How does our predictable everyday world emerge from the hazy, probabilistic rules of quantum mechanics? That puzzle has vexed physicists since quantum theory emerged in the 1920s to explain the behavior of atoms and other infinitesimal things. Now, researchers fiddling with an electron trapped in a diamond have confirmed an essential prediction of a theory that strives to explain this transition from the quantum to classical realm. Called "quantum Darwinism," the theory argues that classical states are simply the quantum ones that are most fit to survive interactions with their environment.
The experimenters "have a beautiful demonstration in a natural environment," says Mauro Paternostro, a theorist at Queen's University in Belfast who was not involved in the work. Physicists caution, however, that the new study is far from the final word on the matter.
Atomic-scale objects behave differently from larger ones. Your coffee cup must be one place or another, but an electron can be in two places at once, in a so-called superposition state. Your cup's position and momentum also exist regardless of whether anyone observes them. Not so for an electron: If you know its position, then its momentum must be undefined, and vice versa, and which attribute the particle has emerges only when it is measured. The measurement "collapses" the electron's quantum state, or wave function—although how that happens remains obscure. Similarly, physicists assume a coffee cup cannot be here and there because something has collapsed its superposition one way or the other—perhaps contact with the surroundings.
Quantum Darwinism claims the truth is more subtle. In the 1980s, Wojciech Zurek, a theorist at Los Alamos National Laboratory in New Mexico, argued that the wave function of a here-and-there cup would inevitably meld with those of surrounding objects. That "entanglement" wouldn't collapse the cup's wave function, but it would obscure the exact relationship between the here and there parts of its quantum state. In quantum theory, that's enough to put the cup in one place or the other.
The right kind of entanglement is key to the theory. The cup must interact with the environment in a way that depends on position rather than, say, momentum. But Zurek says most interactions between a big object and its environment depend on its location. Whether a cup reflects photons into your eye depends on where it is, he notes.
The argument explained why any one observer sees the cup in just one spot, but not why everyone sees the same part of the quantum state and agrees on the location. So in the 2000s, Zurek expanded his idea. As a coffee cup's wave function entangles with more things, including observers, it splits into "here" and "there" branches. All observers' quantum states are entangled with the cup, so everyone agrees on its location. But the argument suggests a radical conclusion: Even if observers agree the cup is "here," the gigantic "there" branch persists, unrealized, like a parallel world.
The classical states of the cup are thus the ones that survive the interaction with the environment to imprint themselves repeatedly on the world—like a highly successful gene or species. "Quantum Darwinism takes this wishy-washy wave function and makes it solid in a sort of bureaucratic way," Zurek says. "You just make a whole bunch of copies of the information."
Now, Fedor Jelezko, a physicist at Ulm University in Germany, Zurek, and colleagues have demonstrated that redundant imprinting of information on the environment. Using laser light and microwaves, they manipulated an electron trapped in an atomic-scale defect in a diamond. The electron acts like a quantum magnet that can point up, down, or both ways, and it naturally interacts with a handful of surrounding magnetic carbon-13 nuclei, which make up about 1% of the carbon in the diamond. Polling the nuclei, the researchers found that the electron redundantly imprinted its state on them, just as quantum Darwinism predicts, they report in a paper in press at Physical Review Letters.
Others have performed similar experiments with photons. But photons don't physically interact and can't imprint their quantum states on each other. So researchers must cook up the entanglement through artificial means. Jelezko's experiment relies on a real physical interaction, says Thao Le, a theorist at University College London (UCL). "It's measuring a much more natural system," Le says.
Does the experiment prove that quantum Darwinism explains the emergence of classical reality? Others say that's going too far. Theorists have tried to bridge the quantum-classical divide in other ways, for example, by trying to derive the rules of quantum mechanics backward, from the behavior of everyday objects. "I don't think this experiment can discriminate between these ideas," says Philippe Grangier, a physicist at the University of Paris-Saclay. Alexandra Olaya-Castro, a theorist at UCL, adds that quantum Darwinism doesn't account for all of the differences between the quantum and classical realms.
Still, the experiment may be a harbinger of things to come, Paternostro says. As experimenters gain control over quantum systems, they will devise new tests for the origin of classical behavior, he says. "I don't think we'll have to wait too long to get some results that will rule out or confirm some of these ideas."