Recent experiments have put relatively large objects into quantum states, illuminating the processes by which the ordinary world emerges out of the quantum one.
Schrödinger’s kittens have never been very cute, and the latest litter is no exception. Images of nebulous clouds of ultracold atoms or microscopic strips of silicon are unlikely to go viral on the internet. All the same, these exotic objects are worth heeding, because they show with unprecedented clarity that quantum mechanics is not just the physics of the extremely small.
“Schrödinger’s kittens,” loosely speaking, are objects pitched midway in size between the atomic scale, which quantum mechanics was originally developed to describe, and the cat that Erwin Schrödinger famously invoked to highlight the apparent absurdity of what that theory appeared to imply. These systems are “mesoscopic” — perhaps around the size of viruses or bacteria, composed of many thousands or even billions of atoms, and thus much larger than the typical scales at which counterintuitive quantum-mechanical properties usually appear. They are designed to probe the question: How big can you get while still preserving those quantum properties?
To judge by the latest results, the answer is: pretty darn big. Two distinct types of experiments — both of them carried out by several groups independently — have shown that vast numbers of atoms can be placed in collective quantum states, where we can’t definitely say that the system has one set of properties or another. In one set of experiments, this meant “entangling” two regions of a cloud of cold atoms to make their properties interdependent and correlated in a way that seems heedless of their spatial separation. In the other, microscopic vibrating objects were maneuvered into so-called superpositions of vibrational states. Both results are loosely analogous to the way Schrödinger’s infamous cat, while hidden away in its box, was said to be in a superposition of live and dead states.
The question of how the rules of quantum mechanics turn into the apparently quite different rules of classical mechanics — where objects have well-defined properties, positions and paths — has puzzled scientists ever since quantum theory was first developed in the early 20th century. Is there some fundamental difference between large classical objects and small quantum ones? This conundrum of the so-called quantum-classical transition was highlighted in iconic fashion by Schrödinger’s thought experiment.
The poor cat is a much-misunderstood beast. Schrödinger’s point was not, as often implied, the apparent absurdity of quantum mechanics if extrapolated up to the everyday scale. The cat was the product of correspondence between Schrödinger and Albert Einstein, after Einstein had criticized the interpretation of quantum mechanics championed by the Danish physicist Niels Bohr and his colleagues.
Bohr argued that quantum mechanics seems to force us to conclude that the properties of quantum objects like electrons do not have well-defined values until we measure them. To Einstein, it seemed crazy that some element of reality depends on our conscious intervention to bring it into being. With two younger colleagues, Boris Podolsky and Nathan Rosen, he presented a thought experiment in 1935 that appeared to make that interpretation impossible. The three of them (whose work now goes by the collective label EPR) noted that particles can be created in states that must be correlated with each other, in the sense that if one of them has a particular value for some property, the other must have some other particular value. In the case of two electrons, which have a property called spin, one spin might point “up” while the other electron’s spin points “down.”
In that case, according to Einstein and his colleagues, if Bohr is right and the actual directions of the spins are undetermined until you measure them, then the correlation of the two spins means that measuring one of them instantly fixes the orientation of the other — no matter how far away the particle is. Einstein called this apparent connection “spooky action at a distance.” But such a phenomenon should be impossible, because Einstein’s theory of special relativity shows that no influence can propagate faster than light.
Schrödinger called this correlation between the particles “entanglement.” Experiments since the 1970s have shown that it is a real quantum phenomenon. But this doesn’t mean that quantum particles can somehow influence one another instantly across space through Einstein’s spooky action. It’s better to say that a single particle’s quantum properties are not necessarily determinate at one fixed place in space, but may be “nonlocal”: fully specified only in relation to another particle elsewhere, in a manner that seems to undermine our intuitive notion of space and distance....MUCH MORE