5
Spooky Life and Quantum Demons
It is often said that however ingenious mankind’s inventions are, nature invariably beats us to it. And it’s certainly true that biology discovered wheels and pumps, scissors and ratchets, long before we did. But that’s not all. Nature also discovered digital information processing billions of years before humans invented the computer. Today, we are on the threshold of a new technological revolution, and one that promises changes as sweeping as those that followed the advent of the digital computer. I am referring to the long-sought-after quantum computer.
The essential idea was captured by Richard Feynman in a futuristic lecture entitled ‘Simulating Physics with Computers’ delivered at the University of California, Berkeley, in 1982.1 Feynman pointed out that when conventional computers are used to model fundamentally quantum mechanical objects such as molecules, they struggle with the sheer computational resources needed to keep track of everything. He conjectured, however, that a (then hypothetical) quantum computer would be up to the job, because it would be simulating something of its own basic kind. In 1985 Oxford physicist David Deutsch took the idea further, working out the precise rules whereby information could be inscribed in the states of atomic and subatomic systems and then manipulated using the standard laws of quantum mechanics.
The secret of a quantum computer is something called superposition. In a conventional (classical) computer a switch is definitely either on or off, representing 1 or 0. In a quantum computer it can be both, so it can represent 1 and 0 at the same time – a ‘superposition’ of 1 and 0. The superposition is not merely a fifty–fifty mix of the two numbers but all possible blends. Physicists refer to such an entity as a qubit (for ‘quantum bit’). Tangle just a few dozen qubits together and you can, in principle, create a device that would outperform the best conventional computer.
It wasn’t long before physicists started scrambling to make such a thing, and today the race to develop and market a fully functional quantum computer is the primary goal of a multibillion-dollar industry involving major government and commercial research programmes around the world. The huge investment is because of the sheer computational power a quantum computer would unleash. Not only would it be able to simulate atomic and molecular processes in detail, it could crack the codes used to encrypt most of our communications and sort through vast databases at lightning speed. If it were made widely available, conventional computer security would be a joke: a quantum computer would jeopardize the intelligence services, diplomatic communications, banking transactions, internet purchases – in fact, anything online that is confidential and requires encryption.
But what has quantum computation got to do with life? Well, in biology, the name of the game is information management. Given that life is so adept at manipulating bits, might it have learned to manipulate qubits too? A number of such claims have been made. Though there is scant evidence that organisms engage in actual quantum computing, it is becoming increasingly clear that life does indeed harness some quantum effects.
QUANTUM THEORY IS SERIOUSLY WEIRD
Einstein once described quantum effects as ‘spooky’. In fact, he thought they were so spooky he steadfastly maintained that quantum mechanics gave a flawed account of nature. It seemed in conflict with his cherished theory of relativity by permitting faster-than-light effectsfn1 and he was uncomfortable with the idea that uncertainty and indeterminism underpinned fundamental phenomena. Today, very few physicists would side with Einstein. Quantum mechanics, with all its spookiness, has embedded itself firmly in mainstream physics. After all, here is a theory that not only explains almost everything from subatomic particles to stars, it has given us indispensable forms of technology such as the laser, the transistor and the superconductor. The problem is not with the extraordinary power of quantum mechanics to explain the world and to drive the growth industries of the twenty-first century. It lies with its seriously weird implications for the nature of reality.
Imagine the following scenarios:
You throw a tennis ball at a window pane and it bounces back. You throw it again, exactly as before, and the ball appears on the far side of the glass without breaking it.
You strike a billiard ball directly towards a pocket. When it reaches the edge of the hole, instead of dropping in it bounces right back at you as if the hole’s near edge were an invisible wall.
A ball is rolling along a gutter in the road towards an intersection. When it gets there it turns the corner on its own, without any need for a sideways kick.
These events would be regarded as miraculous if they occurred in daily life, but they happen all the time at the atomic and molecular level, the domain of quantum physics. Other weird quantum effects with no everyday counterpart include particles such as electrons seemingly being in two places at once,fn2 a pair of photons metres apart spontaneously coordinating their activities (what Einstein called ‘spooky action-at-a-distance’), and a molecule spinning both clockwise and anticlockwise at the same time. Entire books have been written about these peculiar yet real effects. Here I am concerned with one question only: do spooky quantum effects occur in biology?
There is a trivial sense in which all biology is quantum. Life is, after all, applied chemistry, and the shapes, sizes and interactions of molecules all need quantum mechanics to explain them. But that’s not what people have in mind when they talk about ‘quantum biology’. What we really want to know is whether non-trivial quantum processes such as tunnelling (ball-through-the-window-pane effect) or what is referred to as entanglement (spooky action-at-a-distance) have an important role to play in life.
Box 11: Is it a wave? Is it a particle? No, it’s a quantum!
One of the founding discoveries of quantum physics is that waves can sometimes behave like particles. Einstein was the first to suggest this, in 1905, with his hypothesis that light, known to be a wave, trades energy only in discrete packets, called photons. Conversely, electrons, which we normally think of as particles, sometimes behave like waves (as do all particles of matter). It was Schrödinger who wrote down the equation that describes how matter waves behave. This ‘wave-particle’ duality is at the heart of quantum mechanics. Many surprising quantum effects, such as the three ball scenarios I gave above, are consequences of the wave nature of matter. It is futile to argue about whether a photon or an electron is ‘really’ a wave or a particle; experiments can be done in which either of these aspects is manifested, though never both together. One simply has to accept that the denizens of the microworld have no everyday counterpart.
The way quantum waves spread and merge critically affects their physical significance. Imagine dropping two stones together into a smooth pond, a few feet apart. Ripples from each stone spread out and overlap. Where the peak of a wave from one stone meets the peak of the wave from the other, they reinforce to make a higher peak. Where peak meets trough, they cancel. The resulting criss-cross pattern of the merged waves, if nothing disturbs it, is called coherent. But now suppose there is a hailstorm spattering the pond with many additional ripples. The nice orderly wave pattern from the stones will be disrupted. This is called decoherence. Coming to quantum matter waves, many weird quantum effects stem from the coherence of the waves. If coherence disappears, so do most of the non-trivial quantum effects. Like water waves, electron waves will decohere if they are disturbed. Electrons in living matter don’t suffer from hailstorms, but they do have to contend with molecular storms, such as the incessant thermal bombardment by water molecules. Back-of-the-envelope calculations suggest that decoherence will occur extremely rapidly under most biological conditions. But there seem to be escape clauses that permit anomalously slow decoherence rates under some special circumstances.
As a rule, if something gives life an edge, even a slight one, then natural selection will exploit it. If ‘something quantum’ can enable life to go faster, cheaper, better, we might expect evolution to stumble across it and select it. Right away, however, we hit a snag with this glib reasoning. Quantum effects represent a subtle and delicate form of atomic and molecular order. The enemy of all quantum effects is disorder. But life is awash with disorder! That inescapable clamour of randomly agitated molecules, the pervasive depredations of the second law of thermodynamics. Entropy, entropy everywhere! Non-trivial quantum effects can survive in the face of this inexorable thermal noise only under very peculiar circumstances. Visit any laboratory that studies quantum phenomena. You will see gleaming steel chambers, winking electronics, humming cryostats, multitudinous wires and pipes, meticulously aligned laser beams, computers – a plethora of expensive, high-precision, finely tuned equipment. The primary purpose of all that fancy gadgetry is to reduce the disturbance wrought by thermal agitation, either by screening it out – isolating the quantum system of interest from the surroundings – or cooling everything relevant to near absolute zero (about −273oC). A big slice of the fortune pouring into the quantum computer industry is directed to combating the ever-present menace of thermal disruption. It is proving to be very, very hard.
In view of the extraordinary lengths to which physicists have to go in order to evade the effects of thermal noise, it seems incredible that anything spooky would be going on in the messy, relatively high-temperature world of biological organisms. A protein in a cell is about as far from an isolated low-temperature system as one can imagine. But remember the demon. It was designed by Maxwell precisely to conjure order out of chaos, to cheat the second law, to evade the corrosive effects of entropy. Although we know that even a demon can’t violate the letter of the second law, it can certainly violate the spirit of it. And life is replete with demons. Could it be that, among their ingenious repertoire of tricks, life’s demons have also learned how to juggle, not just bits, but qubits too, with a dexterity as yet unmatched by our state-of-the-art laboratories?
TUNNELLING
Stuart Lindsay, a colleague of mine at ASU, is a real-life quantum biologist. The focus of his research is the investigation of how electrons flow through organic molecules, especially the As, Cs, Gs and Ts of DNA fame. The method they have perfected in his lab is to unzip the DNA double helix into single strands and suck one of them, spaghetti-like, through a tiny hole – a ‘nanopore’ – in a plate. Positioned across the hole is a pair of miniature electrodes. As each ‘letter’ transits the hole, electrons go through it, creating a tiny current. Happily, it turns out that the strength and characteristics of the current differs discernibly for each letter, so Lindsay’s set-up can be used as a high-speed sequencing device. He has also found that amino acids can be good electrical conductors too, opening the way to sequencing proteins directly.
When Lindsay first described his work, I confess I was puzzled by why organic molecules would conduct any electricity at all. After all, we use organic substances like rubber and plastic as insulators, that is, as a barrier to electricity. And in fact, at first sight it’s hard to see how electrons would find a path through, say, a nucleotide or an amino acid. The explanation, it turns out, lies with a curious quantum phenomenon known as tunnelling – the ‘ball-through-the window-pane’ effect. Electrons can traverse a barrier even when they have insufficient energy to surmount it; if it weren’t for the wave nature of matter (see Box 11), the electrons would just bounce right back off the organic molecule. The tunnel effect was predicted when Schrödinger presented his famous equation for matter waves in the 1920s and examples were soon found. A type of radioactivity known as alpha decay, first observed in the 1890s, would be incomprehensible were it not for the fact that the emitted alpha particles tunnel their way through the nuclear force barrier of uranium and other radioactive substances. Electron tunnelling forms the basis of many commercial applications in electronics and materials science, including the important scanning tunnelling electron microscope.
Stuart Lindsay can send electrons through organic molecules, but does nature do it too? It does indeed. There is a class of molecules known as metallo-proteins, basically proteins with a metal atom such as iron entombed within (a well-known example is haemoglobin). Metals are good conductors, so that helps. But the phenomenon of tunnelling through organic molecules is actually quite widespread. Which raises a curious question: why do electrons want to traverse proteins anyway? One reason it’s a good thing is metabolism. Enzymes connected with oxygenation, and the synthesis of the all-important energy molecule ATP, hinge on rapid electron transport. Slick electron tunnelling greases the wheels of life’s energy-generating machine. This is not just a happy coincidence; these organic molecules have been honed by evolution. Any old jumble of organic molecules won’t do, at least according to Harry Gray and Jay Winkler at Caltech’s Beckman Institute: ‘Stringent design requirements must be met to transport charges rapidly and efficiently along specific pathways and prevent the off-path diffusion … and the disruption of energy flow.’2
While all this is very interesting physics, there is a fascinating bigger question. Have biomolecules more generally been selected by evolution for efficient quantum ‘tunnellability’? A recent analysis by Gábor Vattay of the Eötvös Loránd University in Hungary and his collaborators suggests that ‘quantum design’ may not be restricted to metabolism but is a generic feature of biology.3 They arrive at this conclusion by studying where on the spectrum between electrical conductors and insulators key biological molecules lie. They claim to have identified a new class of conductors that occupies the critical transition point between the material behaving like an insulator and a disordered metal; many important biomolecules apparently fall into this category. Testosterone, progesterone, sucrose, vitamin D3 and caffeine are among many examples cited by Vattay et al. In fact, they believe that ‘Most of the molecules taking part actively in biochemical processes are tuned exactly to the transition point and are critical conductors.’4 Being poised at the edge of the ability to conduct electricity is likely to be a rather rare property of a molecule, and given the astronomical number of possible molecules that could be formed from the building blocks that life uses, the chances of hitting an arrangement that confers such critical conductance is infinitesimal. Hence there must have been strong evolutionary pressure at work. In this case at least, it looks like biology has indeed spotted a quantum advantage and gone for it.
TRIPPING THE LIGHT FANTASTIC
Quantum biology lay largely in the shadows until 2007, when a dramatic discovery cast light on it – quite literally – and propelled the subject to world attention. A group of scientists at the University of Chicago led by Greg Engel was investigating the physics of photosynthesis.5 Now you might think that photosynthesis is by definition a quantum phenomenon – after all, it involves photons. But that merely puts it into the category of ‘trivial’ quantum effects. The organism – it could be a plant or a photosynthetic bacterium – uses light to make biomass from carbon dioxide and water. In that respect, the photon is simply a source of energy; its quantum aspect is incidental. Where the spooky stuff starts is at the next step. The molecular complex that captures the photon and the reaction centre where the actual chemistry is done are not the same. It’s rather like having solar panels in a field to power a factory located down the road. In biology, there’s always a competition for energy, so it pays to avoid wasting too much of this valuable resource when passing it from place to place, in this case from the light-harvesting molecules to the reaction centre. Scientists have long been mystified about how photosynthesis can accomplish this transfer so efficiently. Now it seems that non-trivial quantum effects could pave the way.
To explain what’s going on I need to invoke another weird quantum property already mentioned in passing: the ability of quantum particles to be ‘in two places at once’. In fact, they can be in many places at once. A corollary of this is that in going from A to B a particle can take more than one route simultaneously. To be precise, it takes all possible routes, and not just the shortest one (see Fig. 16). The strange calculus of quantum mechanics requires one to integrate all available pathways between start and finish: they all contribute to how the particle gets there. This sounds totally mysterious, but it’s not if the particle is viewed as a wave which spreads out rather than as a little blob which doesn’t. Think of a water wave approaching a bollard sticking out from the sea bed. The waves curve around it, some going left, some going right, and they join up on the other side. Quantum waves do the same. Imagination fails us, however, when we try to think in terms of particles: how can a single particle go everywhere at once? How can one envisage that? A popular interpretation of ‘what is going on’ in quantum mechanics is to think (in this example) of each pathway from A to B as representing a separate world. If there is an obstacle in the path of the particle (analogous to the bollard in the water), well, in some worlds the particle goes to the right and in others it goes to the left.
Of course, people ask, ‘But which way did it go really?’ The answer depends on what you mean by ‘really’, which is where discussions of quantum mechanics start to become murky and many people are left behind. Nevertheless, I shall endeavour to explain it. In the old-fashioned approach (going back decades), these alternative worlds (each world containing just one particle trajectory) were considered merely contenders for reality, ghostly virtual worlds that don’t ‘really exist’ but collectively form an amalgam – a superposition – from which ‘the real world’ of experience emerges. For definiteness, consider that an experimenter dispatches an electron from a well-defined point A and detects it at a well-defined point B; well, according to quantum mechanics, it is not possible to say how it got from A to B. There is no ‘fact of the matter’ about the intervening route. Not only is it impossible for the experimenter to know the route, even nature doesn’t know. If you try to station a detector partway between A and B to sneak a look, it totally changes the whole result. The physicist John Wheeler, a doyen of colourful descriptions, liked to say that quantum propagation (between A and B, as I have described it) is ‘like a great smoky dragon’. It has ‘sharp teeth’ and a ‘sharp tail’ (at A and B, where the experimenter receives sharply defined information on the particle’s whereabouts) but in between all is veiled in smoke.
These days, many leading physicists insist that the multiplicity of different quantum worlds are in fact real worlds; they exist in parallel, a point of view known as the many-worlds, or many-universes, interpretation of quantum mechanics. As to why we experience only one world, one has to ask what is meant by ‘we’. Suppose each world has a separate version of you. There are now many worlds and many (nearly identical) yous. Each version of you sees just one world. Whether one (ones?) buys into this fashionable but extravagant interpretation of quantum mechanics doesn’t matter here: one can at least safely say that in going from A to B the particle can try out all routes together.
Does the route fuzziness matter, or is this all philosophical mumbo-jumbo? It certainly does matter, because the alternative paths interfere with each other (like the water waves going around the bollard). Sometimes this interference creates ‘no-go zones’ for the particle (where two merging waves cancel out, peak to trough); conversely, it may facilitate its appearance in another region (where the waves reinforce). Quantum interference effects of just this nature seem to make a difference in the molecular complex responsible for photosynthesis. The Chicago team, soon joined by Graham Fleming and his group at UC Berkeley, focused their attention on green sulphur bacteria. These inconspicuous microbes live in lakes and around deep ocean volcanic vents as far down as 5 kilometres. No sunlight penetrates to that depth, but the hot vents emit a dim red glow and it is from this feeble light source that the bacteria make a living. ‘Feeble’ is the word: it’s been estimated that each photosynthetic complex gets only about one photon a day. That’s a trillion-trillionth of what a plant leaf might expect. With so few photons to go round, green sulphur bacteria need to do the very best with what they can get and, indeed, efficiencies approach 100 per cent, with little or no energy squandered.
Here is how it works. The photons come in, one by one, and are absorbed somewhere within a bundle of light-harvesting antennae, each packed with a type of chlorophyll (200,000 molecules in total). About a picosecond (one trillionth of a second) later the captured energy appears in the chemical reaction centre. To get there, the energy traverses what may be loosely compared to the cable or waveguide that connects the antenna on your roof to your television. (At least it did in the days before optical cable TV.) In the photosynthesis case, the role of the cable is played by a molecular bridge called an FMO complex which is made up of eight molecular subunits 1.5 nanometres apart, each of which is also made of chlorophyll, fixed to a protein scaffold. The photon itself is absorbed and disappears, but its energy is captured (in a form I shall describe shortly) and enters the FMO complex through a molecular structure that biologists engagingly call a ‘baseplate’, where it is received by one of the FMO subunits, and is then passed among the rest like a relay baton, until it reaches a subunit adjacent to the all-important ‘factory’ – the reaction centre – where it is handed over to power the chemical reaction. The whole process is a race against time, to deliver the goods before some external disturbance disrupts it.
The whole set-up may seem a bit complicated and ramshackle, with lots of scope for error, delay and ‘dropping the baton’ on the way. All the molecules involved are big and complicated and they are jiggling about because of thermal agitation; it’s easy to imagine the precious energy, painstakingly harvested and destined as it is for the reaction centre, ending up instead being scattered and dissipated into the messy intervening infrastructure. But that doesn’t happen. The energy arrives unmolested and in record time. It used to be supposed that this energy transport traversed the FMO complex in a series of simple hops (or baton passes), haphazardly accomplished amid the thermal clamour of the molecular milieu. But that looks too hit and miss for such a fine-tuned mechanism. Which is where quantum mechanics comes in.
To give the gist of the quantum explanation, first let me explain in what form this energy is stored. When the photon is absorbed it releases an electron from the antenna molecule (this is the familiar photoelectric effect), leaving behind a positively charged ‘hole’. Because the electron is embedded in a molecular matrix, it doesn’t fly off, free. Instead, it remains loosely bound to the hole in a very large orbit (physicists say it is ‘delocalized’); the arrangement is called an ‘exciton’. The exciton can itself behave in many respects like a quantum particle, with associated wavelike properties, and it is this exciton, not an electron as such, that is passed through the FMO complex. Viewed in terms of pathways, there are many routes the exciton can take and, if quantum coherence is maintained, will take – simultaneously. Loosely speaking, the exciton is able to sift all the options at once and feel out the best possible route to the reaction centre. And then take it. What I am describing is an extraordinary type of demon, a quantum super-demon that ‘knows’ all available pathways at once and can pick the winning one. In more careful physics-speak, the claim is that constructive interference occurs across multiple molecules in the FMO complex so that coherent excitons can optimize the efficiency and deliver the energy to the reaction centre before it can be dissipated into the molecular environment. It takes about 300 femtoseconds (a femtosecond is a thousandth of a trillionth of a second) to get there.
To study this complicated mechanism the Berkeley group used ultra-fast lasers to excite an FMO complex in the lab. They were able to follow the fortunes of the energy as it sashayed through the molecular maelstrom and announced that some sort of ‘quantum beating’ effect – coherent oscillations – did indeed contribute to the high-speed transfer of energy.
The results of these experiments came as a complete surprise because it seemed that the excitons’ carefully balanced dance would be wrecked by thermal agitation. At face value, quantum coherence is maintained for about a hundred times longer than back-of-the-envelope calculations predicted. Although thermal noise was undoubtedly a factor, more recent calculations6 suggest that a little bit of noise can actually be good – that is, it can, paradoxically, boost the efficiency of energy transfer in the right circumstances (doubling it, in this case). And the photosynthetic system seems to have evolved precisely those ‘right circumstances’.
Photosynthesis in plants is more complicated than it is in bacteria, and it is not yet clear whether the quantum effects discovered for the latter are more or less important than in the former. But the Engel–Fleming experiments suggest that quantum-assisted energy transport plays a role in at least one of the basic light-harvesting processes in biology.
SPOOKY BIRDS
‘Doth the hawk fly by thy wisdom, and stretch her wings toward the south?’
– Job 39:26
Although bird navigation had been studied informally for many centuries, it wasn’t until the early 1700s that ornithologists began keeping systematic records. Johannes Leche, professor of medicine at the University of Turku in Finland, noted that the house martin was the first to arrive in those chilly climes – on 6 May, on average – followed by the barn swallow, on 10 May. (I had no idea birds were so punctual.) Direct observation of migratory patterns was later augmented by ringing the birds and, in recent times, tracking by radar and satellites. Today a great deal of information has been gleaned on this extraordinary phenomenon, including some mind-boggling statistics. Arctic terns, for example, can fly more than 80,000 kilometres (49,700 miles) per year, migrating from their breeding grounds in the Arctic all the way to the Antarctic, where they spend the northern winters. The blackpoll warbler, which weighs a mere 12 grams, completes a nonstop flight out over the Atlantic Ocean from New England to the Caribbean, where it spends the winter. Some pigeons reliably find their way home after flights of hundreds of kilometres.
How do these birds do it?
Scientists have discovered that birds use a variety of methods to find their way around, taking account of the orientation of the sun and stars as well as local visual and olfactory cues. But this can’t be the whole story because some birds can navigate successfully at night and in cloudy conditions. Special interest has focused on the Earth’s magnetic field, which is independent of the weather. Experiments with homing pigeons in the early 1970s showed that attaching a magnet to the bird interfered with its ability to orient correctly. But how, exactly, does a bird sense the Earth’s magnetic field, given that it is extremely weak?fn3
A number of physicists claim that it is quantum physics that enables the bird to navigate, by allowing it to see the field. Evidently, there has to be some sort of compass inside the creature, coupled to its brain so it can perform in-flight corrections. Tracking that compass down hasn’t been easy, but in the past few years a plausible candidate has emerged, and it depends on quantum mechanics – in fact, on one of its oddest features.
All fundamental particles of matter possess a property called ‘spin’. The idea of spinning bodies is of course familiar and simple enough – the Earth itself spins. Imagine an electron as a scaled-down Earth, shrunk to a point in fact, but retaining its spin. Unlike planets, every electron has exactly the same amount of spin, as it does electric charge and mass; it is a basic property they have in common. Of course, electrons go round inside atoms too, and in that manner their speed and direction may vary, depending on which atom and which energy level (orbit) they occupy. But the fixed spin I am talking about is intrinsic to the electron, and the full designation is, unsurprisingly, ‘intrinsic spin’.
What has this got to do with birds? Well, electrons also possess electric charge (they are the archetypal electrically charged particle, which is why they are called electrons). As Michael Faraday discovered in 1831, a moving electric charge creates a magnetic field. Even if an electron isn’t moving from place to place, it is still spinning, and this spin creates a magnetic field around it: all electrons are tiny compasses. So, given that electrons are magnetic as well as electric, they will respond to an external magnetic field much as a compass needle does. That is, the electron will feel a force from the external field that will try to twist it so the poles oppose (north–south). There is, however, a complication. Unlike a compass needle, an electron is spinning. When an external force acts on a spinning body, it doesn’t just swing round and line up, it gyrates – a process called ‘precession’. That is, the spin axis itself rotates about the line of the applied force. Readers familiar with inclined spinning tops (which precess about the vertical due to the Earth’s gravity) will know what I mean.
An isolated electron with nothing more to do than feel the force of Earth’s magnetism will execute such a gyration about 2,000 times a second in this case. However, most electrons are employed in atoms, going round and round the nucleus, and the internal electric and magnetic fields of the atom itself, arising from the nucleus and other electrons, swamp the Earth’s feeble field, which has negligible effect by comparison. But if an electron is displaced from the atom, it’s a different story. That can happen if the atom absorbs a photon. The atom’s magnetism weakens rapidly with distance from the nucleus, so the Earth’s field becomes relatively more important for the behaviour of the electron. The ejected electron will therefore gyrate differently.
The bird’s eyes are being assailed by photons all the time – it’s what eyes are for. So here is an opportunity for avian electrons to serve as tiny compasses to steer the bird, but only if there is a way for the bird to know what the ejected electrons are doing. Somehow, the light-disrupted electrons have to engage in some chemistry to send a signal to the bird’s brain with information about their activities. The bird’s retina is packed with organic molecules; researchers have zeroed in on retinal proteins dubbed ‘cryptochromes’ to do the job I am describing.7 When a cryptochrome electron is ejected by a photon, it doesn’t cut all its links with the molecule it used to call home. This is where Einstein’s spooky action-at-a-distance comes in, used here in the service of the bird. The electron, though ejected from its atomic nest, can still be entangled with a second electron left behind in the protein atom, but, because of their different magnetic environments, the two electrons’ gyrations get out of kilter with each other. This state of affairs doesn’t last for long; the electron and the positively charged molecule (called a free radical) left behind are stand-out targets for chemical action. (The finger of blame for many medical conditions from diabetes to cancer is pointed at free radicals running amok within cells.) According to the theory of the avian compass, these particular free radicals react either with each other (by recombining), or with other molecules in the retina, to form neurotransmitters, which then signal the bird’s brain. This neuro-transmission reaction rate will vary according to the specifics of the spooky link and its mismatched gyrations of the two electrons, which is a direct function of the angle between the Earth’s magnetic field and the cryptochrome molecules. So in theory, the bird might actually be able to see the magnetic field imprinted on its field of vision. How useful!
Is there any evidence to support this spooky-entanglement story? Indeed there is. A research group at the University of Frankfurt has experimented with captive European robins, which migrate from Scandinavia to Africa, and shown that their direction-finding abilities definitely depend on the wavelengths and intensity of the ambient light, as the theory predicts.8 Their experiments suggest that the birds combine visual and magnetic data when making decisions on which way to go. The Frankfurt group also tried doubling the ambient magnetic field strength. This initially disrupted the bird’s directional sense, but the clever little creatures sorted it all out in about an hour and somehow recalibrated their magnetic apparatus to compensate.
The real clincher came with experiments done at UC Irvine by Thorsten Ritz, in which the birds were zapped by radio frequency (MHz) electromagnetic waves. Beaming the waves parallel to the geomagnetic field had no effect, but when they were beamed vertically the birds became confused.9 Combining the results of many experiments with different frequencies and ambient light conditions shows the presence of a resonance – a familiar phenomenon in which the energy absorbed by a system spikes at a certain frequency, for example, the opera singer who shatters a wine class when striking the right note. A resonance is exactly what one would expect if the quantum explanation is right, because the radio waves are tuned to typical transition frequencies for organic molecules and would likely interfere with the formation of the all-important spooky entanglement.
The era of quantum ornithology has arrived!
QUANTUM DEMONS UP YOUR NOSE
The sense of smell could provide another terrific example of biological quantum demons at work. Even humans, who don’t rank very highly on the scale of olfactory prowess, can distinguish very many different odours. A skilled perfumer (called ‘a nose’ in the trade) can discriminate between hundreds of subtly different fragrances with a discernment comparable to that of a master wine taster.
How does it work? The basic story is this. Inside the nose are legions of molecular receptors – molecules sporting cavities of many different specific shapes. If a molecule in the air has a complementary shape, it will bind to the corresponding receptor, like a lock and key. Once the docking process happens, a signal is sent to the brain: ‘Chanel No. 5!’ or similar. Of course, I’m simplifying: odour identification usually involves combining signals from several different receptors. Still, it is clear that olfactory receptors behave like classic Maxwell demons – they sort molecules very precisely by their shapes (rather than speeds – same basic idea) and reject the rest, thus filtering and communicating the information to the brain for the benefit of survival (admittedly, probably not in the case of Chanel, but detecting smoke might qualify).
However, the simple lock-and-key model clearly has shortcomings. Molecules of similar size and shape can smell very different. Conversely, very different molecules can smell similar. It is all very enigmatic. Evidence points to a finer level of discrimination – a demon with sharper senses. An old idea – decades old, in fact – is that, in addition to a molecule’s size and shape, its vibrational signature might come into the story. Molecules can (and do) wobble around (thermal agitation, remember), and just as musical instruments have distinctive tones produced by the specific admixture of harmonics, so too do the vibrational patterns of molecules. A buffeted airborne molecule will thus arrive at its nasal docking station jittering about, and a receptor designed to ‘pick up the vibes’ would provide a useful additional level of discrimination. The mechanism was left vague, however, until 1996, when Luca Turin, then at University College London, proposed that quantum mechanics might be at play; specifically, quantum tunnelling of an electron from odorant molecule to receptor.10 Turin proposed that the tunnelling electron is coupled to the vibrational states of the odorant molecule (that’s a routine mechanism in molecular physics), and further, that the electron energy levels in the receptor molecule are tuned to specific vibrational frequencies of the odorant molecule. The electron that tunnels serves to communicate the docking molecule’s identity by absorbing a quantum of energy from the vibration (known to physicists as a phonon – a quantum of sound) and delivering it to the receptor. If the electron’s energy matches the receptor’s energy level structure, tunnelling is facilitated and a metaphorical light goes on in the nose.
Turin’s proposal gave a boost to the vibration theory of smell and offered a possible explanation for otherwise puzzling similarities and differences in smells – it’s all down to vibrational patterns rather than the shapes of molecules as such. The theory also offered the advantage of being testable. One check is to try to alter the vibrational modes of the odorant molecule while leaving its chemistry (and also its shape) unchanged. That can be done by substituting various atoms for their isotopes. For example, deuterium, whose nucleus consists of a proton and a neutron, is about twice as heavy as normal hydrogen but chemically identical. Switching a hydrogen atom for a deuterium atom will leave the molecular shape the same but it will alter the vibrational frequencies in an obvious way: heavier atoms move more slowly for the same energy, so vibrate at lower frequencies. And experiments did indeed seem to confirm that the act of deuterating molecules changes the smell, but the results remain controversial and ambiguous.11 More recently, Turin did the experiment with fruit flies and discovered that they can distinguish between an odorant molecule containing hydrogen and the same molecule containing deuterium. The experimenters also trained the insects to avoid the deuterated molecules, and found that the flies also steered clear of an unrelated molecule with vibrational modes matching that of the deuterated odorant. All this bolsters the theory that quantum tunnelling of vibrational information is key to how flies smell, at least.
QUANTUM BIOLOGY: HERE TO STAY?
For almost a century, quantum mechanics was like a Kabbalistic secret. But today – largely because of quantum computing – the Schrödinger’s cat is out of the bag, and all of us are being forced to confront the exponential Beast that lurks in the current picture of the world.
– Scott Aaronson12
Niels Bohr once remarked that anyone who isn’t shocked by quantum mechanics hasn’t understood it. And shocking it is. While quantum mechanics explains matter brilliantly, it shreds reality. The words ‘quantum’ and ‘weird’ inevitably go together. Weird like being in two places at once, or being teleported through barriers or visiting parallel worlds – things that would be utterly bizarre if they happened in daily life. But they occur all the time in the micro-world of atoms and molecules. With so much quantum magic on offer, you’d expect life to be on to it. And it is! As I have described in this chapter, in the last few years evidence has grown to suggest that several important biological processes might be exploiting some aspects of quantum weirdness. They offer tantalizing hints that quantum magic could be all over life. If quantum biology amounts to more than a handful of quirky phenomena, it could transform the study of life as profoundly as molecular biology has done over the past half-century.
When Schrödinger delivered his famous Dublin lectures, quantum mechanics was newly triumphant. It had explained many of the properties of non-living matter. Moreover, it seemed to many physicists of the day that quantum mechanics was sufficiently powerful and sufficiently weird to be able to explain living matter too. In other words, it was hoped that quantum mechanics, or possibly some new ‘post-quantum mechanics’ still to be worked out, might embed a type of ‘life principle’ hitherto concealed from us by the sheer complexity of living matter. In his lectures Schrödinger did make use of some routine technical results in quantum mechanics to address the question of how biological information can be stored in a stable form, but he didn’t attempt to invoke the sort of weird quantum effects I have described in this chapter to explain life’s remarkable properties.
In the decades that followed, few biologists paid much attention to quantum mechanics, most being content to appeal to classical ball-and-stick models of chemistry to explain everything in biology. But in the last few years there has been a surge of interest in quantum biology, although some of the more extravagant claims have given the subject a somewhat suspect status. The key question is whether, if there are indeed non-trivial quantum shenanigans going on in living matter, they are just quirky anomalies or the tip of a quantum iceberg that encompasses all life’s vital processes. The case studies I have described by no means exhaust all the possible quantum biology effects that have been investigated. The fundamental problem, as will be apparent from the tortuous explanations I have given, is that biology is bewilderingly complex. There is plenty of room for subtle quantum effects to lurk within that complexity but, conversely, there is plenty of room for simple quantum theoretical models to mislead.
The problem in making a case for quantum biology is that the two words, ‘quantum’ and ‘biology’, describe domains in tension. Quantum effects are most conspicuous in isolated, cold, simple systems, whereas biology is warm and complex with lots of strongly interacting parts. Quantum mechanics is all about coherence. External disturbances are the enemy of coherence. But as I have explained in the earlier chapters, life loves noise! Biology’s demons harness thermal energy to create and to move. Living matter is full of commotion; molecules mill around and bang into each other continually, hook up and shake each other, exchange energy, rearrange their shapes. This pandemonium can’t be shut out in live organisms, as it can be in the carefully controlled environment of a physics lab. Nevertheless, there is a fertile middle ground in which noise and quantum coherence coexist for long enough for something biologically useful to happen.13
Quantum biology is not just of interest in explaining life, it could also teach quantum engineers some very lucrative tricks. The main focus of quantum engineering today is quantum computing. Consider this statistic. A quantum computer with just 270 entangled particles (entangled = spookily linked) could deploy more information-processing power than the entire observable universe harnessed as a conventional (bit-manipulating) computer. That’s because a quantum computer’s power rises exponentially with the number of entangled components, so a mere 270 entangled subatomic particles have 2270 states, which is about 1081 (compare 1080 atomic particles in the universe). If all those states could be manipulated, it would yield godlike computational power. If a tiny collection of particles has the potential to process mind-numbing amounts of information, would we not expect to see such processing manifested somewhere in nature? And the obvious place to look is biology.
Several years ago there were claims that the molecular machinery implementing the genetic code might be a type of quantum computer.14 Although there is little supporting evidence that DNA executes true quantum computation, it is possible that some form of quantum-enhanced information processing is going on. Maxwell’s demon evades the degrading effects of entropy and the second law by turning random thermal activity into stored bits of information. A quantum Maxwell demon could stave off the same degrading thermal effects that destroy quantum coherence and turn random external noise into stored qubits. If life has evolved such demons able to preserve quantum coherence long enough for the genetic machinery to manipulate the stored qubits, then significant information-processing speed-up might occur. Even a slight boost would confer an advantage and be selected by evolution.
Nevertheless, I should finish this chapter on a cautionary note. All of the putative quantum biology effects I have discussed have been hotly debated.15 Some early claims were overblown, and more experiments are needed before any definitive conclusions can be drawn. The complexity of biological systems often precludes any simple way to untangle wavelike quantum effects from familiar classical vibrational motion, leaving most of the experiments done so far open to alternative interpretations. The jury, it seems, is still out.16
What about the speculation that biology implements actual quantum computation? A long while ago, when dwelling on the profundity of quantum computation, I was struck by a curious thought. It is difficult to imagine any non-living, naturally occurring system doing quantum computation, so I was prompted to ask why, if life is not availing itself of this opportunity for exponentially enhanced information processing, the possibility even exists. Why do the laws of physics come with informational capabilities beyond anything that Shannon imagined, if nature hasn’t made use of it anywhere in the universe? Has this untapped informational potential sat unexploited by nature for 13.8 billion years just for human engineers to cash in on?
I fully realize that what I have just written is in no sense a scientific argument; it is a philosophical (some might say theological) one. I raise it because in my experience as a theoretical physicist I have found that if well-established physical theory predicts that something is possible, then nature invariably seems to make use of it. One need think no further than the Higgs boson, predicted by theory in 1963 and discovered really to exist in 2012. Other examples include antiparticles and the omega minus particle. In all cases there was a well-defined place for such a thing in nature and, sure enough, they are out there. Of course, there are many speculative theories that make predictions not borne out by experiment, so my argument is only as good as the reliability of the theory concerned. But quantum mechanics is the most reliable theory we have, and its predictions are almost never questioned. Quantum mechanics has a place for exponential godlike information management; has nature overlooked to fill it? I don’t think so.