
Quantum Light Harvesting Hints at Entirely New Form of Computing - digisth
http://www.technologyreview.com/view/522016/quantum-light-harvesting-hints-at-entirely-new-form-of-computing/
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Xcelerate
This is a really cool article! I've never heard of plants doing something like
this. However, the article is written kind of sloppily.

> Quantum states are highly fragile — sneeze and they disappear in a puff of
> smoke.

No, quantum states never disappear. They just evolve in time. However, it is
hard to _isolate_ a system that you are assigning a state vector from its
surroundings.

> They say the processes behind light harvesting are a special blend of the
> quantum and the classical.

This kind of misleadingly suggests that reality has separate quantum and
classical components. That's not the case; everything is quantum. It's just
that some things that occur cannot be described accurately with classical
physics.

> Because energy can exist in a superposition of states, it can travel a
> variety of routes around the network at the same time. And when it finds the
> correct destination, the superposition collapses, leaving the energy at the
> reaction centre.

 _Light_ travels. Energy is just a number that's conserved in non-relativistic
QM. Also, while I suppose you could describe light as traveling through every
path at once (and I know a lot of physicists like to describe it this way
too), it sounds much more mysterious than it needs to be to me. I would just
say: "as the wavefunction evolves in time, its probability amplitude spreads
throughout space".

> Another is the quantum zeno effect, the paradoxical phenomenon in which an
> unstable state never changes if it is watched continuously.

This makes it sound like its conscious or something. There's no paradox here.
"Watching" is just a series of measurements. Measurements collapse a state
into an eigenstate. If you perform these frequently enough, the system doesn't
have time to decohere, so it just keeps collapsing into the same eigenstate.
It would be less like watching the pot and more like pulling it off the stove
each time its about to boil.

Anyway, I sound nitpicky, but I remember that before I learned QM I was always
bothered by the fact that people tried to make it sound mysterious and vague
for some reason when its actually quite objective and concrete.

~~~
loup-vaillant
My turn to nitpick:

> _as the wavefunction evolves in time, its probability amplitude spreads
> throughout space_
    
    
      sed 's/probability//'
    

Probability is a real number embodied in a state of mind[1]. Amplitude are (as
far as current physics believes) a complex number out there in the world. I
know, squared norm and Born statistics, but still.[2]

[http://lesswrong.com/lw/oj/probability_is_in_the_mind/](http://lesswrong.com/lw/oj/probability_is_in_the_mind/)

[http://lesswrong.com/lw/r5/the_quantum_physics_sequence/](http://lesswrong.com/lw/r5/the_quantum_physics_sequence/)

~~~
darkmighty
Oh god who writes those?

It's a convention in physics to call it probability amplitude because, you
know, it _makes sense_ , as a direct analogous to energy.

([http://en.wikipedia.org/wiki/Probability_amplitude](http://en.wikipedia.org/wiki/Probability_amplitude))

If you want to learn quantum mechanics there are far better places than
sketchy, hand waving articles. I'd start here:

[http://www.youtube.com/watch?v=hUJfjRoxCbk](http://www.youtube.com/watch?v=hUJfjRoxCbk)

~~~
Jach
What makes sense about a probability that can take on negative or complex
values and collections of which don't sum to 1? Convention is no excuse for
bad terminology or notation, especially for beginners to the subject who may
not have the math or physics background to appreciate the connection with the
continuity equation. You lose nothing by removing the word "probability" and
avert possible confusion. "Amplitude" and either "squared modulus" or
"amplitude density" work fine.

I think Feynman's QED is mandatory reading for an intro to quantum thinking
(and Feynman in general is fantastic). I also think his several comments on
wave v. particle indicate another abuse of conventional terminology. Sometimes
he'll insist light is not a wave but a particle, but it's actually a screwy
kind of particle and you need to think about arrows and paths. ("Collapse" is
another word that'd be better off going away...)

My favorite beginner-level approach to QM nowadays, at least for beginners who
are familiar with linear algebra, is actually Scott Aaronson's (edit: who is
an expert in the field, and I believe has even commented on some of Eliezer's
quantum posts...) (quantum) computer scientist approach:
[http://www.scottaaronson.com/democritus/lec9.html](http://www.scottaaronson.com/democritus/lec9.html)
It introduces QM as "What if we tried to make something like probability
theory but allowing negative or complex values?" It wouldn't _be_ probability,
but it would _be like_ probability. Once you set up the math, the usual
"weird" quantum effects of experiments can be shown as a direct consequence of
the math.

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hughes
I wish I knew more about quantum physics.

With most science-related articles, I know enough to be able to tell what's
journalistic hype and what contains an element of truth. But with _anything_
about quantum physics or quantum computing, I am completely at a loss as to
whether the ideas presented are even real, never mind having a hope of
understanding them.

~~~
loup-vaillant
Here is an understandable and serious 40.000 feet bird's eye view of quantum
physics:
[http://lesswrong.com/lw/r5/the_quantum_physics_sequence/](http://lesswrong.com/lw/r5/the_quantum_physics_sequence/)

Having reading it, I'm confident this post is crap. The abstract of the
scientific article behind it looks much better, though.

~~~
Double_Cast
I started reading the sequence a while ago. I'm confident I understood
everything, up until the dialogue between the aliens who split their brains.
There were simply too many streams of consciousness for me to follow. At that
point, I decided to quit and come back to it after a few months. I find that
sometimes, looking at a problem with a fresh mind is enough to solve it
instantly.

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jheriko
the writing smacks of pseudoscience sadly... the idea of 'a mix of quantum and
classical' implies a lack of understanding of what quantum mechanics is and
specifically the correspondence principle.

when did these two things become separate in anything other than some
categorisation of our theoretical understanding of things?

The entire article is coloured with it which makes it hard to glean any
valuable information from. The analogy (?) about light bouncing around
proteins, superpositions and collapses is especially confusing.

If you look at the paper it does seem genuine and without the hype and bad
analogies. (although there is some quite bad grammar, and i am far enough from
subjects i am comfortable with that this could be psuedoscience).

What I get from this is 'we have done work on a very specific example of
quantum computing in nature - that work provides evidence that such approaches
are viable'

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andy_ppp
This article is very exciting, it almost makes you wonder if the universe is
set up to keep adding new possibilities to how much information can in theory
be processed by biological systems... e.g. the brain. Anyway if this research
is true it's incredible that nature has worked out such a clever hack.

I've not seen many other examples of biology using quantum states - given the
article there must be quite a few I assume?

~~~
vanderZwan
Well, some scientists propose that smell uses quantum physics:

[http://www.ted.com/talks/luca_turin_on_the_science_of_scent....](http://www.ted.com/talks/luca_turin_on_the_science_of_scent.html)

(as I understand, Luca Turin's explanation isn't complete either - again it's
a mix of different systems)

There's also the theory that birds detect magnetic fields through quantum
effects:

[http://www.newscientist.com/article/mg19826544.000-do-
birds-...](http://www.newscientist.com/article/mg19826544.000-do-birds-see-
with-quantum-eyes.html)

Actually, I wonder if it's a coincidence that both of these are sensors.

I can't think of any other examples at the moment, but why _would 't_ life
evolve to exploit quantum effects where it can?

~~~
sbirchall
Quantum Bees:
[http://discovermagazine.com/1997/nov/quantumhoneybees1263](http://discovermagazine.com/1997/nov/quantumhoneybees1263)

I know it's Discover and probably total crap, but I just love the idea.

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TrainedMonkey
This is actually interesting. If their theory is correct plants have
essentially loss less and super fast way for photons to traverse a random
network^1.

1: They claim it is random network, I am not sure if we can call specific
protein truly random. There is also question of size, scale of this effect
might be small enough that there will be no applicable technology that could
utilize it yet.

~~~
VladRussian2
in the original article they clear that it isn't a photon, it is exciton that
does the traveling:

[http://arxiv.org/pdf/1311.4688v1.pdf](http://arxiv.org/pdf/1311.4688v1.pdf)

"In these systems excitons initiated by the incoming photons should travel
really fast throughout a chain of chromophores in order to reach the reaction
center where they are converted to chemical energy."

>loss less and super fast way for photons to traverse a random network^1.

My understanding is it isn't exactly loss less. It is a trade-off. The
"natural" QM way of simultaneous multi-path travel is the super fast way of
finding the reaction center. The issue is that many of these multi-paths may
interfere and as result some degree of "entrapment" happens significantly
delaying the arrival at the reaction center (Anderson localization - on a
lattice draw all possible ways from A to B, including loops, etc... and
whenever paths pass through the same edge with different direction double the
edge time on each path and given lattice complex enough the propagation would
almost stall - very-very rough illustration). To minimize the delay, the
nature seems to came up with workaround - [slightly] breaking the phases on
different paths and it seems that coherence is still preserved so we still
have multi-path simultaneity while the above mentioned Anderson localization
is decreased because of phase mismatch. This phase breaking is result of
interaction which causes some energy loss/dissipation.

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dchichkov
I think this article is auto-generated or pseudoscience.

