Big quantum

Here’s a little piece I wrote for Prospect, who deemed in the end that it was too hard for their readers. But I am sure it is not, dear blogspotter, too hard for you.

If you think quantum physics is hard to understand, you’re probably confusing understanding with intuition. Don’t assume, as you fret over the notion that a quantum object can be in two places at once, that you’re simply too dumb to get your mind around it. Nobody can, not even the biggest brains in physics. The difference between quantum physicists and the rest of us is that they’ve elected to just accept the weirdness and get on with the maths – as physicist David Mermin puts it, to ‘shut up and calculate.’

But this pragmatic view is losing its appeal. Physicists are unsatisfied with the supreme ability of quantum theory to predict how stuff behaves at very small scales, and are following the lead of its original architects, such as Bohr, Heisenberg and Einstein, in demanding to know what it means. As Lucien Hardy and Robert Spekkens of the high-powered Perimeter Institute in Canada wrote recently, ‘quantum theory is very mysterious and counterintuitive and surprising and it seems to defy us to understand it. And so we take up the challenge.’

This is something of an act of faith, because it isn’t obvious that our minds, having evolved in a world of classical physics where objects have well-defined positions and velocities, can ever truly conceptualize the quantum world where, apparently, they do not. That difference, however, is part of the problem. If the microscopic world is quantum, why doesn’t everything behave that way? Where, once we reach the human scale, has the weirdness gone?

Physicists talk blithely about this happening in a ‘quantum-to-classical transition’, which they generally locate somewhere between the size of large molecules and of living cells – between perhaps a billionth and a millionth of a metre (a nanometre and a micrometre). We can observe subatomic particles obeying quantum rules – that was first done in 1927, when electrons were seen acting like interfering waves – but we can’t detect quantumness in objects big enough to see with the naked eye.

Erwin Schrödinger tried to force this issue by placing the microcosm and the macrocosm in direct contact. In his famous thought experiment, the fate of a hypothetical cat depended on the decay of a radioactive atom, dictated by quantum theory. Because quantum objects can be in a ‘superposition’ of two different states at once, this seemed to imply that the cat could be both alive and dead. Or at least, it could until we looked, for the ‘Copenhagen’ interpretation of quantum theory proposed by Bohr and Heisenberg insists that superpositions are too delicate to survive observation: when we look, they collapse into one state or the other.

The consensus is now that the cross-over from quantum to classical rules involves a process called decoherence, in which delicate quantum states get blurred by interacting with their teeming, noisy environment. An act of measurement using human-scale instruments therefore induces decoherence. According to one view, decoherence imprints a restricted amount of information about the state of the quantum object on its environment, such as the dials of our measuring instruments; the rest is lost forever. Physicist Wojciech Zurek thinks that the properties we measure this way are just those that can most reliably imprint ‘copies’ of the relevant information about the system under inspection. What we measure, then, are the ‘fittest’ states – which is why Zurek calls the idea quantum Darwinism. It has the rather remarkable corollary that the imprinted copies can be ‘used up’, so that repeated measurements will eventually stop giving the same result: measurement changes the outcome.

These are more than just esoteric speculations. Impending practical applications of quantum superpositions, for example in quantum cryptography for encoding optical data securely, or super-fast quantum computers that perform vast numbers of calculations in parallel, depend on preserving superpositions by avoiding decoherence. That’s one reason for the current excitement about experiments that probe the contested ‘middle ground’ between the unambiguously quantum and classical worlds, at scales of tens of nanometres.

Andrew Cleland and coworkers at the University of California have now achieved a long-sought goal in this arena: to place a manufactured mechanical device, big enough to see sharply in the electron microscope, in a quantum superposition of states. They made a ‘nanomechanical resonator’ – a strip of metal and ceramic almost a micrometer thick and about 30 micrometres long, fixed at one end like the reed of a harmonica – and cooled it down to within 25 thousandths of a degree from absolute zero. The strip is small enough that its vibrations follow quantum rules when cold enough, which means that they can only have particular frequencies and energies (heat will wash out this discreteness). The researchers used a superconducting electrical circuit to induce vibrations, and they report in Nature that they could put the strip into a superposition of two states – in effect, as if it is both vibrating and not vibrating at the same time.

Sadly, these vibrations are too small for us to truly ‘see’ what an object looks like that is both moving and not moving. But even more dramatic incursions of quantum oddness might be soon in store. Last year a team of European scientists outlined a proposal to create a real Schrödinger’s cat, substituting an organism small enough to stand on the verge of the quantum world: a virus. They suggested that a single virus suspended by laser beams could be put into a superposition of moving and stationary states. Conceivably, they said, this could even be done with tiny, legged animals called tardigrades or ‘water bears’, a few tenths of a millimetre long. If some way could be devised to link the organism’s motion to its biological behaviour, what then would it do while simultaneously moving and still? Nobody really knows.