How many of us have heard, or maybe even made, the statement that “quantum mechanics is weird”? As human beings that evolved at classical energy and lengths scales, we are so used to the fact that things look “classical” that the actual workings of our quantum Universe constantly have us in awe, confuse us, and sometimes even appall us.
Take, for instance, the frequently maltreated cat. If any two of us look at the same cat, we will both conclude that we are looking at a cat. Well, actually, we conclude that we both “perceive” a cat, and we will agree about its state of well-being. From a fundamental point of view, the question has to be: why? The answer originates in the fact that any fraction of photons that we intercept with our eyes carries the same, classical information about the lovely beast. The more formal analysis of the emergence of this classical objectivity is known as Quantum Darwinism, as it relies on Darwinian fitness of certain states—their ability to not just survive immersion in the environment, but create, multiple “offspring” of the information about themselves in the photon (and other) environments, where they can be accessed by observers such as us.
Quantum Darwinism shows how the perception of objective classical reality arises via selective amplification and the spreading of information in our fundamentally quantum universe. Quantum Darwinism goes beyond decoherence, as it recognizes that the many copies of the system’s pointer states are imprinted on the environment: agents acquire data indirectly, by intercepting environment fragments (rather than directly measuring systems of interest). The data disseminated through the environment provide us with shared information about stable, effectively classical pointer states. Humans rely primarily on the photon environment, eavesdropping on “objects of interest” by intercepting tiny fractions of photons that contributed to decoherence.