Two decades ago, scientists seemed on the verge of understanding biology in a new, quantum way.
Life unfolds over an incomprehensible span of scales, from our planet-enveloping biosphere at one end, to individual cell-building biomolecules at the other. Even at its most microscopic, though, biology doesn’t really reach down to the quantum realm, in which particles act like waves, become entangled with one another, and exist in superpositions of multiple states at once. But scientists in the field of quantum biology are searching for ways that organisms might be able to push quantumness into the space, time, and temperature domains relevant to life, to make use of its strange properties.
In photosynthesis, for example, organisms use specialized pigments and proteins to harvest light with nearly perfect quantum efficiency; they convert almost every incoming photon into useful chemical energy. In 2007, new evidence suggested that life might accomplish this feat by taking advantage of a quantum effect called coherence. The result buoyed the controversial idea that, despite being a warm, wet, and decidedly classical environment, a living cell could maintain — and even exploit — fragile quantum states.
Gregory Scholes, a chemist at Princeton University, was initially enthusiastic about the result. He and colleagues followed up with experiments on photosynthesizing proteins and pigments and came away with similar conclusions. But today, Scholes is skeptical that quantum effects play a role in life. In fact, he’s convinced that the way forward for quantum biology might not be quantum at all. Rather than taking advantage of genuine quantum effects, Scholes proposes, life might be imitating them instead. In several papers published over the past three years, Scholes and colleagues have shown that complex networks of classical objects can conspire to produce phenomena that mathematically mimic quantum objects.
Don’t be fooled: The states that these networks produce are not truly quantum; they’re only “quantumlike.” They arise when many interacting, oscillating parts add up to a collective whole whose behavior obeys the same mathematics that makes predictions about the quantum world.
“Maybe quantum biology, at the biggest scales, means using 3 1/2 billion years of evolution to work out how to get the functionality that you could get from quantum systems,” Scholes said.
Researchers in the foundations of quantum mechanics have been exploring how to classically re-create certain aspects of the quantum world for decades, said Markus Müller, a physicist at the Institute for Quantum Optics and Quantum Information in Vienna. What Scholes has done, Müller said, is show how quantumlike behavior can emerge from relatively unremarkable complex networks — of the sort that abound in nature.
“Classical systems can mimic some of the key features of quantum information,” said Sabre Kais, a quantum chemist developing quantum computing algorithms for complex systems at North Carolina State University. “This is an exciting new direction.”
The Bounds of Quantum Biology
The temptation to recast life’s mysteries as quantum in nature is about as old as quantum mechanics itself. In a 1929 lecture, the quantum pioneer Niels Bohr made the vague but enticing statement that quantum mechanics, which at the time was just starting to find its mathematical footing, “may perhaps be of decisive importance, particularly in the discussion of the position of living organisms in our picture of the world.”
Bohr’s contemporary Pascual Jordan spent several decades writing on Quantenbiologie, or quantum biology, arguing that life has a unique ability to amplify the strange indeterminism of the quantum world to macroscopic scales, and claiming this as the basis of human thought and free will. J.B.S. Haldane, a geneticist and evolutionary biologist, echoed Jordan in a 1934 paper arguing that the ability to scale up quantum indeterminacy was what made life special. (Jordan, who joined the Nazi Party and its paramilitary forces in 1933, damaged the credibility of quantum biology by attempting to link it to Nazism.)
These early proponents of quantum biology sought explanations for the puzzling properties of life at the classical scale in the counterintuitive laws of physics at the quantum scale. A classical particle can be in only one place, in one way, at a time; a quantum particle is smeared out across all the places and ways it could potentially be, in a kind of wave of possibility. This smear is described mathematically by a wave function which, like a classical wave, has peaks and troughs.
Until a particle is observed, all of its possible configurations effectively exist at once. They’re stacked up in superposition, like overlapping waves of water or sound. And like overlapping waves, stacked quantum states add up, cancel one another out, or otherwise transform each other. When quantum states in superposition have tidy enough wave properties to affect each other this way, they’re called coherent. Coherent quantum systems can also become entangled with each other, essentially merging into a single, unified entity with a shared wave function.
Quantum states are delicate things; they’re easily destroyed when jostled against the outside world. Even a tiny amount of environmental noise, like the atomic jiggle of heat, can trigger decoherence, a collapse into classical behavior. (Quantum computers aren’t supercooled just for fun.) And in the interior of a cell, decoherence should be basically instantaneous — quantumness shouldn’t survive long enough in such an environment to have any bearing on biology.
It’s true that, even in a cell, very, very small particles like hydrogen atoms can “quantum tunnel,” popping up beyond energetic barriers that would otherwise slow them down or prevent them from crossing. And there’s evidence that tunneling in certain enzymes could explain their speedy reaction rates. But that’s not really what quantum biologists are after, Scholes said. There’s no long-lived coherence involved in this kind of tunneling. The quantumness involved is simple, fleeting, and kind of unavoidable — even in a flask of dead chemicals, some tiny particles will tunnel through energetic barriers. The question is whether life can do what dead chemistry can’t: hold quantum states in coherence long enough to use coherence itself as a resource.
A Light-Harvesting Letdown
Scientists first toyed with the idea that quantum coherence could explain the remarkable efficiency of photosynthesis as early as the 1930s.
Photosynthetic organisms use arrays of pigments and proteins called light-harvesting complexes to soak up light. When a photon strikes one of these complexes, its electromagnetic energy is absorbed and boosts an electron in the complex into an excited state. This quasiparticle — called an exciton — heads toward a reaction center, where it is transformed into chemical energy that can perform the key steps of photosynthesis.
