A quantum computer is a serious piece of hardware. My colleagues and I build quantum computers from superconducting systems, quantum dots, lasers operating on nonlinear crystals, and the like. Although the part of a quantum computer that actually performs the calculation is too small to be seen even under a microscope, the apparatus used to address and control the quantum computer typically takes up an entire laboratory full of equipment. In order to keep their sensitive components shielded from the environment, many quantum computers have to operate at very low temperatures, sometimes a few thousandths of a degree above absolute zero.
So in the spring of 2007 when the New York Times reported that green sulphur-breathing bacteria were performing quantum computations during photosynthesis, my colleagues and I laughed. We thought it was the most crackpot idea we had heard in a long time. Closer examination of the paper , published in Nature, however, showed that something decidedly non-crackpot was going on.
Photosynthesis converts light from the Sun into chemically useful energy inside cells. In photosynthesis, particles of light called photons are absorbed by light-sensitive molecules called chromophores (“light carriers” in ancient Greek), which are arranged in a tightly bound structure called an antenna photocomplex. When a photon is absorbed, a quantum particle of energy called an exciton is generated. (An exciton isn’t a particle in the traditional sense, but it acts enough like a particle that physicists find it useful to treat it as one. Such mathematical likenesses are called “quasi-particles.”) The exciton hops from chromophore to chromophore inside the photocomplex until it arrives at the reaction center, an agglomeration of molecules that take in the exciton and transform its energy into a form that the living system can put to use to perform cellular metabolism, grow, and reproduce. The great majority of the energy used by living systems once came from photosynthesis: Every calorie that you consume came originally from excitons that hopped through the antenna photocomplex of a photosynthetic organism.
By zapping complexes of photosynthetic molecules with lasers, the authors of the paper were able to show that the excitons use quantum mechanics to make their journey through the photocomplex more efficient. The experimental evidence was strong and compelling. The authors also speculated that the excitons were performing a particular quantum computation algorithm called a quantum search, in which the wave-like nature of propagation allows the excitons to zero in on their target. As it turns out, the excitons were performing a different kind of quantum algorithm called a quantum walk, but the “crackpot” fact remained: Quantum computation was helping the bacteria move energy from point A to point B.
How could tiny bacteria be performing the kind of sophisticated quantum manipulations that it takes human beings a room full of equipment to perform? Natural selection is a powerful force. Photosynthetic bacteria have been around for more than a billion years, and during that time, if a little quantum hanky panky allowed some bacteria to process energy and reproduce more efficiently than other bacteria, then quantum hanky panky stuck around for the next generation. Nature is also the great nanotechnologist. Living systems operate on the basis of molecular mechanisms, where atoms and energy are channeled systematically through molecular complexes within the cell. The molecules in turn are assembled using the laws of quantum mechanics—quantum weirdness is always lurking just around the chemical corner. These quantum changes can either help or hinder energy transport. Natural selection ensures that the role of quantum weirdness in cellular energy transport is a beneficial one.
How can quantum weirdness assist in energy transport? The answer lies in a phenomenon called wave-particle duality. Wave-particle duality means that waves—light and sound—are at bottom composed of particles—photons and phonons. Conversely, things that we think of as particles, such as electrons, atoms, or for that matter soccer balls, have waves associated with them.
The quantum wave of a soccer ball is about the same size as the ball itself, and doesn’t extend halfway down the soccer field (although the ball can sometimes seem to be on Lionel Messi’s left and right foot at the same time). But the wave corresponding to a particle can be much larger than the particle itself. While a single exciton consists of an excited electron within a chromophore, the wave corresponding to a propagating exciton can extend over many chromophores.
Does that make sense? Of course not! Quantum mechanics is fundamentally strange and counterintuitive: Quantum particles don’t behave like soccer balls.
To see how the wavelike nature of excitons can assist in their propagation, first visualize a classical kind of exciton dynamics. Imagine that each chromophore is a lilypad, and the exciton is a frog hopping randomly between neighboring lilypads. The frog starts at the edge of the circular lilypond. How long does it take to get to the pond’s center? Because the frog is hopping from lilypad to lilypad at random, it sometimes moves towards the center of the pond, but it is equally likely to move to the left or right, or even backward. The frog will land on a substantial fraction of all the lilypads in the pond on its way to the center. The number of hops the frog has to take to get to the center is proportional to the number of lilypads in the pond.
Now consider a quantum frog. The frog’s initial wave is circularly symmetric at the edge of the lilypond and propagates inward, like a backward version of the wave created when you drop a stone in the center of a pond. The time it takes for the wave to travel from the shore to the center of the pond is proportional to the radius of the lilypond. But the radius of the lilypond goes as the square root of the number of lilypads in the pond, because the number of lilypads is proportional to the area of the pond, i.e., the radius squared. The wavelike nature of propagation in quantum mechanics—the “quantum hop”—has the potential to get the frog to the center of the pond much more quickly than the “classical hop.” So, for example, if the frog hops once a minute, and a classical frog takes 100 minutes to get to the center, then the quantum frog takes only 10 minutes.
In green sulphur-breathing bacteria, the antenna photocomplex through which the excitons propagate is like the lilypond for the quantum frog: The waves corresponding to the excitons are spread out over many chromophores, and wavelike propagation allows excitons to move more quickly from chromophore to chromophore than classical hopping would allow.
Together with Alan Aspuru-Guzik and Patrick Rebentrost at Harvard, my MIT colleague Masoud Mohseni and I constructed a general theory of how quantum walks in photosynthesis can use the wavelike nature of quantum mechanics to attain maximum efficiency . It turns out that wavelike transport is not always the best strategy. To understand why, suppose that the lilypond is full of rocks sticking up out of the water. As the wave moves through the pond, it scatters off the rocks. As a result, the wave never reaches the middle of the pond, which remains calm and protected. This is a phenomenon called destructive interference. Although the wave can propagate a short distance, eventually the random waves scattered off the rocks interfere with the overall wave’s propagation, effectively stopping it in its tracks. The quantum frog becomes completely stuck: A classical hopping strategy would have been more efficient. In the antenna photocomplex, the “rocks” are microscopic irregularities and molecular disorder that scatter the quantum wave as it tries to pass through.
By constructing detailed quantum mechanical models, my collaborators and I were able to identify the optimal strategy for the interplay between wavelike propagation and classical hopping in photosynthesis. Over short distances, the wavelike propagation is more effective than random hopping. The exciton travels like a wave right up to the distance at which destructive interference causes it to get stuck. At this point, the fact that living systems are hot, wet environments comes into play: The environment effectively gives the exciton a whack that gets it unstuck and makes it perform a classical hop, which frees up the exciton to propagate again. (The technical term for this whack is “decoherence.”) Then the process repeats. The wave propagates until it gets stuck; the environment gives it a whack; the exciton hops. Eventually, the exciton reaches the reaction center in the minimum possible time. Expressed in terms of our quantum frog, the rule is simple: Wave until you get stuck, then hop.
The birds, the bees, and the fruit flies
Photosynthetic plants and bacteria are masters of the minutiae of quantum mechanics, manipulating quantum coherence and decoherence to attain almost 100% energy transport efficiency. If quantum hanky panky is so effective, are there other living beings that take advantage of quantum effects to live better and have more offspring? Only in the case of photosynthesis have scientists actually found the smoking gun (or maybe the smoking photon). However, there are several other organisms in which quantum mechanisms apparently play an important role.
European robins are sensitive to the Earth’s magnetic field, which helps them during migration. Do they have a tiny compass in their heads, a piece of magnetite that swivels back and forth to point out magnetic north? Apparently not. Instead the evidence suggests another light-activated quantum mechanism. A photon excites an electron, which swivels around in the Earth’s magnetic field; the rate at which the electron decays from its excited state depends on how far it has swiveled. Since the robins need light to detect magnetic north, the next time you see a flock of them stumbling around at night, you know what is going on.
I smell a quantum
Quantum mechanics may also be involved in the sense of smell. Scientists have long believed that smell operates via a lock and key mechanism, an idea Linus Pauling first proposed more than 50 years ago. The molecule to be sniffed—the “odorant”—locks into a receptor in the olfactory apparatus that can only fit that particular key. The receptor then unlocks or changes its configuration, leading to a flow of ions sufficiently large to trigger a neuron to fire.
The problem with this model is that olfactory receptors are not very specific: They can be unlocked by many different keys. This has led some researchers to propose that the receptors are sensitive not only to the shape of the odorant molecule, but to its vibrational frequencies—that is, its “sound.” The combination of shape and sound provides a unique signature for the molecule. For the vibrational theory of smell to hold, however, the underlying dynamics of the molecule in the receptor must be intrinsically quantum mechanical: The receptor must respond to individual phonons—quasi-particles of sound—generated by the molecule. Experiments show that fruit flies are indeed sensitive to vibrational frequencies of molecules, supporting the hypothesis of quantum smell. When trained to sense a molecule with a particular vibrational frequency they are attracted to it like flies to…phonons.
Who is quantum?
Where else might quantum mechanics play a role in life? Because light is made up of photons, interactions between living systems and light represent a good place to look. Our eyes are capable of detecting single photons by a highly quantum mechanical mechanism: A molecule in the retina absorbs a single photon, and uses its energy to release the flow of tens of thousands of ions, stimulating a neural response. Neural impulses in the brain are probably too coarse and classical to support the wave-like quantum dynamics that hold sway in photosynthesis, but at the level of individual synapses, the neurotransmitter binding mechanism might well benefit from the same types of quantum dynamics that apparently enhance smell.
As scientists delve deeper into the details of molecular dynamics in living systems, they are likely to see more examples of quantum mechanics at work. We don’t yet know exactly what aspects of biology benefit from quantum mechanics. But we do know one thing: The unquantized life is not worth living.
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In this 90-minute webcast, Seth Lloyd, Thorsten Ritz, and Paul Davies discuss the intersection of biology and quantum mechanics.