The standard journalistic shorthand for quantum theory calls it 'the theory that describes very small objects such as atoms'. It's convenient, generally pertinent, and historically reasonable, but sadly that doesn't stop it from being wrong. (Whether it will stop me from using it is another matter.)

The notion most students of quantum theory are given is that its limits are a matter of spatial scale: quantized energy levels get ever more closely spaced as the system size increases, until eventually they blur into a continuum. There is some truth in that, but it's not the end of the matter. For one thing, it doesn't explain the most striking distinction between classical and quantum physics, which is not discreteness of energies but the fact that superpositions of states — being in two places at once, say — make perfect sense in the quantum world but perfect nonsense in the classical. The usual explanation for classicality now invokes decoherence, a dispersal of quantum behaviour as a quantum object interacts with its environment.

The fundamental problem of quantum superpositions at macroscopic scales was made clear in Schrödinger's gedanken experiment with a cat, a box, and a bottle of poison opened by the decay of an atom. Again, students are often given a handwaving argument for why the cat cannot be in a superposition of live and dead states: the quantum weirdness is somehow washed out by decoherence in the chain of interactions leading from the atom to the many-particle animal and its environment.

It's sobering, then, to realize that the cat's fate is still disputed. Wojciech Zurek of the Los Alamos National Laboratory, a former student of the late John Wheeler, remembers a talk on these issues at a condensed-matter conference some time ago. “At one point the speaker asked the audience, 'So do you really believe that the cat in the box, assuming perfect isolation, is in a dead/alive superposition?'”, he says. “The vote was an overwhelming disbelief in the suspension of the cat between life and death.”

But when Zurek saw the same question raised at another meeting around 2005 involving researchers in the burgeoning new field of quantum information, “people had no problem with the cat in a superposition.” This younger audience seemingly embraced quantumness, however classically paradoxical.

Surely materials scientists needn't worry about such foundational issues, but can take the pragmatic 'shut up and calculate' view that quantum theory is just a handy tool for deducing the properties of condensed matter? Not necessarily. For instance, nanoelectromechancial systems such as nanoscale resonating arms, potential high-precision force and displacement sensors, now operate at scales where quantum effects could remain evident even in structures containing billions of atoms. One must work incredibly hard to see them, cooling the devices to millikelvin temperatures. But in principle these structures could reveal the haziness in position imposed by the uncertainty principle, and the strange prospect of microscopically visible objects ('Schrödinger's kittens') being effectively in two places at once, showing that quantum mechanics is not sizeist after all.