“Imagine a broken drying machine that spits out pairs of unmatched socks. They come in complementary contrasts: if one is red, the other is green. Or, if one is white, the other is black, and so on. We don’t know which of these options we’ll get until we look—but we do know that if we find one is red, we can be sure the other is green. Whatever the actual colors are, they are correlated with one another.”
“Now imagine the quantum mechanical version of this same machine. According to the Copenhagen interpretation of quantum mechanics developed in the mid-1920s by Niels Bohr, Werner Heisenberg, and collaborators, quantum socks in a correlated state (where the color of one is linked to the color of the other) don’t actually have any fixed colors until we look. The very act of looking at one quantum sock determines the color of the other. If we look in one way, the first sock might be red (and the other therefore green). If we look in another, the first is white (and the other black).”
“Crudely, you could say that in these correlated pairs the colors of the socks are characteristics that extend well beyond the socks themselves. The color of a given sock is not local, that is, not contained in the properties of just that one sock. The two colors are said to be entangled with each other.”
“The physicist Erwin Schrödinger described entanglement as the key to quantum behavior, and used it to construct a famous paradox. It begins with an unfortunate cat that Schrödinger imagined trapped inside a box, into which a lethal poison was released by the outcome of some quantum event. Because the event was quantum, it could be in what physicists call a superposition state: both triggering the poison release, and not triggering it. These superpositions are not unusual for tiny objects like atoms that are firmly in the quantum realm. But, because Schrödinger entangled the event with a large cat, the result is the paradoxical conclusion that the cat is both killed and not killed. The conventional resolution to the paradox was to claim that making a measurement on a superposition state, like the live–dead cat, forces a choice, so that the superposition collapses the cat—indeed, in effect the whole universe—into one state or another: The cat is either dead or alive, but not both. In that view, we can never see the live–dead cat.”
“But what was the state of the cat before we looked? According to the Copenhagen interpretation, the question has no meaning. Reality, it maintains, is what we can observe and measure, and it makes no sense to wonder about what things are really like before we make those observations. Others, most prominently Albert Einstein, couldn’t accept this. They stuck with the classical “realist” view, which says that everything has particular, objective properties, whether we look or not. Einstein and two young colleagues, Boris Podolsky and Nathan Rosen, came up with a version of the “quantum drying machine” thought experiment to try to demonstrate how quantum theory led to a paradox, in which a measurement in one place instantly affected an object in another place. But in the 1980s, measurements of laser photons showed that entanglement really does work that way—not because of “faster-than-light” communication, but because quantum properties can be genuinely non-local, spread over more than one particle.”
“One key to these kittens becoming cats has been learning how to maintain quantum coherence, or roughly, the ability for the peaks and troughs of wave-like quantum particles to stay synchronized. As a quantum state evolves, it gets entangled with its environment, and quantum coherence can leak away into the surroundings. One might very crudely imagine it to be a little like the way heat in a hot body gets dissipated into a cooler surrounding environment.”
“Another way to think of it is to say that information gets increasingly local. The point about quantum systems is that non-local correlations mean you can’t know everything about some part of it by making measurements just on that part. There’s always some residual ignorance. In contrast, once we have established that a sock is red or green, there’s nothing left to be known about what color it is. Wojciech Zurek of Los Alamos National Laboratory in New Mexico has formulated an expression for the ignorance that remains once the state of the measuring apparatus has been determined, which he calls quantum discord. For a classical system, the discord is zero. If it is greater than zero, the system has some quantumness to it.”
“Decoherence bleeds away discord. Quantum phenomena are converted to ones that obey classical rules: no superpositions, no entanglement, no non-locality, and a time and a place for everything.”
“If we could totally suppress decoherence, would that get us all the way to a full-size Schrödinger cat? It might not be that simple. This is because, to know that you’d made one, you’d have to look at it. Sure, the act of entangling a system with a measuring apparatus could itself decohere it—but the problem might be even worse than that. Physicists Johannes Kofler, now at the Max Planck Institute for Quantum Optics in Garching, and Caslav Brukner of the University of Vienna proposed in 2007 that the very act of studying a large quantum system experimentally may induce the emergence of classical behavior even without any decoherence. Measurement itself can turn quantum multiplicity into classical uniqueness.”
“Kofler and Brukner showed that, when a measurement is “coarse-grained,” so that the resolution is insufficient to distinguish several closely spaced quantum states of a very large system, the quantum-mechanical equations describing how it evolves in time collapse into the classical equations of mechanics devised by Isaac Newton. “We can rigorously show that under the coarse-grained measurements, entanglement or nonlocal features of many-particle states are washed out,” says Brukner. Classical physics emerges from quantum physics when measurement becomes fuzzy, as it always must for “big” systems: ones composed of many particles with many possible states.”
“Hyunseok Jeong of Seoul National University in South Korea and his collaborators have shown that even here there’s an aspect of measurement that destroys quantum behavior. In addition to some inevitable fuzziness in what we measure, says Jeong, there is also a degree of ambiguity about exactly when and where we measure: what he calls the measurement references. This too has the effect of making a quantum system appear to behave like a classical one.”
“Kofler says that decoherence and coarse-graining of measurements offer two complementary routes to the classical world. “If you have sufficiently strong decoherence, you get classicality independent of your measurements,” he says. “And if you have coarse-grained measurement, you get classicality independent of the interaction with the environment.””
“This picture offers a striking resolution of the Schrödinger’s cat puzzle. We could never see it in a live–dead superposition, Brukner says, not because it can’t exist as such, or because of decoherence, but because, well, we just couldn’t actually see it. “Even if somebody would prepare a Schrödinger-cat state in front of us, we would not be able to reveal it as such without having an instrument of sufficient precision.” That’s to say, any measurement we could actually make on the cat wouldn’t show anything that couldn’t equally be explained by a classical picture. Even for the oscillators of optomechanical devices, detecting genuine superposition states will be challenging, involving positional differences of just fractions of an ångstrøm (10-10 meters). For such reasons, “it is quite challenging to test these ideas in a real experiment,” Jeong admits. Even so, he optimistically adds, “I hope to see my idea be tested in a laboratory in the near future.””
“Some theories, such as the gravitational-collapse idea of Penrose and Diósi, predict that for large enough particles the interference should vanish.”
“Some researchers have argued that such a superposition could persist in the nerve signal sent from the retina to the brain, so that fleeting “perceptual superpositions” are possible.”