Quantum Biology: Challenges of a Speculative Science

What is Quantum Biology?

Robin, licensed under creative commons from Ross Elliott

Quantum Biology provides one of the only possible explanations for how birds like the Robin can detect the Earth’s weak magnetic field, using light sensitive proteins in their eyes| Image by Ross Elliott, used under Creative Commons license

Quantum Biology is an area founded on the idea that the weird features unique to quantum mechanics can survive in living creatures, and that they directly affect their function and behaviour. Or, to run that sequence in reverse, systems in nature might have evolved to take advantage of quantum mechanics. Key processes discussed are the efficiency of photosynthesis, and the sensitivity of bird compasses, both of which perform better than expected by classical and semi-classical theories. It’s a big claim, and has faced a lot of rightful skepticism. In this post I’d like to summarise a little about the field, its challenges, and what it can still offer.

The current view is one where quantum mechanics is much more siloed off; defining the behaviour of chemical bonds and reaction rates along with other aspects, but not going beyond that. Nobody is used to seeing a ping pong ball suddenly phase through a table, we just don’t expect quantum weirdness on large scales. Quantum Biologists have to say how these effects can survive for long enough times and have large enough effect for biological systems to even see them, let alone take advantage of them.

What are these unique quantum behaviours? Most commonly they discuss such phenomena as quantum tunnelling, entanglement, coherence, and delocalisation. Tunnelling is the quantum behaviour where objects have a small but non-zero chance of passing right through obstacles, rather than needing enough energy to hop over them. Entanglement is about two or more separate objects acting as one system together, so changing any part instantly and simultaneously creates changes in other parts. Coherence relates to how pure the states are, and how quickly that decays. Lastly, delocalisation is simply the ability for something like an electron to be associated with more than one position, or one chemical bond, at any given time.

The Photosynthesis problem

Given all the above, a fundamental challenge was to find a system with a distinctive quantum fingerprint. In 2007 a paper by Engel and Fleming appeared, saying it observed long-ranged coherence in FMO, a molecule which acts like a convoluted wire for energy in photosynthesis. Using ultrashort laser pulses they observed energy move through the system, and spotted a wavelike ‘beat’ spread over the wire, suggesting coherence was present for long enough to help transfer energy efficiently across the whole wire. The beat of the wave was the quantum fingerprint they had been looking for, quantum mechanics in a natural system.

The paper was massive, gaining 2600+ citations, and was appropriately followed by a flurry of research not only in ultrafast dynamics and spectroscopy, but also in Open Quantum systems, a field that explicitly looks at how quantum mechanical systems behave when exposed to their surroundings (imagine Schrödinger’s cat, but the box isn’t very well made). The search was on not only to understand these systems, but to find more of them. My own research is in the theory of open quantum systems, but we’ll get onto that later, first I need to complicate the narrative a little.

In February of this year I was able to attend a school in Trieste on Ubiquitous Quantum Mechanics, and one of the speakers was Michael Thorwart from the University of Hamburg. He was discussing open quantum systems, and how he had repeated the experiment in 2017, and disagreed with the results. Saying the beating could be explained by semi-classical methods, and that the data had perhaps been misinterpreted.

I talked to him afterwards,  and asked what he might say to someone like me, just starting in the field and interested in seeing whether an open quantum systems approach could be relevant to biology. In short, he said not to get my hopes up. Or at least, to be thorough if I thought I found anything. While good work had been done since in Open Quantum systems in the decade since, he was skeptical of claims made that quantum effects could influence biology. Surprisingly, I was okay with that, and have chirpily carried on with my research since. Why?

Asking the right question

The first and most obvious reason is that science doesn’t strictly have to be right, it just needs to be less wrong. And even if Quantum Biology turns out to not quite be right about photosynthesis, it has helped bring attention to little understood areas of biology, and whether right or wrong, we will learn something. Additionally, anything learned about quantum phenomena in open systems can further inform theory, and possibly be applied to artificial systems, even if it can’t quite happen in nature.

In short, I am interested and motivated by the questions, but I can’t presume the answers, that’s not honest. And open, clear honesty is the way I believe we should approach questions in research. Still, there are plenty of open questions in biology, as described in a 2012 review paper by Lambert et al. For example, all the classical models explaining how birds detect the earth magnetic field seem to be far too insensitive. The best candidate currently is the ‘radical pair’ model, which would have to maintain quantum coherences and entanglement for tens of microseconds, longer than any artificial system has managed.

Whichever way this shakes out, either finding an alternative to the radical pair model, or finding a way to maintain coherences and entanglements for much longer times, we stand to gain a lot of understanding. And for me that is the crux of it, you can ask interesting questions, but you can’t fully anticipate the answers.

My work right now is looking at when quantum mechanical effects can boost the transport of energy through 1D and 2D systems. Looking at how much these systems can withstand and still maintain their quantum advantage, and also seeing how these processes could be directed on energy gradients (see the video above). Obviously, I’d like this to apply to something in biology, whether photosynthesis or electron transport chains, that is just a cool idea. But the main thing is that I’m motivated enough to ask the question and discover some part of an answer. And from that point, be honest about what results from it, and use that to inform the next question, and so on. I’m happy for my intuition to be wrong, either way it’s learning.

Further Reading

For further reading on the state of quantum biology I recommend:

For those who want to know more about the history of the field, I can recommend the History of Quantum Biology by Jim Al-Khalili and Johnjoe McFadden

Papers:

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems, Engel, G. S. et al. Nature 446, 782-786 (2007) (link)

Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer, Duan, H.-G. et al. PNAS 114 (32) 8493-8498 (2017) (link)

Quantum biology, Lambert,N., Chen,Y-H., Cheng, Y-C., Li, C-M.,Chen, G-y., & Franco Nori. Nature Physics volume 9, pages 10–18 (2013) (link)

Sustained Quantum Coherence and Entanglement in the Avian Compass, Gauger, E. M., Rieper, E., Morton, J. J. L., Benjamin, S. C. & Vedral, V. Phys. Rev. Lett. 106, 040503 (2011) (link)