Laboratory Bell tests use carefully isolated photons and electrons, and it is easy to come away thinking entanglement is an exotic effect that exists only in such settings. The truth is the reverse. Entanglement arises naturally whenever quantum systems interact, so it is common throughout nature. What is rare, and hard, is keeping it isolated and under control.

Entanglement in atoms, molecules and materials

The two electrons in a helium atom are entangled: the atom has a definite state, but neither electron has a state of its own. In a chemical bond, the electrons shared between atoms are typically entangled, and the stability of the bond depends on how their states combine. In some materials, vast numbers of electrons are entangled together. In a superconductor, electrons pair up and move as a coordinated whole, which lets current flow without resistance.

The strangeness Bell exposed is part of the ordinary fabric of matter.

Life and entanglement: an open question

Some researchers have proposed that living things use quantum effects. One leading idea concerns migrating birds such as the European robin, which can sense the direction of Earth's magnetic field. The radical pair hypothesis proposes that birds sense magnetic fields through pairs of electrons whose entangled spins respond to the field, inside a protein in the eye called cryptochrome. The evidence is suggestive but not conclusive. A similar wave of excitement about quantum effects in photosynthesis followed experiments in 2007, but it has been tempered by later studies suggesting the effects are short-lived or classical in origin. Bold claims about quantum biology deserve interest and caution.

Decoherence: why the world looks classical

If entanglement is everywhere, why don't we see tables in superpositions, or Schrödinger's cat both alive and dead? A big part of the answer is decoherence, a process studied from 1970 onward by the German physicist H. Dieter Zeh and later developed by Wojciech Zurek and many others.

The idea is that no large object is ever truly isolated. A dust grain floating in air is struck constantly by air molecules and bathed in light. Each collision entangles the grain's position with the state of the molecule or photon that hit it. Decoherence is the process by which a system becomes entangled with its environment, so its quantum behavior spreads out into countless untracked particles. Once that happens, the grain viewed by itself shows no sign of superposition. Interference effects vanish because the information they need is scattered through the environment.

For large objects, this is astonishingly fast. The bigger and warmer an object, and the more it interacts with its surroundings, the faster decoherence destroys its visible quantum behavior. For a dust grain in air, estimates give times far shorter than any clock could measure.

Decoherence explains why quantum effects are hidden in everyday life. Physicists disagree, however, about whether it fully solves the puzzle of measurement, since on some views it explains why outcomes look definite without explaining why one particular outcome occurs. Decoherence explains why superpositions become invisible, but whether it settles the measurement problem is disputed, a debate that belongs to QUAN 202.

The engineering challenge

Decoherence is also the main enemy of quantum technology. Experimenters fight it by cooling systems to near absolute zero, holding them in vacuum and shielding them from stray light. Every quantum technology that relies on entanglement must protect it from decoherence long enough to use it. Much of the field's progress is the story of winning that fight a little more each decade.

A question worth asking

Entanglement's history carries a lesson beyond physics. For about thirty years the EPR question was widely treated as philosophy unworthy of serious attention. It took a physicist working on it in his spare time, and then experimenters willing to risk their careers, to show it could be tested. Their work, later opened to a hundred thousand members of the public, is a reminder that a question dismissed by authority may still be worth asking, and that evidence can settle it.

Worked example: a trapped ion and a dust grain

Compare two objects. The first is a single charged atom, an ion, held by electric fields inside a vacuum chamber and cooled by lasers until it barely moves. Almost nothing touches it. Such an ion can be held in a superposition for seconds or longer and entangled with a second ion.

The second object is a dust grain about a thousandth of a millimeter wide, floating in a room. Every second, enormous numbers of air molecules strike it, and each one carries away a little information about where the grain is. Any superposition of the grain being in two places is lost almost at once, long before anyone could test it.

Between these extremes lie the experiments that probe the boundary. In 1999 Zeilinger's group in Vienna showed that molecules of sixty carbon atoms could interfere like waves. A few years later the same group heated similar molecules until they glowed, and found that the interference faded as they emitted more light: each emitted photon carried information into the environment. Decoherence could be turned up and down like a dial.