

This is not a scientific theory, it's pure conjecture. Believing in something despite no evidence-based reason to do so is religion, not science.

What makes you say that Tegmark has no clue about it? What do you know about what he knows?
He's a quite gifted professor. (although as it happens, decoherence is not his main research topic either)
Tegmark's opinion on 'quantum brain' nonsense is the mainstream scientific viewpoint. It's just that he's the the only one who's bothered to sit down and do the math.

The advent of "Quantum computing" has not changed fundamental quantum theory one whit. It's based on quantum theory. You think we just wished quantum computers into existence? They came about by being predicted from current theory.
You can't choose to believe in the predictions of a theory when you like the outcomes and ignore them when you don't.

What's your point? That there's a lot of research going on involving coherent states? I am not surprised with that result. And it does not change anything.

You can't just naïvely list a bunch of examples of entangled/coherent states and assert, without any consideration of the physics involved, that it therefore should be possible in liquid at room temperature. Decoherence is interaction with the environment. If you want something to remain in an entangled state, then you have to limit that interaction. Solid substances, low temperatures, relatively non-interacting properties such as nuclear spin, and just generally few degrees of freedom.

Pointing to a long coherence time for a certain property in one environment as evidence for long coherence times for a completely different property and environment is just plain dumb.

Our knowledge of decoherence has improved a lot in the last 30 years. But the basics of how and why it occurs have not. What's changed, is that our experimental techniques have improved, we've developed "optical traps" and such. We've gotten better at isolating systems. The boundaries have certainly been pushed, but these are boundaries created by technical limitations, not theoretical ones.

So I can give you a rationale for why we don't think entangled states are significantly/directly involved in physiological functions. Which is more than I can do for the Flying Spaghetti Monster.

unusualname

Yeah, well as I said, since you seem pretty convinced about what is ruled out by decoherence in biological systems there's no point arguing.

Tegmark's argument was made in 1999 when neither he nor anyone else had much clue about entanglement behaviour exposed to noisy environments. Presenting impossibility arguments before the science is fully understood isn't any more helpful than making an educated guess on what might be possible. Except the latter is at least stimulating to ponder.

Let's come back to this in 5 years and see what's developed, I'll wager that Tegmark's argument is discredited due to the usual lack of imaginative foresight that has plagued scientific history.

alxm

I dispute that statement. I think most physicists would dispute that statement.

Anyway, here's a more recent, popular-scientific overview (2007), then. Written by people who (unlike Tegmark) work specifically with quantum computing.

unusualname

To dispute it you have to show what scientific results about entanglement robustness in noisy environments were known in 1999, eg by posting a link to a paper.

I posted a link to half a dozen new results that have just been published in the last month, and that was a quick 5 minute search.


That popular 2007 paper is an interesting read, however they make unjustified assumptions about what might constitute a "thought" (how do they know?) and they are really just arguing that large scale quantum-computation is unlikely in the brain, which seems reasonable, I don't really think we possess "quantum computers" either, the evolutionary benefit of factoring large numbers doesn't seem great for example.

But as to what constitutes a "thought" or "conscious awareness", that's a mystery, but can you not envisage the possibility that millions of continuous successive short term quantum entangled events might be involved in all that activity that occurs in the brain? (ie there may be non-trivial quantum events not related to physical chemistry in the brain)

I'm not sure who you're more annoyed with, me for suggesting something so outlandish or evolution for having the affront to evolve a non-trivial quantum process in photosynthesis.

After all, decoherence is a statistical phenomena and evolution is essentially a creator of form from randomness, it ought to have discovered ways to minimise decoherence effects.

alxm

Yes, but I can find a dozen or so papers on, say, the chemistry of water published this year.
A lot of papers means a lot of people are studying the thing in question, it doesn't necessarily imply any major upheavals are taking place. (Conversely, a revolutionary paper might come quite out of the blue)