A handful of spins in diamond have shone new light on one of the most enduring mysteries in physics – how the objective reality of classical physics emerges from the murky, probabilistic quantum world. Physicists in Germany and the US have used nitrogen-vacancy (NV) centres in diamond to demonstrate “quantum Darwinism”, whereby the “fittest” states of a system survive and proliferate in the transition between the quantum and classical worlds.

In the past, physicists tended to view the classical and quantum worlds as being divided by an abrupt barrier that makes a fundamental distinction between the familiar macroscopic (classical) and the unfamiliar microscopic (quantum) realms. But in recent decades that view has changed. Many experts now think that the transition is gradual, and that the definite classical states we measure come from probabilistic quantum states progressively (although very quickly) losing their coherence as they become ever more entangled with their environment.

Quantum Darwinism, put forward by Wojciech Zurek of Los Alamos National Laboratory in New Mexico, argues that the classical states we perceive are robust quantum states that can survive entanglement during decoherence. His theoretical framework posits that the information about these states will be duplicated many times and disseminated throughout the environment. Just as natural selection tells us that the fittest individuals in a species must survive to reproduce in great numbers and so go on to shape evolution, the fittest quantum states will be copied and appear classical. This redundancy means that many individual observers will measure any given state as having the same value, so ensuring objective reality.