We have a new way to explain why we agree on the nature of reality

Our world appears to be essentially a blur at the quantum level, but we don’t experience it that way. Researchers have now developed a recipe for measuring how quickly the objective reality we experience emerges from this obscurity, strengthening the case that an evolutionary-inspired framework can explain why it arises in the first place.

In the quantum realm, each object—such as a single atom—exists in a cloud of possible states and occupies a well-defined or “classical” state only after it has been measured or observed. But we observe strictly classical objects without existentially fuzzy parts, and the mechanism that does this has long puzzled physicists.

In the year 2000, Wojciech Zurek at Los Alamos National Laboratory in New Mexico proposed “quantum Darwinism” where a process similar to natural selection would ensure that the states of the objects we see are those that are “fittest” among all of the many states that could exist, and therefore best replicated through their interactions with the environment en route to the observer. When two observers who only have access to fragments of physical reality agree on something objective, it is because they are both observing one of these identical copies.

Steve Campbell at University College Dublin and his colleagues have now shown that different observers are likely to agree on an objective reality, even if the way they gather information about an object—the way they observe it—is not the most sophisticated or optimally accurate.

“If one observer picks up a fragment, he can choose to make whatever measurement he wants. I can pick up another fragment and I can choose to make whatever measurement I want. So how does classical objectivity arise? That’s where we started,” he says.

The researchers reformulated the problem of the emergence of objectivity as a problem of quantum sensing. If an objective fact is, for example, the frequency with which an object shines with light, then observers must obtain accurate information about that frequency, much like a computer equipped with a light sensor. At its best, the setup could capture super-precise measurements and quickly reach a definitive conclusion about the frequency of light—a scenario quantified by a mathematical formula called “quantum Fisher information,” or QFI. In the new work, the researchers used the QFI as a benchmark against which to compare how different, less precise observational schemes reach the same, precise conclusions, a team member says Gabriel Landi at the University of Rochester in New York State.

Surprisingly, the team’s calculations showed that for large enough fragments of physical reality, even observers making imperfect measurements could eventually gather enough information to reach the same conclusions about objectivity as the ideal QFI standard.

“A dumb measurement can actually work just as well as a much more sophisticated measurement,” says Landi. “That’s one way to see the emergence of classicism: when fragments become large enough, observers begin to agree even on simple measurements.” The work thus offers another step towards understanding why, when observing our macroscopic world, we agree on its physical properties, such as the color of a cup of coffee.

“The work highlights that perfect, ideal measurements are not required,” he says Diego Wisniacki at the University of Buenos Aires, Argentina. He says that QFI underlies quantum information theory but has not previously been introduced to quantum Darwinism, so it could bridge this still rather theoretical quantum framework with well-established experiments—for example, in quantum devices with light or superconducting qubits.

“This is another ‘brick’ in our understanding of quantum Darwinism,” he says G. Massimo Palma at the University of Palermo, Italy. “And it is the way. [of studying it] which comes close to an experimental description of what you actually observe in the laboratory.”

The model the researchers used in their study is very simple, so while their method may open the door to new experiments, calculations for more complex systems will be needed to put quantum Darwinism on an even firmer footing, he says. “It would be a really great breakthrough if we could go beyond simple toy models,” says Palma.

Landi says scientists are already interested in turning their theoretical investigations into an experiment—for example, with qubits made of trapped ions, where they could see how the time scale for the emergence of objectivity compares with the specific times during which these qubits are known to retain their quantum.

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