It’s a long way from the weird world of quantum physics to the comfortably familiar world described by the classical laws of physics. The quantum world has particles that can be entangled across vast distances or even have a range of possible states. The classical world, on the other hand, has objects like baseballs that move with a definite speed and direction.
It seems like these two worlds — the large and the small — would have fundamentally different rules. But some physicists are trying to bridge the “quantum-classical transition” by describing how quantum mechanics becomes classical mechanics — specifically by looking at how particles interact with their surrounding environment.
One approach to this is known as quantum Darwinism (QD). As the name suggests, this idea has something to do with survival of the fittest, as described by Darwin’s natural selection. But instead of finches or gazelles surviving to reproduce and pass on their genes, we have the “fittest” quantum properties making the most copies of themselves.
At the heart of quantum Darwinism is measurement. In the macro-world that we inhabit and experience, measurement is very straightforward — a ball falling from the top of a building accelerates at an easily described rate. In the quantum world, though, particles can have a range of possible states, what’s known as a “superposition.” Particles don’t exist in all those states at once; but when a particle is measured, one state emerges as the measured outcome.
Superpositions are very fragile and can be easily disrupted by a noisy environment, in which a quantum particle can interact with many other particles. This is why we always see a particle in a single state, rather than in a quantum superposition. This process of a particle losing its “quantumness” is known as decoherence. In large systems with many particles, decoherence happens very fast; so quantum behavior is hard to observe in these kinds of systems.
Even though decoherence erases the quantum behavior of a particle, multiple people can still observe the same properties of a quantum object. In an article on Quanta, Wojciech Zurek, a Polish-American physicist at Los Alamos National Laboratory in New Mexico, argues that in order for this to happen, two things need to be true.
First, quantum objects have certain “pointer states” which are more likely to survive in a noisy environment. These might include a particle’s location, speed or quantum spin. So while the quantumness of some properties are squashed by interacting with particles in the environment, other properties can resist decoherence — and be measured.
Second, an observer has to be able to get information from the quantum object, which means the object has to imprint this information on its environment. Zurek argues that this happens when many replicas of certain pointer states are made. This allows multiple observers to measure the same value of the property — which is a hallmark of classical behavior.
Pointer states that are best able to survive in a noisy environment and also best able to create replicas in the environment are the ones that can be measured by multiple observers. Because they can be measured, these “fittest” states bridge the gap from the quantum world to the classical world.
This is all great in theory, but does it have real-world implications? Physicists think so. Several groups have attempted to demonstrate quantum Darwinism in the laboratory using simple quantum systems.
Two groups created a quantum system consisting of a single photon with an “environment” of several other photons. The property of interest — or pointer state — was the photon’s polarization. The two teams found that they could obtain information about the photon’s polarization by measuring just one photon in the environment — the information from the photon of interest had been imprinted on the surrounding photons.
Another group ran a similar experiment with a single nitrogen atom embedded in the crystal lattice of a diamond — which is made of carbon atoms. The researchers found that information about the nitrogen atom’s spin was recorded as multiple copies in the surrounding lattice, showing up as changes in the nuclear spin of the nearby atoms.
Both of these experiments show that certain pointer states are more likely to survive within an environment and that multiple copies of the information is imprinted on the environment. As a result, multiple observers could access the same information even if they measured different particles surrounding the particle of interest.
These experiments are simplified systems, so they may not represent what happens in more complex systems or in noisier environments. However, our understanding of the quantum-classical transition is likely to improve as physicists’ ability make delicate measurements like the ones in these experiments improves.