Let’s kick the Quantum Beat

As a newbie in the field of biophysics, I had to take some course in biology. In my case the course, luckily, was addressed to physicists, and one of the suggested reading was Schroedinger’s “What is life?”. I did not know that Schroedinger wrote about this topic and, of course, I was curious and puzzled. I wanted to read it, and if you want to find a book nowadays, you usually type amazon.com and look for it. The problem with this approach is that Amazon, though really helpful, is sneaky and starts this evil suggestions thing in which I usually get trapped. This time the trap was more than well placed. Among the suggested readings, there was this: Life on the Edge: the Coming of Age of Quantum Biology. 

Now, if there is something you need to know about me is that the mere word Quantum must trigger something in my mind and you immediately have my fully devoted attention.

“I would like to have 200 gr of brown bread, please.”

“Classic or Quantum?”

“…”

“Would you like to have the Classic brown bread or the Quantum one, then?”

“Give me 5 kg of the quantum one!”

And it may works to let me buy things. So my mind turned blank about Schroedinger’s one (now in my wishlist) and I bought Life on the Edge instead.

Now, the book is an interesting collection of science facts, expressed in an incredibly enjoyable narrative way. Beautiful scenarios open just in front of your eyes, between the lines. You will encounter the Robin with its sophisticated magnetic inclination compass migrating south. The same that many others animals seems to share, from other birds to butterflies. You will accompany a prehistoric artist during the painting of a bison in the Chauvet cave.

Imagine yourself in 20 years time: what do you think would be the latest PC shape you will find yourself interacting with? From humongous boxes as big as rooms to pocket-size tablets, calculation power has improved and taken unimaginable shapes. Would it be a projection on you arm? Wearable smart t-shirts? A brain to eyes implant? What if I tell you that it could take the form of a Ficus benjamina in your living room…

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A Ficus benjamina, a.k.a. your next quantum computer.

I bet you are puzzled, and you’ll be more by the end of this blog post. First I have to go a bit into the realm of quantum (happiness flowing through my veins).

You all probably know, by studies or by memes on facebook, about the poor Schroedinger cat, who was never alive or dead but always in a superpositions of the two states, until someone opened its box. The cat must have feel awful, being a complex warm system where is very unlikely that such a phenomenon would really happen – we’ll get to that later. Despite how bold this example may sound, it easily for our mind to retain the concept. Now, hold this concept of a quantum state to actually BE multiple states at the same times, until we observe it and force it to fall in one single state. Hold it very tightly, and don’t let it go no matter how crazy it may sound.

Lets try to use this counter-intuitive concept for computation. What if we can represent the world not only by 0 an 1, but by a superpositions of states? The answer is theoretically easy: more information could be stored and process simultaneously, but experimentally not practical to achieve. A qubit is how we call this new state of bits. This can be very helpful in optimisation problems, when you need to find the lowest energy solution of a system; basically any time you need to explore a large spectrum of possibilities, but eventually just choosing one. With classic computing, we are forced to try each of the scenarios and then choose the best (some tricks can be used to reduce the number of scenarios to just the most likely to happen, but the issue stays complex), resulting in a very time and resources consuming problem. With quantum computing, instead, we would explore many possibilities at once since they can be encoded by the superposition of states, and then let them collapse at one state: the one we want and we look for. If you are a cartographer wanting to map the shortest path to reach a country far far away, you would need to try each of the paths before you can choose the best. But if you are a quantum cartographer, then you may be able to walk all the path at the same time, making it much faster to find the best.*

There have been several proposed designs for quantum computers, all of them not only involving edge-cutting science (I mean, we talk about teleportation! – Something on this soon to come), but mainly being achievable just under very controlled conditions. In fact we want the superposition of states to collapse just when a solution is found, not a moment before. Commercial options are now available, like the D-Wave Two, working under the astonishing temperature of 0.015 Kelvin degree, more than 4 times colder than the average human could withstand for more than 1h before to die. To exaggerate: if you bring Schrodry, your Schroedinger’s cat, with you on a night trip to Antarctica, the fact that the air in the box is still so “warm” could force your beloved Schrodry to be, i.e., dead much faster than you opening the box to look into it. In fact, before you have time to force it to collapse into a single state, the molecules inside the box would vibrate because of the temperature -everything vibrates unless at 0 Kelvin- and eventually one of then will bump on the cat: this simple interaction is seen by the cat as a measurement, and then it will decide whether to be dead or alive once and for all. This is why you should keep your Schrodry at 0 Kelvin to make it al least 1/2 alive forever (Disclaimer: this works for purely quantum cats not currently available in commerce – Please, do not try at home with classic felines).

We are getting closer to the Ficus benjamina! In fact, I am going to save you the 8 figures bill to buy a D-Wave Two, by telling you that your beloved indoor plant is already computing at the quantum level. At the cosy environment temperature of your living room!

When a photon hits on a chlorophyll molecule (or any other of the light-receiving molecules), it deposits its energy by exciting an electron of the molecules into a state knows as exciton (not a lot of fantasy in naming particles), the coesistence of an electron made free from the molecules thanks to the photon and its positively charged absence, or hole. The exciton is of no use within the receiver itself: it needs to travel to the reaction site where its stored energy is used to transform carbon dioxide into sugars.

In many ways this mechanism exploits the principles of quantum mechanics, i.e. we have electrons that behave as waves travelling from molecule to molecule to reach the reaction centre. What I haven’t told you is the that the exciton is actually a quite fragile quantum state, and if it does not reach its destination soon it will get reabsorbed. This observation is crucial, because in absence of other explanation, the only way for the exciton to find the reaction site is to move through the molecules randomly. This takes a while, and the exciton is more likely to die during its quest. But plants survive on our planet, so something else needs to happen.

This something else is superposition of states. A group of MIT researchers first found evidence of it and published the results in benchmark paper in Nature dated 2007. They used a technique called 2D Fourier transform electronic spectroscopy: three short laser pulses were sent on a photosynthetic complex, and the signal generated by their interaction with the photosynthetic molecules is recorded by a detectors. What they observed was not only the well-expected presence of the exciton, but its oscillations over a period of time of at least 600 femtosecond, 20 trillion smaller than the time human spend to blink, but still quite impressive if you think that without superposition we would have seen no oscillation at all. These oscillations comes from the interference from the exciton taking one path in the molecule, and the same exciton taking simultaneously another path, and the same exciton taking simultaneously another path, and the same exciton… The exciton is in a superposition of states, each exploring a different path to reach the reaction centre. The path are not very different in size, therefore their waves are almost in tune, generating a signal that is not fare away from the one you can hear tuning a guitar. A Quantum Beat! This make the exciton to explore its landscape much faster and more efficiently, therefore increasing its chances of success. This explain why we still observe photosynthesis on our planet.

What is particularly astonishing is the fact that plants work without problems above 20 degree, while the engineers at D-Wave have to cool down to 0.015 Kelvin!  While one could argue that the MIT people were observing this peculiar signature of quantum coherence working at the low temperature of 77 Kelvin with a complex and not a real plant, additional papers showing the same results have been added under the tag search for quantum biology, one even studying algae (a more evolved system) at room temperature.

The molecules in the box in Antarctica at -60 were moving too much for the cat to maintain its superposition, so what are the chlorophyll molecules doing instead? Exactly the same thing, but the plants, maybe thanks to the wisdom of evolution, actually took advantage of it and their random fluctuations help to release the exciton when it gets trapped by specific “trapping” sites present in the photosynthetic network of molecules. This is not only impressive, but a very good lesson for every physicists, suggesting how much we still have to learn by observation of nature.

This amazing behaviour of plants is in everything equivalent to quantum computing, but we may emulate it also for other applications, such as energy harvesting.

My two cents:

  • I love the fact that eventually everything can be explained through physics
  • I am still quite puzzled, as many scientists in the community, about the evidence of quantum phenomena on the temperature and size scale of biology. Not only is very unlikely, but incredibly difficult to observe. Nevertheless, being enough technological advanced that this is possible is intriguing, and it makes this age a good moment for physicists to propose solutions for unsolved problems in biology that would sound crazy or not feasible. Dare to think big absurd theories!
  • Interdisciplinary scientists must really be superheroes, or at least this is the feeling I get when reading this edge-cutting research.

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  • Read Life on the Edge. Not only explained the Quantum Beat much better than I just tried to do, but also it explores many other examples that could equally blow your mind. And it does everything guiding you by hand through the basic of biology and quantum mechanics.
  • Bonus: The song linked to this video starts with the use of stinky gas: quantum biology seems to help also with explaining the way our nose recognise smells! Read further to discover how! Also when you have finished your readings, watch Cowboy Bebop, one of the best anime of all times 😉

 

 

Other interesting readings include:

Physics of life: the dawn of quantum biology – Nature, 2011 [some bits of this blog post have been strongly influenced by this paper]

Quantum biology – Nature Physics, 2014 [An excellent review]

And for the bravest, the most relevant original papers:

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems

Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature


 

* I cheated a bit! This example is just one of the calculation of a quantum computer. Also the D-Wave Two, although is technically a quantum computer, is capable of just one specific type of computation, called Quantum Annealing. More details on their youtube channel: https://www.youtube.com/user/dwavesystems

 

 

 

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