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The quantum world is a weird one. In theory and to some extent in practice its tenets demand that a particle can appear to be in two places at once—a paradoxical phenomenon known as superposition—and that two particles can become “entangled,” sharing information across arbitrarily large distances through some still-unknown mechanism.
Perhaps the most famous example of quantum weirdness is Schrödinger’s cat, a thought experiment devised by Erwin Schrödinger in 1935. The Austrian physicist imagined how a cat placed in a box with a potentially lethal radioactive substance could, per the odd laws of quantum mechanics, exist in a superposition of being both dead and alive—at least until the box is opened and its contents observed.
As far-out as that seems, the concept has been experimentally validated countless times on quantum scales. Scaled up to our seemingly simpler and certainly more intuitive macroscopic world, however, things change. No one has ever witnessed a star, a planet or a cat in superposition or a state of quantum entanglement. But ever since quantum theory’s initial formulation in the early 20th century, scientists have wondered where exactly the microscopic and macroscopic worlds cross over. Just how big can the quantum realm be, and could it ever be big enough for its weirdest aspects to intimately, clearly influence living things? Across the past two decades the emergent field of quantum biology has sought answers for such questions, proposing and performing experiments on living organisms that could probe the limits of quantum theory.
Those experiments have already yielded tantalizing but inconclusive results. Earlier this year, for example, researchers showed the process of photosynthesis—whereby organisms make food using light—may involve some quantum effects. How birds navigate or how we smell also suggest quantum effects may take place in unusual ways within living things. But these only dip a toe into the quantum world. So far, no one has ever managed to coax an entire living organism—not even a single-celled bacterium—into displaying quantum effects such as entanglement or superposition.
So a new paper from a group at the University of Oxford is now raising some eyebrows for its claims of the successful entanglement of bacteria with photons—particles of light. Led by the quantum physicist Chiara Marletto and published in October in the Journal of Physics Communications, the study is an analysis of an experiment conducted in 2016 by David Coles from the University of Sheffield and his colleagues. In that experiment Coles and company sequestered several hundred photosynthetic green sulfur bacteria between two mirrors, progressively shrinking the gap between the mirrors down to a few hundred nanometers—less than the width of a human hair. By bouncing white light between the mirrors, the researchers hoped to cause the photosynthetic molecules within the bacteria to couple—or interact—with the cavity, essentially meaning the bacteria would continuously absorb, emit and reabsorb the bouncing photons. The experiment was successful; up to six bacteria did appear to couple in this manner.
Marletto and her colleagues argue the bacteria did more than just couple with the cavity, though. In their analysis they demonstrate the energy signature produced in the experiment could be consistent with the bacteria’s photosynthetic systems becoming entangled with the light inside the cavity. In essence, it appears certain photons were simultaneously hitting and missing photosynthetic molecules within the bacteria—a hallmark of entanglement. “Our models show that this phenomenon being recorded is a signature of entanglement between light and certain degrees of freedom inside the bacteria,” she says.
According to study co-author Tristan Farrow, also of Oxford, this is the first time such an effect has been glimpsed in a living organism. “It certainly is key to demonstrating that we are some way toward the idea of a ‘Schrödinger’s bacterium,’ if you will,” he says. And it hints at another potential instance of naturally emerging quantum biology: Green sulfur bacteria reside in the deep ocean where the scarcity of life-giving light might even spur quantum-mechanical evolutionary adaptations to boost photosynthesis.
There are many caveats to such controversial claims, however. First and foremost, the evidence for entanglement in this experiment is circumstantial, dependent on how one chooses to interpret the light trickling through and out of the cavity-confined bacteria. Marletto and her colleagues acknowledge a classical model free of quantum effects could also account for the experiment’s results. But, of course, photons are not classical at all—they are quantum. And yet a more realistic “semiclassical” model using Newton’s laws for the bacteria and quantum ones for photons fails to reproduce the actual outcome Coles and his colleagues observed in their laboratory. This hints that quantum effects were at play in both the light and the bacteria. “It’s a little bit indirect, but I think it’s because they’re only trying to be so rigorous in ruling out things and claiming anything too much,” says James Wootton, a quantum computing researcher at IBM Zurich Research Laboratory who was not involved in either paper.
The other caveat: the energies of the bacteria and the photon were measured collectively, not independently. This, according to Simon Gröblacher of Delft University of Technology in the Netherlands who was not part of this research, is somewhat of a limitation. “There seems to be something quantum going on,” he says. “But…usually if we demonstrate entanglement, you have to measure the two systems independently” to confirm any quantum correlation between them is genuine.
Despite these uncertainties, for many experts, quantum biology’s transition from theoretical dream to tangible reality is a question of when, not if. In isolation and collectively, molecules outside of biological systems have already exhibited quantum effects in decades’ worth of laboratory experiments, so seeking out these effects for similar molecules inside a bacterium or even our own bodies would seem sensible enough. In humans and other large multicellular organisms, however, such molecular quantum effects should be averaged out to insignificance—but their meaningful manifestation within far smaller bacteria would not be too shocking. “I’m a little torn about how surprising [this finding] is,” Gröblacher says. “But it’s obviously exciting if you can show this in a real biological system.”
Several research groups, including those led by Gröblacher and Farrow, are hoping to take these ideas even further. Gröblacher has designed an experiment that could place a tiny aquatic animal called a tardigrade in superposition—a proposition much more difficult than entangling bacteria with light owing to a tardigrade’s hundreds-fold–larger size. Farrow is looking at ways to improve on the bacterial experiment; in the next year he and his colleagues hope to entangle two bacteria together, rather than independently with light. “The long-term goals are foundational and fundamental,” Farrow says. “This is about understanding the nature of reality, and whether quantum effects have a utility in biological functions. At the root of things, everything is quantum,” he adds, with the big question being whether quantum effects play a role in how living things work.
It might be, for example, that “natural selection has come up with ways for living systems to naturally exploit quantum phenomena,” Marletto notes, such as the aforementioned example of bacteria photosynthesizing in the light-starved deep sea. But getting to the bottom of this requires starting small. The research has steadily been climbing toward macrolevel experiments, with one recent experiment successfully entangling millions of atoms. Proving the molecules that make up living things exhibit meaningful quantum effects—even if for trivial purposes—would be a key next step. By exploring this quantum–classical boundary, scientists could get closer to understanding what it would mean to be macroscopically quantum, if such an idea is true.
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